Female reproductive tract microbiota and recurrent pregnancy loss: a nested case-control study

Microbial imbalances linked to early pregnancy loss: a comparative analysis of vaginal microbiota

OBJECTIVE

To explore the role and related functions of vaginal microbiota in early pregnancy loss.

METHODS

This study was a case-control study with a comparison group (reference group). We recruited 178 women, including 73 who had experienced at least one early clinical pregnancy loss and 105 patients with one live birth and no history of pregnancy loss. Data on demographics, disease history, menstrual and reproductive history was collected. The case group patients were sampled immediately upon presenting with pregnancy loss at their first visit. The reference group patients underwent samples when they chose to participate voluntarily. All vaginal discharge was performed DNA Preparation and Metagenomics Sequencing. DNA extraction was performed using the phenol/trichloromethane method and the DNA fragments were then size-selected to 300-700 bp using magnetic beads. The selected fragments were repaired and ligated with indexed adaptors. The captured DNA was amplified again by PCR and circularized to create a single-stranded circular (ssCir) library. The ssCir library was subsequently amplified through rolling circle amplification (RCA) to produce DNA nanoballs (DNBs). The DNBs were then loaded onto a flow cell and sequenced using the DNBSEQ Platform. Nonparametric tests, including Kruskal-Wallis and Wilcoxon tests, were employed. Relative abundance between groups was compared, and differential species selection was performed using the LEfSe software with linear discriminant analysis.

RESULTS

1. PCoA analysis based on Bray-Curtis distances at the species level revealed a difference between the groups (p = 0.011). At the genus level, α-diversity, assessed using the Shannon, Simpson, and Inverse Simpson indices, indicated higher bacterial richness and diversity in the control group (Shannon: mean 0.554 vs. 0.383, p = 0.0044; Simpson: mean 0.254 vs. 0.179, p = 0.0043; Inverse Simpson: mean 1.636 vs. 1.414, p = 0.0043); At the genus level, 107 microbial genera were identified, 18 of which displayed statistically significant differences. At the species level, 23 microbial species showed significant differences between the two groups. 2. We analyzed the differences in the most abundant phyla, genera, and species, with a particular focus on the top 20 most abundant genera and species. Firmicutes and Proteobacteria were significantly more prevalent among patients with pregnancy loss (PL). Among the top 20 most abundant genera, Streptococcus and Porphyromonas were significantly more abundant in patients with PL, whereas Bifidobacterium was significantly more prevalent in the reference group. Among the 20 most abundant species, Lactobacillus crispatus was significantly more prevalent in patients with PL, whereas common in the control group. 3. Principal Coordinates Analysis (PCoA) of Bray-Curtis distances, highlight their distinct clustering patterns, suggesting a notable difference between the metabolic pathways of the two groups. Key pathways with a negative correlation to PL include those related to amino acid biosynthesis, lipid metabolism, and nucleotide biosynthesis.

CONCLUSION

Our study highlights the association between vaginal microbiota dysbiosis and EPL, identifying specific microbial taxa that may contribute to pregnancy loss. These findings underscore the importance of the vaginal microbiome in reproductive health and open up new avenues for research into microbiome-based diagnostics and therapies. By integrating microbial, immune, and environmental data, future research has the potential to uncover the mechanisms underlying EPL and develop targeted interventions to improve pregnancy outcomes.

Endometrial factors and pregnancy loss frequency in recurrent pregnancy loss patients: comparing RT-PCR microbiology, microbial cultures, and immunohistochemistry of endometrium biopsy

The objective of this study is to investigate the presence of bacteria in endometrial samples from patients with recurrent pregnancy loss (RPL) and explore potential correlations between bacterial presence, chronic endometritis, and previous pregnancy loss history. Endometrial samples from 90 RPL patients were analysed using RT-PCR to detect 10 specific bacterial species. A subgroup of 65 patients underwent additional microbial culture and immunohistochemistry for plasma cell identification. Correlations between bacterial presence, chronic endometritis, and the number of previous pregnancy losses were evaluated. We detected at least one out of 10 chosen bacteria DNA by RT-PCR in 24.4% (22/90) of endometrial samples. Patients with PCR-identified bacteria had a significantly higher number of previous pregnancy losses (median 3 vs 2, p = 0.01). No correlation was observed between bacterial presence and chronic endometritis diagnosis. A significant correlation was found between bacterial detection by PCR and microbial culture (p = 0.03), though culture methods detected fewer positive cases. In RPL patients, detecting DNA from at least one of 10 selected bacterial species by RT-PCR correlates with a higher number of previous pregnancy losses. However, this bacterial presence does not correlate with chronic endometritis diagnosis based on the CD138 immunohistochemistry-identified plasma cell count. These findings suggest a potential role of endometrial bacteria in RPL that may be independent of the classical inflammatory response associated with chronic endometritis.

Correlation between Lactobacillus of Vaginal Microbiota and the Pregnancy Outcomes for Patients Experiencing Recurrent Miscarriage

The etiology of recurrent miscarriage (RM) is complex, with the vaginal microbiota (VM) being an important factor associated with RM. We aimed to establish the VM composition in both patients with RM and healthy women and further investigate relationship between the subsequent pregnancy outcomes of patients with RM and VM, to explain the potential mechanism of VM in RM to some extent. A cohort study compared the VM between 34 patients with RM and 15 healthy women using a sequencing technique based on Type IIB restriction enzymes for the microbiome (2bRAD-M). Further comparison was made between 11 patients with clinical miscarriages (CM) and 13 patients with ongoing pregnancies (OP) in the RM group who conceived naturally. To determine the VM composition, the 2bRAD-M library was prepared, and sequence and bioinformatics analyses were conducted. The composition of the VM exhibited notable differences between the non-RM and RM groups, with significant findings for alpha diversity (p < 0.05) and beta diversity (p = 0.01). Further analysis between the RM-OP and RM-CM groups revealed a significant difference in Lactobacillus (97.81% ± 2.71% vs. 53.37% ± 46.42%, p = 0.03). Other uncommon species, such as Cutibacterium acnes (C. acnes) (p = 0.04) were found significantly increase in the RM-CM group. Functional annotation analysis revealed 47 related signaling pathways between the two groups. The results of this study indicate that Lactobacillus is associated with subsequent miscarriages and that C. acnes is closely related to pregnancy outcomes of patients with RM.

Human embryo implantation: The complex interplay between endometrial receptivity and the microbiome

The endometrial and vaginal microbiota have co-evolved with the reproductive tract and play a key role in both health and disease. However, the difference between endometrial and vaginal microbiota, as well as their association with reproductive outcomes in women undergoing frozen embryo transfer, remains unclear. 120 women who underwent in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) and whole embryo freezing were enrolled. The vaginal and uterine microbiome were sequenced during the first frozen thawed embryo transfer. Based on whether or not they were pregnant after embryo transfer, women were assigned into two groups, and the microbiome of their reproductive tract was compared. The V3-V4 regions of the 16S rRNA gene were examined in the samples using the Next Generation Sequencing method. In the vagina, the non-pregnant group had higher bacterial species richness and diversity, with significantly lower Lactobacillus levels (91.66 % & 74.50 %) and higher Gardnerella levels (3.92 % & 12.12 %) than pregnant group (P < 0.05). In the uterine cavity, the diversity of uterine microbiome between pregnant group and non-pregnant group showed no significant differences. However, dramatic decrease in Lactobacillus (37.27 % & 33.45 %) and Pseudomonas (9.80 % & 4.08 %) were observed in the non-pregnant group (P < 0.05). There may be a correlation between the composition of female reproductive tract microbiome and the reproductive outcomes of patients with frozen-thawed embryo transfer. Lactobacillus-dominated microbiome in reproductive tract is more likely to be associated with higher clinical and ongoing pregnancy rate.

The investigation and management of recurrent early pregnancy loss: a Canadian Fertility and Andrology Society clinical practice guideline

This guideline defines recurrent early pregnancy loss (REPL) as two or more losses that occur before 10 weeks gestational age and includes non-consecutive and biochemical losses. Investigations should be considered on an individual basis and may include an evaluation of genetic, anatomical, endocrinological, structural and male-associated factors. Based on the findings and available resources, options for management may include preimplantation genetic testing (PGT) for aneuploidies or PGT for chromosomal structural rearrangements, progesterone supplementation and supportive care. This guideline emphasizes a personalized approach to the problem of REPL, recognizing an overall promising prognosis for this patient population and the avoidance of treatment options that have not been shown to be of benefit.

Expert Opinion on the Use of Probiotics in Recurrent Pregnancy Loss

Recurrent pregnancy loss (RPL) involves multiple consecutive miscarriages in early pregnancy, affecting a significant number of Indian women and placing substantial physical and emotional stress on expecting couples. This expert consensus aims to highlight probiotics as a promising option for enhancing fertility and supporting successful pregnancy outcomes, offering hope to individuals and couples affected by RPL. A group of fourteen experts with diverse expertise in gynecology, obstetrics, and fertility from across India gathered between June 29 and June 30, 2024. According to the experts, advanced maternal age emerges as an independent risk factor for miscarriage, with increased risks among older Indian women. The major contributors to RPL include thyroid disease and polycystic ovarian disease. Experts emphasize that the vaginal microbiome dysbiosis, characterized by the reduced dominance of Lactobacilli, is associated with adverse pregnancy outcome, such as preterm birth, early pregnancy loss, and increased events of RPL. Oral probiotic supplementation, particularly strains like L. acidophilus and L. rhamnosus, may improve embryo implantation, reduce miscarriage risk, and support pregnancy maintenance. A healthy lifestyle choice and minimal use of antibiotics are important in creating a positive reproductive outcome. The present expert opinion supports the potential benefits of probiotics, particularly Lactobacillus species, in managing RPL and improving reproductive outcomes. By promoting a balanced microbiota, reducing inflammation, and modulating immune responses, probiotics may play a critical role in enhancing reproductive success.

Correlation between uterine microbiota and pregnancy outcomes of embryo transfer in overweight and obese women

Objective

Currently, it has been reported that uterine microbiota affected pregnancy outcomes in assisted reproductive technology, but little was known in obese women. Thus, this study aims to explore how body weight affects pregnancy outcomes by comparing the differences in uterine microbiota between women of normal weight and those with obesity or overweight.

Methods

The study included 45 embryo transfer cycles. Patients were divided into two groups based on body mass index (BMI): normal-weight group, BMI ≤23.9 kg/m² (Con group, n=31) and obesity/overweight group, BMI >23.9 kg/m² (OwOb group, n=14). Microbial samples were collected from the catheter tips and analyzed using RNA Ribosomal 16S.

Results

In 45 women, the β-human chorionic gonadotropin (β-hCG) positivity rate and clinical pregnancy rate 10-12 days after embryo transfer were higher in the Con group. At the genus level, the relative abundance of Lactobacillus in the Con group was 2.2 times higher than that in the OwOb group. The Alpha diversity in the OwOb group was significantly higher than in the Con group (p=0.028). After regrouping based on β-hCG results 10-12 days post-transfer, in the Con-negative group (the group with negative β-hCG in the Con group, n=16) the relative abundances of pathogenic bacteria such as Klebsiella (p<0.001), Parasutterella (p=0.004), Dialister (p=0.01), and Gardnerella (p=0.029) were significantly higher than in the Con-positive group (the group with positive β-hCG in the Con group, n=15).

Conclusion

Overweight and obese women possess a distinct uterine microbiota, characterized by a lower relative abundance of Lactobacillus and an increased relative abundance of pathogenic bacteria, along with specific genera strongly associated with obesity. In individuals with higher BMI, pathogenic bacteria are more likely to impair endometrial receptivity, ultimately leading to adverse pregnancy outcomes. Therefore, modulating the uterine microbiota in overweight/obese women may improve assisted reproductive technology success rates.

Exploring the Immunological Aspects and Treatments of Recurrent Pregnancy Loss and Recurrent Implantation Failure

Recurrent pregnancy loss (RPL) is defined as the occurrence of two or more consecutive pregnancy losses before 24 weeks of gestation. It affects 3-5% of women who are attempting to conceive. RPL can stem from a variety of causes and is frequently associated with psychological distress and a diminished quality of life. By contrast, recurrent implantation failure (RIF) refers to the inability to achieve a successful pregnancy after three or more high-quality embryo transfers or at least two instances of egg donation. RIF shares several causative factors with RPL. The immunological underpinnings of these conditions involve alterations in uterine NK cells, reductions in M2 macrophages and myeloid-derived suppressor cells, an increased Th1/Th2 ratio, a decreased Treg/Th17 ratio, the presence of shared ≥3 HLA alleles between partners, and autoimmune disorders. Various therapeutic approaches have been employed to address these immunological concerns, achieving varying degrees of success, although some therapies remain contentious within the medical community. This review intends to explore the immunological factors implicated in RPL and RIF and to analyze the immunological treatments employed for these conditions, which may include steroids, intravenous immunoglobulins, calcineurin inhibitors, anti-TNF antibodies, intralipid infusions, granulocyte colony-stimulating factor, and lymphocyte immunotherapy.

Pregnancy and perinatal outcomes in women with recurrent pregnancy loss-A case-control study

INTRODUCTION

Recurrent pregnancy loss (RPL), defined as two or more pregnancy losses, might be associated with elevated obstetrical and perinatal risks in the following pregnancies. RPL and pregnancy problems related to placental development may have similar etiological features. This study explores the incidences of pregnancy and perinatal outcomes in women with RPL.

MATERIAL AND METHODS

This retrospective case-control study investigated the outcomes of the next subsequent singleton pregnancy after thorough RPL examination (n = 360) in Helsinki University Hospital, Finland, in 2007-2016. Data for cases and four control women for each case, matched for age, parity, delivery month, year, and hospital (n = 1440), were retrieved from the Medical Birth Register. Primary outcomes were pregnancy and delivery complications, perinatal outcomes, and characteristics of pregnancy follow-up. Secondary outcomes were maternal and neonatal diagnoses. Associations between RPL and outcomes were estimated with risk ratios with 95% confidence intervals (CI). In sub-analyses, we compared the outcomes of secondary RPL with multipara controls and women with unexplained or explained RPL.

RESULTS

Women with RPL had a higher risk for gestational hypertension (3.1% vs. 1.4%, risk ratio [RR] 2.20 [Confidence interval (CI) 1.06-4.55], p = 0.03), preterm birth (8.9% vs. 5.8%, RR 1.54 [CI 1.04-2.28], p = 0.04), malpresentation of the fetus (3.3% vs. 1.5%, RR 2.18 [CI 1.09-4.37], p = 0.02), premature rupture of membranes (5.6% vs. 2.4%, RR 2.35 [CI 1.37-4.04], p = 0.002), and had more prenatal visits than controls. Mode of delivery was comparable between the study groups, although RPL women had more induced labor (28.1% vs. 22.2%, RR 1.26 [CI 1.04-1.53], p = 0.02). Mean birthweight was lower (3387 ± 680 g) in RPL women's newborns than in the control group (3482 ± 564 g, p = 0.02), and the risk of umbilical artery pH <7.10 (6.7% vs. 3.6%, RR 1.85 [CI 1.15-2.95], p = 0.03) was higher. Risk ratio for small for gestational age was higher in the secondary RPL group than in multipara controls (5.1% vs. 2.0%, RR 2.50 [CI 1.15-5.42], p = 0.02).

CONCLUSIONS

Women with a history of RPL seem to have higher risks in their subsequent pregnancies and should therefore be monitored carefully. These findings support the theory of placental development being the common nominator behind hypertensive pregnancy disorders and RPL.

The Role of the Vaginal and Endometrial Microbiomes in Infertility and Their Impact on Pregnancy Outcomes in Light of Recent Literature

The Human Microbiome Project (HMP), initiated in 2007, aimed to gather comprehensive knowledge to create a genetic and metabolic map of human-associated microorganisms and their contribution to physiological states and predisposition to certain diseases. Research has revealed that the human microbiome is highly diverse and exhibits significant interpersonal variability; consequently, its exact impact on health remains unclear. With the development of next-generation sequencing (NGS) technologies, the broad spectrum of microbial communities has been better characterized. The lower female genital tract, particularly the vagina, is colonized by various bacterial species, with Lactobacillus spp. predominating. The upper female genital tract, especially the uterus, was long considered sterile. However, recent studies have identified a distinct endometrial microbiome. A Lactobacillus-dominated microbiome of the female genital tract is associated with favorable reproductive outcomes, including higher success rates in natural conception and assisted reproductive technologies (ART). Conversely, microbial imbalances, or dysbiosis, marked by reduced Lactobacilli as well as an increased diversity and abundance of pathogenic species (e.g., Gardnerella vaginalis or Prevotella spp.), are linked to infertility, implantation failure, and pregnancy complications such as miscarriage and preterm birth. Dysbiosis can impair the vaginal or endometrial mucosal barrier and also trigger pro-inflammatory responses, disrupting essential reproductive processes like implantation. Despite growing evidence supporting the associations between the microbiome of the female genital tract and certain gynecological and obstetric conditions, clear microbial biomarkers have yet to be identified, and there is no consensus on the precise composition of a normal or healthy microbiome. The lack of standardized protocols and biomarkers limits the routine use of microbiome screening tests. Therefore, larger patient cohorts are needed to facilitate comparative studies and improve our understanding of the physiological microbiome profiles of the uterus and vagina, as well as how dysbiosis may influence clinical outcomes. Further research is required to refine diagnostic tools and develop personalized therapeutic strategies to improve fertility and pregnancy outcomes.

Pharmacological and non-pharmacological interventions for improving endometrial receptivity in infertile patients with polycystic ovary syndrome: a comprehensive review of the available evidence

Direct and indirect evidence suggests that endometrial receptivity may play a crucial role in the reduced fertility rate of women with polycystic ovary syndrome (PCOS). Various pharmacological and non-pharmacological strategies with potential effects on endometrial receptivity in patients with PCOS have been proposed. The aim of this study was to summarize the rationale and the clinical and experimental evidence of interventions tested for improving endometrial receptivity in infertile patients with PCOS. A systematic review was conducted by consulting electronic databases. All interventions with a potential influence on endometrial receptivity in infertile patients with PCOS were evaluated, and their main biological mechanisms were analysed. In total, 24 interventions related to endometrial receptivity were identified. Notwithstanding a strong biological rationale, no intervention aimed at improving endometrial receptivity in women with PCOS is supported by an adequate body of evidence, limiting their use in clinical practice. Further high-quality research is needed in this field to limit potentially ineffective and unsafe add-on treatments in infertile patients with PCOS.

The association between lifestyle factors and the composition of the vaginal microbiota: a review

PURPOSE

The vaginal microbiota offers valuable insights into women's sexual health and the risk of developing sexually transmitted infections (STIs) and bacterial vaginosis. Despite the public health implications of changes in the vaginal environment, existing data on this topic remain sparse.

METHODS

Following the PRISMA statement guidelines, we consulted five bibliographic databases, focusing on five main daily habits and behaviors. We included only studies published up to October 2023, investigating the influence of personal hygiene, sexual behaviors, hormonal contraception, smoking, alcohol consumption, and psychosocial stress on the vaginal microbiota using next-generation sequencing.

RESULTS

Based on our inclusion criteria, we incorporated 37 studies into this review. Hormonal contraception and personal hygiene were found to promote eubiosis of the vaginal microbiota. In contrast, sexual behaviors, smoking, alcohol consumption, and psychosocial stress were associated with an increased susceptibility to bacterial vaginosis, STIs, and severe pelvic inflammatory diseases due to a modified vaginal microbiota. Black ethnicity emerged as a confounding factor, with this population showing unstable vaginal microbiota. Oral contraception and a stable male sexual partner were found to favor Lactobacillus colonization, acting as a protective factor. Conversely, non-hormonal contraception and unprotected or non-penile/vaginal sexual activity increased the incidence of vaginal inflammation and bacterial vaginosis by disturbing the vaginal microbiota and reducing Lactobacillus abundance.

CONCLUSION

Daily habits and lifestyle can influence the composition of the vaginal microbiota, thereby affecting vaginal health. Disturbances in the vaginal microbiota could be associated factors for STIs and vaginosis. Therefore, prioritizing more appropriate management of the vaginal microbiota is crucial.

A prospective cohort study examining the association between the periconceptual vaginal microbiota and first-trimester miscarriage in Kenyan women

BACKGROUND

Studies evaluating the association between the vaginal microbiota and miscarriage have produced variable results.

OBJECTIVE

This study evaluated the association between periconceptual and first-trimester vaginal microbiota and women's risk for miscarriage.

METHODS

At monthly preconception visits and at 9-12 weeks gestation, women collected vaginal swabs for molecular characterisation of the vaginal microbiota. Participants who became pregnant were followed to identify miscarriage versus pregnancy continuing to at least 20 weeks gestation.

RESULTS

Forty-five women experienced miscarriage and 144 had pregnancies continuing to ≥20 weeks. A principal component analysis of periconceptual and first-trimester vaginal bacteria identified by 16S rRNA gene PCR with next-generation sequencing did not identify distinct bacterial communities with miscarriage versus continuing pregnancy. Using taxon-directed quantitative PCR assays, increasing concentrations of Megasphaera hutchinsoni, Mageeibacillus indolicus, Mobiluncus mulieris and Sneathia sanguinegens/vaginalis were not associated with miscarriage. In exploratory analyses, these data were examined as a binary exposure to allow for multivariable modelling. Detection of Mobiluncus mulieris in first-trimester samples was associated with miscarriage (adjusted relative risk [aRR] 2.14, 95% confidence interval [CI] 1.08, 4.22). Additional analyses compared women with early first-trimester miscarriage (range 4.7-7.3 weeks) to women with continuing pregnancies. Mobiluncus mulieris was detected in all eight (100%) first-trimester samples from women with early first-trimester miscarriage compared to 101/192 (52.6%) samples from women with continuing pregnancy (model did not converge). Detection of Mageeibacillus indolicus in first-trimester samples was also associated with early first-trimester miscarriage (aRR 4.10, 95% CI 1.17, 14.31).

CONCLUSIONS

The primary analyses in this study demonstrated no association between periconceptual or first-trimester vaginal microbiota and miscarriage. Exploratory analyses showing strong associations between first-trimester detection of Mobiluncus mulieris and Mageeibacillus indolicus and early first-trimester miscarriage suggest the need for future studies to determine if these findings are reproducible.

Microbiota and Recurrent Pregnancy Loss (RPL); More than a Simple Connection

Recurrent Pregnancy Loss (RPL) affects 1-2% of women, and its triggering factors are unclear. Several studies have shown that the vaginal, endometrial, and gut microbiota may play a role in RPL. A decrease in the quantity of Lactobacillus crispatus in local microbiota has been associated with an increase in local (vaginal and endometrial) inflammatory response and immune cell activation that leads to pregnancy loss. The inflammatory response may be triggered by gram-negative bacteria, lipopolysaccharides (LPS), viral infections, mycosis, or atypia (tumor growth). Bacterial structures and metabolites produced by microbiota could be involved in immune cell modulation and may be responsible for immune cell activation and molecular mimicry. Gut microbiota metabolic products may increase the amount of circulating pro-inflammatory lymphocytes, which, in turn, will migrate into vaginal or endometrial tissues. Local pro-inflammatory Th1 and Th17 subpopulations and a decrease in local Treg and tolerogenic NK cells are accountable for the increase in pregnancy loss. Local microbiota may modulate the local inflammatory response, increasing pregnancy success. Analyzing local and gut microbiota may be necessary to characterize some RPL patients. Although oral supplementation of probiotics has not been shown to modify vaginal or endometrial microbiota, the metabolites produced by it may benefit patients. Lactobacillus crispatus transplantation into the vagina may enhance the required immune tolerogenic response to achieve a normal pregnancy. The effect of hormone stimulation and progesterone to maintain early pregnancy on microbiota has not been adequately studied, and more research is needed in this area. Well-designed clinical trials are required to ascertain the benefit of microbiota modulation in RPL.

A Standard Pipeline for Analyzing the Endometrial Microbiome

The endometrial microbiome is a rapidly advancing field of research, particularly in obstetrics and gynecology, as it has been found to be linked with obstetric complications and potential impacts on fertility. The diversity of microorganisms presents in the endometrium, along with their metabolites, can influence reproductive outcomes by modulating the local immune environment of the uterus. However, a major challenge in advancing our understanding of the endometrial microbiota lies in the heterogeneity of available studies, which vary in terms of patient selection, control groups, collection methods and analysis methodologies. In this study, we propose a detailed pipeline for endometrial microbiome analysis, based on the most comprehensive prospective of 64 studies that have investigated the endometrial microbiome up to the present. Additionally, our review suggests that a dominance of Lactobacilli in the endometrium may be associated with improved reproductive prognosis, including higher implantation rates and lower miscarriage rates. By establishing a standardized pipeline, we aim to facilitate future research, enabling better comparison and correlation of bacterial communities with the health status of patients, including fertility-related issues.

Vaginal Microbiome and Pregnancy Complications: A Review

Background/Objectives: There are indications that the microbial composition of the maternal mucosal surfaces is associated with adverse events during pregnancy. The aim of this review is to investigate the link between vaginal microbiome alterations and gestational complication risk. Methods: This comprehensive literature review was performed using Medline and Scopus databases. The following search algorithm was used, "Pregnancy Complications" [Mesh] AND (Vagin*), and after the literature screening, 44 studies were included in the final review. Results: The studies that were included investigated the association between vaginal microbial composition and preterm birth, miscarriage, preeclampsia, ectopic pregnancy, gestational diabetes mellitus, chorioamnionitis, and preterm premature rupture of membranes. In most of the studies, it was well established that increased microbial diversity is associated with these conditions. Also, the depletion of Lactobacillus species is linked to most of the gestational complications, while the increased relative abundance and especially Lactobacillus crispatus may exert a protective effect in favor of the pregnant woman. Several pathogenic taxa including Gardnerella, Prevotella, Sneathia, Bacterial Vaginosis-Associated Bacteria-2, Atopobium, and Megasphera seem to be correlated to higher maternal morbidity. Conclusions: Vaginal microbiome aberrations seem to have an association with pregnancy-related adverse events, but more high-quality homogenous studies are necessary to reliably verify this link.

The microbiome in recurrent pregnancy loss - A scoping review

Recurrent pregnancy loss (RPL) is a troubling condition that affects couples worldwide. Despite extensive research efforts, many RPL cases remain unexplained, highlighting the need for novel approaches to unravel its underlying mechanisms. Recent advances in microbiome research have shed light on the potential role of the microbiome in reproductive health and outcomes. Based on a systematic literature research, this review aims to comprehensively explore the current understanding of the microbiome's involvement in RPL, focusing on the vaginal, endometrial, and gut microbiomes. Evidence from the available studies is examined to explain the relationship between the microbiome and RPL. Furthermore, we discuss the diagnostic potential of the microbiome, therapeutic interventions, and future directions in microbiome research for RPL. Understanding the complex interactions between the microbiome and reproductive health holds promise for developing targeted interventions to help patients today diagnosed as unexplained.

The endometrial microbiota and early pregnancy loss

The human endometrium is a dynamic entity that plays a pivotal role in mediating the complex interplay between the mother and developing embryo. Endometrial disruption can lead to pregnancy loss, impacting both maternal physical and psychological health. Recent research suggests that the endometrial microbiota may play a role in this, although the exact mechanisms are still being explored, aided by recent technological advancements and our growing understanding of host immune responses. Suboptimal or dysbiotic vaginal microbiota, characterized by increased microbial diversity and reduced Lactobacillus dominance, has been associated with various adverse reproductive events, including miscarriage. However, the mechanisms linking the lower reproductive tract microbiota with pregnancy loss remain unclear. Recent observational studies implicate a potential microbial continuum between the vaginal and endometrial niche in patients with pregnancy loss; however, transcervical sampling of the low biomass endometrium is highly prone to cross-contamination, which is often not controlled for. In this review, we explore emerging evidence supporting the theory that a dysbiotic endometrial microbiota may modulate key inflammatory pathways required for successful embryo implantation and pregnancy development. We also highlight that a greater understanding of the endometrial microbiota, its relationship with the local endometrial microenvironment, and potential interventions remain a focus for future research.

Clinical Relevance of Vaginal and Endometrial Microbiome Investigation in Women with Repeated Implantation Failure and Recurrent Pregnancy Loss

Recent studies have investigated if and how the vaginal and endometrial microbiome might affect endometrial receptivity and reproductive health. Although there is no consensus on the existence of a core uterine microbiome yet, evidence shows that the dominance of Lactobacillus spp. in the female reproductive tract is generally associated with eubiosis and improved chances of successful implantation and an ongoing pregnancy. Conversely, vaginal and endometrial dysbiosis can cause local inflammation and an increase of pro-inflammatory cytokines, compromising the integrity and receptivity of the endometrial mucosa and potentially hampering successful embryonic implantation. This review provides a critical appraisal of the influence of the vaginal and endometrial microbiome as parts of the female reproductive tract on fertility outcomes, focusing on repeated implantation failure (RIF) and recurrent pregnancy loss (RPL). It seems that RIF as well as RPL are both associated with an increase in microbiome diversity and a loss of Lactobacillus dominance in the lower female reproductive system.

Longitudinal analysis of vaginal microbiota during IVF fresh embryo transfer and in early pregnancy

IMPORTANCE

Infertility is a global public health issue which leads many couples to seek fertility treatments, of which in vitro fertilization (IVF) is considered to be the most effective. Still, only about one-third of the women achieve live birth after the first IVF embryo transfer (IVF-ET). Factors affecting embryo implantation are poorly known, but the female reproductive tract microbiota may play a key role. Our study confirms the beneficial role of vaginal lactobacilli, especially Lactobacillus crispatus, in the probability of achieving clinical pregnancy and live birth following IVF-ET. Our findings regarding the intra-individual shift of vaginal microbiota between non-pregnancy and pregnancy states are novel and provide new information about the dynamics of microbiota in the early steps of human reproduction. These findings may help clinicians in their attempts to optimize the conditions for ET by microbiota screening or modulation and timing the ET when the microbiota is the most favorable.

Effect of L. crispatus M247 Administration on Pregnancy Outcomes in Women Undergoing IVF: A Controlled, Retrospective, Observational, and Open-Label Study

The aim of our study was to retrospectively evaluate whether the oral administration of L. crispatus (M247) could increase pregnancy and live birth rates in women undergoing assisted reproductive technology procedures. Enrolled women (N = 160) were divided into two groups: treated (N = 80) or untreated (N = 80) with the probiotic strain. The odds ratio (OR) for a treated woman to have a clinical pregnancy (CP) was 1.56. In women aged 30-40 years, M247 increased the probability of a CP in correlation with the progressive rise in BMI, reaching 47% (35% in controls) with a BMI of 35 (OR: 2.00). The CAID statistics showed that in a woman of the blastocyst subgroup, below 43 years, with a BMI over 18.6, treatment with M247 increased the chance of a CP from 28.4% to 44.5% (OR: 2.08; p < 0.05). Considering live births, the rate of the probiotic group was 12.5% versus 7.5% (OR: 1.76). Considering only the blastocyst subgroup, the treatment increased the number of live births by 200% (OR: 3.64; p = 0.05). As confirmed also by statistical indices NNT, NNH, and LHH, the use of M247 demonstrated a risk-benefit ratio to the full advantage of the benefits.

Role of the vaginal microbiome in miscarriage: exploring the relationship

Miscarriage is a devastating pregnancy loss that affects many women worldwide. It is characterized as a spontaneous miscarriage that occurs before 20 weeks of gestation which affects more than 25% of pregnancies. While the causes of miscarriage are complex and multifactorial, recent research has suggested a potential role of the vaginal microbiota. The vaginal microbiome is a dynamic ecosystem of microbes that are essential for preserving vaginal health and avoiding infections. Vaginal dysbiosis has been accompanied with numerous adverse pregnancy complications, such as preterm birth. However, the effect of the vaginal microbiome in miscarriage is not fully understood. This review aims to investigate the link between vaginal microbiota and miscarriage. Also, we investigate the various mechanisms through which the vaginal microbiota may affect miscarriage. Additionally, we examine the implications of these research findings, specifically the possibility of vaginal microbiome screening and targeted interventions to prevent miscarriage.

Cervical and Vaginal Microbiomes in Early Miscarriages and Ongoing Pregnancy with and without Dydrogesterone Usage

Emerging evidence suggests that the reproductive tract microbiota is a key modulator of local inflammatory and immune pathways throughout pregnancy and may subsequently impact pregnancy outcomes. In this study, our objective was to analyze the cervical and vaginal microbiomes during early pregnancy among three groups: women with healthy ongoing pregnancies, women undergoing dydrogesterone treatment, and those who experienced miscarriages. The experiment involved 51 women at 8-11 weeks of gestation. The microbiome was examined using 16S rRNA sequencing on the Ion Torrent PGM platform. Across all groups, Lactobacillus iners was predominant, suggesting that the vaginal community type CST III is common among the majority of participants. Notably, our data highlighted the significant roles of Gardnerella vaginalis and Mycoplasma girerdii in the pathogenesis of early miscarriage. Conversely, L. iners and Bifidobacterium longum have a protective effect in early pregnancy. Moreover, dydrogesterone intake appeared to influence notable differences between the cervical and vaginal microbiomes. Overall, our study enhanced our understanding of the cervical and vaginal microbiome composition in the eastern European population during early pregnancy.

Vaginal microbiome in obesity and its impact on reproduction

A number of reproductive outcomes have been increasingly found to be affected by the vaginal microbiota. Obesity has become a global epidemic, affecting increasing numbers of reproductive-age women, and has been shown to be a risk factor for a number of adverse female health outcomes. A healthy vaginal microbiome is characterized by Lactobacillus-dominance, in particular Lactobacillus crispatus; obesity has been found to be associated with higher diversity and a lower likelihood of Lactobacillus-dominance. In this review, we summarize the evidence on the vaginal microbiome in obese women and the impact on reproductive outcomes such as conception rates, early pregnancy, and preterm birth. We further explore the mechanisms by which obesity may result in an altered microbial composition and highlight future avenues for therapeutic targeting of the vaginal microbiota.

A Narrative Review Discussing the Obstetric Repercussions Due to Alterations of Personalized Bacterial Sites Developed within the Vagina, Cervix, and Endometrium

BACKGROUND

The reproductive tract microbiota that evolved as an integrative component has been studied intensively in the last decade. As a result, novel research, clinical opportunities, and perspectives have been derived following the close investigation of this microecological environment. This has paved the way for an update to and improvement of the management strategies and therapeutic approaches. However, obscurities, contradictions, and controversies arise regarding the ascension route from the vagina to the endometrium via the cervix, with finality in adverse obstetric outcomes.

METHODS

Starting from these considerations, we aimed to gather all existing data and information from four major academic databases (PubMed, ISI Web of Knowledge, Scopus, and ScienceDirect) published in the last 13 years (2010-2023) using a controlled vocabulary and dedicated terminology to enhance the coverage, identification, and sorting of potentially eligible studies.

RESULTS

Despite the high number of returned entries (n = 804), only a slight percentage (2.73%) of all manuscripts were deemed eligible following two rounds of evaluation. Cumulatively, a low level of Lactobacillus spp. and of other core microbiota members is mandatory, with a possible eubiosis-to-dysbiosis transition leading to an impairment of metabolic and endocrine network homeostasis. This transposes into a change in the pro-inflammatory landscape and activation of signaling pathways due to activity exerted by the bacterial lipopolysaccharides (LPSs)/endotoxins that further reflect a high risk of miscarriage in various stages. While the presence of some pathogenic entities may be suggestive of an adverse obstetric predisposition, there are still pros and cons of the role of specific strains, as only the vagina and cervix have been targeted as opposed to the endometrium, which recently started to be viewed as the key player in the vagina-cervix-endometrium route. Consequently, based on an individual's profile, diet, and regime, antibiotics and probiotics might be practical or not.

CONCLUSIONS

Resident bacteria have a dual facet and are beneficial for women's health, but, at the same time, relaying on the abundance, richness, and evenness that are definitory indexes standing as intermediaries of a miscarriage.

Recurrent Pregnancy Loss Etiology, Risk Factors, Diagnosis, and Management. Fresh Look into a Full Box

Recurrent pregnancy loss is a complex health challenge with no universally accepted definition. Inconsistency in definitions involves not only the number of spontaneous abortions (two or three) that are accepted for recurrent pregnancy loss but the types of pregnancy and gestational age at miscarriage. Due to the heterogeneity of definitions and criteria applied by international guidelines for recurrent pregnancy loss, the true incidence of recurrent miscarriage, which is reported to range from 1% to 5%, is difficult to estimate. Moreover, the exact etiology of recurrent pregnancy loss remains questionable; thus, it is considered a polyetiological and multifactorial condition with many modifiable and non-modifiable factors involved. Even after thoroughly evaluating recurrent pregnancy loss etiology and risk factors, up to 75% of cases remain unexplained. This review aimed to summarize and critically analyze accumulated knowledge on the etiology, risk factors, relevant diagnostic options, and management approach to recurrent pregnancy loss. The relevance of various factors and their proposed roles in recurrent pregnancy loss pathogenesis remains a matter of discussion. The diagnostic approach and the management largely depend on the etiology and risk factors taken into consideration by a healthcare professional as a cause of recurrent miscarriage for a particular woman or couple. Underestimation of social and health consequences of recurrent pregnancy loss leads to compromised reproductive health and psychological well-being of women after miscarriage. Studies on etiology and risk factors for recurrent pregnancy loss, especially idiopathic, should be continued. The existing international guidelines require updates to assist clinical practice.

The Female Reproductive Tract Microbiota: Friends and Foe

We do not seem to be the only owner of our body; it houses a large population of microorganisms. Through countless years of coevolution, microbes and hosts have developed complex relationships. In the past few years, the impact of microbial communities on their host has received significant attention. Advanced molecular sequencing techniques have revealed a remarkable diversity of the organ-specific microbiota populations, including in the reproductive tract. Currently, the goal of researchers has shifted to generate and perceive the molecular data of those hidden travelers of our body and harness them for the betterment of human health. Recently, microbial communities of the lower and upper reproductive tract and their correlation with the implication in reproductive health and disease have been extensively studied. Many intrinsic and extrinsic factors influences the female reproductive tract microbiota (FRTM) that directly affects the reproductive health. It is now believed that FRTM dominated by Lactobacilli may play an essential role in obstetric health beyond the woman's intimate comfort and well-being. Women with altered microbiota may face numerous health-related issues. Altered microbiota can be manipulated and restored to their original shape to re-establish normal reproductive health. The aim of the present review is to summarize the FRTM functional aspects that influence reproductive health.

Cervicovaginal microbiome in patients with recurrent pregnancy loss

There have been few studies concerning an association between unexplained recurrent pregnancy loss (RPL) and the microbiome. A recent study including 67 patients demonstrated that an increase in Ureaplasma species in the endometrium raised the risk of miscarriage with an euploid karyotype. While endometrial sampling is invasive, cervicovaginal sampling is not. We compared vaginal and cervical microbiomes with a 16 S ribosomal RNA sequence between 88 patients with unexplained RPL and 17 healthy women with no history of miscarriage. We prospectively assessed risk factors for maternal colonization at a subsequent miscarriage without an aneuploid karyotype in patients. Cervicovaginal bacteria were dominated by Lactobacillus iners, Gardnerella vaginalis, Atopobium vaginae and Bifidobacterium breve in Japanese population. The proportions of Delftia and unknown bacteria in the cervix were significantly higher in patients with RPL than in controls. Streptococcus, Microbacterium, Delftia, Anaerobacillus and Chloroplast in the cervix were significantly higher in patients with a history of chorioamnionitis compared to the controls. The abundance of Cutibacterium and Anaerobacillus in the cervix was significantly higher in patients who had subsequently miscarried compared to those who gave birth. The miscarriage rate in patients with higher proportions of both Cutibacterium and Anaerobacillus (66.7%, 2/3) was significantly greater than that of patients who lacked these bacteria (9.2%, 6/65, adjusted odds ratio 16.90, 95% confidence interval 1.27-225.47, p = 0.032). The presence of certain bacteria could be a predictor of subsequent miscarriage without an aneuploid karyotype. The cervicovaginal microbiome might be useful for investigating a possible cause of RPL.

Microbiome Changes in Pregnancy Disorders

The human microbiota comprises all microorganisms, such as bacteria, fungi, and viruses, found within a specific environment that live on our bodies and inside us. The last few years have witnessed an explosion of information related to the role of microbiota changes in health and disease. Even though the gut microbiota is considered the most important in maintaining our health, other regions of the human body, such as the oral cavity, lungs, vagina, and skin, possess their own microbiota. Recent work suggests a correlation between the microbiota present during pregnancy and pregnancy complications. The aim of our literature review was to provide a broad overview of this growing and important topic. We focused on the most significant changes in the microbiota in the four more common obstetric diseases affecting women's health. Thus, our attention will be focused on hypertensive disorders, gestational diabetes mellitus, preterm birth, and recurrent miscarriage. Pregnancy is a unique period in a woman's life since the body undergoes different adaptations to provide an optimal environment for fetal growth. Such changes also involve all the microorganisms, which vary in composition and quantity during the three trimesters of gestation. In addition, special attention will be devoted to the potential and fundamental advances in developing clinical applications to prevent and treat those disorders by modulating the microbiota to develop personalized therapies for disease prevention and tailored treatments.

Endometrial Microbiota and Immune Tolerance in Pregnancy

Recent studies have demonstrated that the uterus has its own microbiota. However, there is no consensus on endometrial microbiota composition, thus its role in the healthy uterine environment is still a frontier topic. Endometrial receptivity is key to embryo implantation, and in this specific context immunological tolerance against fetal antigens and the tightly regulated expression of inflammatory mediators are fundamental. According to recent evidence, endometrial microbiota may interact in a very dynamic way with the immune system during the peri-conceptional stage and later during pregnancy. For this reason, a condition of dysbiosis might lead to adverse pregnancy outcomes. The aim of this review is to summarize the evidence on the molecular mechanisms by which the endometrial microbiota may interact with the immune system. For this purpose, the link between dysbiosis and reproductive disorders, such as infertility, recurrent pregnancy loss (RPL), and preterm birth, will be discussed. In conclusion, the most recent findings from molecular analyses will be reported to illustrate and possibly overcome the intrinsic limitations of uterine microbiota detection (low endometrial biomass, high risk of contamination during sampling, and lack of standardization).

Association between the vaginal and uterine microbiota and the risk of early embryonic arrest

The aim of this study was to explore the microecological distribution and differences in the uterus and vaginal microbiome in women with early embryonic arrest and those with normal pregnancy by high-throughput sequencing. We systematically sampled the vaginal and uterine microbiomes of 56 pregnant women, namely, 38 patients with early embryonic arrest and 18 pregnant women with normal pregnancy-induced abortion. We obtained colonization data by 16S rRNA gene amplicon sequencing. In the vagina, Lactobacillus, Bacteroidetes and Helicobacter exhibited significant differences between the groups. We further found that Lactobacillus iners, Lactobacillus crispatus, Lactobacillus gasseri and Lactobacillus jensenii were the most dominant Lactobacillus species and that L. iners was significantly different between the groups. Receiver operating characteristic (ROC) curve analysis confirmed that Ensifer had the highest predictive value for early embryonic arrest. In the uterine cavity, we determined that Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria were the dominant bacteria at the phylum level and that Bacteroides, Pseudarthrobacter, Lactobacillus and Ralstonia were the dominant genera. Further classification of Lactobacillus revealed that L. iners, L. crispatus, L. gasseri, and L. jensenii were the main species. There was a significant difference in L. jensenii between the normal pregnancy group and early embryonic arrest group. Random forest analysis revealed 18 different genera in the uterus, and ROC curve analysis indicated that Candidatus Symbiobacter, Odoribacter, Blautia, Nocardioides and Ileibacterium had a certain predictive value.