A comparison of pregnancy-specific risk scoring systems for venous thromboembolic pharmacoprophylaxis in hospitalized maternity patients

OBJECTIVE

Venous thromboembolism (VTE) remains a leading cause of maternal mortality. The American College of Obstetricians and Gynecologists (ACOG) and the Royal College of Obstetricians & Gynecologists (RCOG) have proposed pregnancy-specific risk scoring guidelines for antepartum (AP) and postpartum (PP) thromboprophylaxis. We compared the impact of scoring thresholds and their potential preventative effect.

STUDY DESIGN

We conducted a retrospective cohort study of hospitalized maternity patients over a 4-month period. Patients were assigned an AP and PP risk score using each guideline. Hospitalization-associated VTE was accessed over a 6-year period. Comparison was by Fischer's exact and Chi Square tests.

RESULTS

638 women were included. Of AP patients, 20% met pharmacoprophylaxis criteria for baseline characteristics and 100% for length of stay using RCOG, and 12% met phrarmacoprophylaxis criteria using ACOG (p < .001). For PP patients, 53% met criteria for RCOG compared to 24% using ACOG (p < .001). If pharmacoprophylaxis were performed at a threshold 1 point above recommendation, 7% of AP patients and 11% of PP women would meet ACOG criteria. This increased ACOG threshold captured all cases of VTE following hospitalization.

CONCLUSION

In our population, using ACOG prophylaxis guidelines at an increased threshold would have potentially prevented all hospitalization related VTE without excessive anti-coagulation.

Pharmacologic intervention for the management of retained placenta: a systematic review and meta-analysis of randomized trials

BACKGROUND

Retained placenta affects 2% to 3.3% of all vaginal deliveries and is one of the leading causes of postpartum hemorrhage worldwide. Despite the prevalence of this condition, there is limited guidance on its management.

OBJECTIVE

A systematic review and meta-analysis were performed to evaluate the efficacy of pharmacologic interventions for the management of retained placenta.

STUDY DESIGN

PubMed, ClinicalTrials.gov, Cochrane Library, Web of Science, and Scopus were searched for full-text publications in English. Search terms included "retained placenta" AND "treatment" OR "therapy" OR "disease management" OR "Pitocin" OR "misoprostol" OR "Cytotec" OR "dinoprostone" OR "nitroglycerin" OR "carbetocin" OR "ergotamine," with no restriction on publication dates. Only randomized controlled trials were included. The primary outcome was the need for manual extraction of the placenta or dilation and curettage. Reviewers evaluated the quality of included articles using the Cochrane Collaboration's tool for assessing the risk of bias. Pooled risk ratios were estimated based on random- and fixed-effects analyses. Interstudy heterogeneity was considered when I²≥50%.

RESULTS

The literature search identified 29 randomized controlled trials that met the inclusion criteria (2682 subjects). The most commonly used agent across the studies was oxytocin administered via umbilical vein injection; there was high heterogeneity among these studies (I²=62%). Oxytocin was inferior to carbetocin (risk ratio, 1.61; 95% confidence interval, 1.03-2.52) and prostaglandins (risk ratio, 2.63; 95% confidence interval, 1.18-5.86) for the primary outcome. For oxytocin, prostaglandin agents, and nitroglycerin, there was a trend toward favoring the study drug for the primary outcome compared with control or placebo. Compared with placebo or control, estimated blood loss was lower if pharmacologic interventions were administered, with a mean difference of 121.5 mL (95% confidence interval, -185.7 to -52.3). There was no difference in postpartum hemorrhage or the need for blood transfusion between pharmacologic interventions and placebo or control.

CONCLUSION

Pooled estimates for oxytocin via umbilical vein injection, prostaglandin agents, and nitroglycerin performed favorably compared with placebo or control for the management of retained placenta. Carbetocin and prostaglandin agents were superior to oxytocin in reducing the need for manual extraction or dilation and curettage.

Fetal Exosomal Platelet-activating Factor Triggers Functional Progesterone Withdrawal in Human Placenta

In most mammals, labor is heralded by the withdrawal of progesterone. In humans, circulating progesterone levels increase as gestation advances while placental expression of progesterone receptor A (PR-A) declines. As a result of PR-A downregulation, the non-canonical NF-κB pathway is activated, an event implicated in triggering labor. Here, we sought to identify fetal-derived mediator(s) that represses placental PR-A in human placenta leading to activation of pro-labor signaling. Lipidomic profiling demonstrated enrichment of platelet-activating factor (PAF) in exosomes originating from the human fetus. Exposure of primary cytotrophoblasts to fetal exosomes from term pregnancies reduced PR-A expression by > 50%, and PAF also reduced PR-A message levels in a dose-dependent manner. Notably, fetal exosomes from preterm pregnancies had lower PAF levels and no effect on PR-A expression. Synthetic PAF-induced DNA methylation increases by 20% at the PR-A promoter, leading to recruitment of corepressors and downregulation of PR-A in cytotrophoblast. Furthermore, suppression of PR-A by PAF-stimulated expression of the pro-labor genes, corticotropin-releasing hormone (CRH) and cyclooxygenase-2 (COX-2), which was reversed by disruption of the DNA methyltransferases 3B and 3L. Taken together, PAF represents a novel fetal-derived candidate for initiation of labor by stimulating methylation and repression of PR-A and activating pro-labor signaling in trophoblast.

Assessing resilience of healthcare infrastructure exposed to COVID-19: emerging risks, resilience indicators, interdependencies and international standards

In the moment of preparation of this paper, the world is still globally in grip of the Corona (COVID-19) crisis, and the need to understand the broader overall framework of the crisis increases. As in similar cases in the past, also with this one, the main interest is on the “first response”. Fully appreciating the efforts of those risking their lives facing pandemics, this paper tries to identify the main elements of the larger, possibly global, framework, supported by international standards, needed to deal with new (emerging) risks resulting from threats like Corona and assess the resilience of systems affected. The paper proposes that future solutions should include a number of new elements, related to both risk and resilience. That should include broadening the scope of attention, currently focused onto preparation and response phases, to the phases of “understanding risks”, including emerging risks, and transformation and adaptation. The paper suggests to use resilience indicators in this process. The proposed approach has been applied in different cases involving critical infrastructures in Europe (energy supply, water supply, transportation, etc., exposed to various threats), including the health system in Austria. The detailed, indicator-based, resilience analysis included mapping resilience, resilience stress-testing, visualization, etc., showing, already before the COVID-19, the resilience (stress-testing) limits of the infrastructures. A simpler (57 indicator based) analysis has, then been done for 11 countries (including Austria). The paper links these results with the options available in the area of policies, standards, guidelines and tools (such as the RiskRadar), with focus on interdependencies and global standards—especially the new ISO 31,050, linking emerging risks and resilience.

Characterization of Perineum Elasticity and Pubic Bone-Perineal Critical Distance with a Novel Tactile Probe: Results of an Intraobserver Reproducibility Study

BACKGROUND

Tactile imaging provides biomechanical mapping of soft tissues. Objective biomechanical and anatomical assessment of critical structures within the vagina and pelvis may allow development and validation of a clinical tool that could assist with clinical decisions regarding obstetrical procedures and mode of delivery. Objective: To assess intraobserver reproducibility of measurements of perineal elasticity and pubic bone-perineal critical distance with a novel tactile probe in pregnant women.

METHODS

An Antepartum Tactile Imager (ATI) was designed with a vaginal probe resembling a fetal skull. The probe comprises 128 tactile sensors on a double curved surface and measures 46 mm in width and 72 mm in length. The probe has a motion tracking sensor that allows acquisition of 3D tactile images. There were two arms of the study. In the first arm, biomechanical mapping of the perineum and pelvic bone location was performed in 10 non-pregnant women for purposes of demonstrating safety and feasibility. In the second arm, biomechanical mapping was performed in 10 pregnant women to explore intraobserver reproducibility. Each subject had two standardized examinations over 3 - 5 minutes by the same observer. Examination comfort and pain levels were assessed by post-procedure survey. Reproducibility was analyzed by intraclass correlation coefficients (ICC) with 95% confidence intervals and Bland-Altman plots. Bias and the 95% limits of agreement were also calculated.

RESULTS

The safety and feasibility arm of the study demonstrated high degree of safety and tolerability and reliable acquisition of tactile signals. In the reproducibility arm, 10 pregnant women were recruited at mean gestational age of 34.2 ± 6.5 weeks. The mean perineum elasticity (Young's modulus, E) was 9.8 ± 5.9 kPa, and the mean pubic bone-perineal critical distance (D) at 20 kPa load was 34.6 ± 6.2 mm. The ICC was 0.97 [95% confidence interval (CI) 0.91, 0.99] and 0.82 [CI 0.44, 0.95] for E and D respectively, consistent with excellent intrarater agreement. The bias and the 95% limits of agreement of E were -6.3% and -29.4% to +16.7%, respectively. The bias and the 95% limits of agreement of D were -2.6% and -25.3% to +20.2%, respectively.

CONCLUSIONS

The tactile imaging data obtained in the study reproducibly characterized perineal elasticity and pubic bone-perineal critical distance. Further evaluation of this tool in clinical settings is warranted.

Cervical Characterization with Tactile-Ultrasound Probe

BACKGROUND

Premature cervical softening and shortening may be considered an early mechanical failure that predisposes to preterm birth. Preliminary clinical studies demonstrate that cervical elastography may be able to quantify this phenomenon and predict spontaneous preterm delivery.

OBJECTIVE

To explore a new approach for cervix elasticity and length measurements with tactile-ultrasound probe.

METHODS

Cervix probe has tactile array and ultrasound transducer designed to apply controllable load to cervix and acquire stress-strain data for calculation of cervical elasticity (Young's modulus) and cervical length for four cervix sectors. Average values, standard deviations, intraclass correlation coefficients and the 95% limits of agreement (Bland-Altman plots) were estimated.

RESULTS

Ten non-pregnant and ten pregnant women were examined with the probe. The study with non-pregnant women demonstrated a reliable acquisition of the tactile signals. The ultrasound signals had a prolonged appearance; identification of the internal os of the cervix in these signals was not reliable. The study with pregnant women with the gestational age of 25.4 ± 2.3 weeks demonstrated reliable data acquisition with real-time visualization of the ultrasound signals. Average values for cervical elasticity and standard deviations of 19.7 ± 15.4 kPa and length of 30.7 ± 6.6 mm were calculated based on two measurements per 4 sectors. Measurement repeatability calculated as intraclass correlation coefficients between two measurements at the same cervix sector on pregnant women was found to be 0.97 for cervical elasticity and 0.93 for the cervical length. The 95% limits of agreement of 1) cervical elasticity were from -22.4% to +14.9%, and 2) cervical length from -13.3% to +16.5%.

CONCLUSIONS

This study demonstrated clinically acceptable measurement performance and reproducibility. The availability of stress-strain data allowed the computation of cervical elasticity and length. This approach has the potential to provide cervical markers to predict spontaneous preterm delivery.

A Bibliometric Analysis of Top-Cited Journal Articles in Obstetrics and Gynecology

IMPORTANCE

Citation analysis is a bibliometric method that uses citation rates to evaluate research performance. This type of analysis can identify the articles that have shaped the modern history of obstetrics and gynecology (OBGYN).

OBJECTIVES

To identify and characterize top-cited OBGYN articles in the Institute for Scientific Information Web of Science's Science Citation Index Expanded and to compare top-cited OBGYN articles published in specialty OBGYN journals with those published in nonspecialty journals.

DESIGN, SETTING, AND PARTICIPANTS

Cross-sectional bibliometric analysis of top-cited articles that were indexed in the Science Citation Index Expanded from 1980 to 2018. The Science Citation Index Expanded was queried using search terms from the American Board of Obstetrics and Gynecology's 2018 certifying examination topics list. The top 100 articles from all journals and the top 100 articles from OBGYN journals were evaluated for specific characteristics. Data were analyzed in March 2019.

MAIN OUTCOMES AND MEASURES

The articles were characterized by citation number, publication year, topic, study design, and authorship. After excluding articles that featured on both lists, top-cited articles were compared.

RESULTS

The query identified 3 767 874 articles, of which 278 846 (7.4%) were published in OBGYN journals. The top-cited article was published by Rossouw and colleagues in JAMA (2002). Top-cited articles published in nonspecialty journals were more frequently cited than those in OBGYN journals (median [interquartile range], 1738 [1490-2077] citations vs 666 [580-843] citations, respectively; P < .001) and were more likely to be randomized trials (25.0% vs 2.2%, respectively; difference, 22.8%; 95% CI, 13.5%-32.2%; P < .001). Whereas articles from nonspecialty journals focused on broad topics like osteoporosis, articles from OBGYN journal focused on topics like preeclampsia and endometriosis.

CONCLUSIONS AND RELEVANCE

This study found substantial differences between top-cited OBGYN articles published in nonspecialty vs OBGYN journals. These differences may reflect the different goals of the journals, which work together to ensure optimal dissemination of impactful articles.

Fetal lung C4BPA induces p100 processing in human placenta

The non-canonical NF-κB signaling may be a central integrator of a placental clock that governs the length of human pregnancy. We sought to identify fetal signals that could activate this NF-κB pathway in the placenta, and in turn, contribute to the onset of labor. Proteomics analysis of exosomes purified from fetal cord arterial blood revealed a total of 328 proteins, among which 48 were more significantly abundant (p < 0.01) in samples from women who delivered following elective Cesarean-section at term (39 to 40 weeks of estimated gestational age, EGA) compared to those who had elective Cesarean deliveries near term (35 to 36 weeks of EGA). Computational, crystal structural, and gene functional analyses showed that one of these 48 proteins, C4BPA, binds to CD40 of placental villous trophoblast to activate p100 processing to p52, and in turn, pro-labor genes. These results suggest that fetal C4BPA-induced activation of non-canonical NF-κB in human placenta may play a critical role in processes of term or preterm labor.

Integrated RNA-seq and ChIP-seq analysis reveals a feed-forward loop regulating H3K9ac and key labor drivers in human placenta

BACKGROUND

Chromatin alterations are important mediators of gene expression changes. We have recently shown that activated non-canonical NF-κB signaling (RelB/p52) recruits histone acetyltransferase CBP and deacetylase HDAC1 to selectively acetylate H3K9 (H3K9ac) to induce expression of corticotropin-releasing hormone (CRH) and prostaglandin-endoperoxide synthase-2 (PTGS2) in the human placenta. Both of these genes play a role in initiating parturition in human pregnancy.

METHODS

We performed chromatin immunoprecipitation followed by gene sequencing (ChIP-seq) in primary term human cytotrophoblast (CTB) with use of antibodies to RelB, CBP, HDAC1 and H3K9ac. We further associated these chromatin alterations with gene expression changes from mid-trimester to term in CTB by RNA sequencing (RNA-seq).

RESULTS

We detected a genome-wide differential gene enrichment between mid-trimester and term human placenta. Pathway analysis identified that cytokine-cytokine receptor interaction, NF-κB, and TNF are the leading pathways enriched in term placenta and associated with these chromatin alterations.

DISCUSSIONS

Our analysis has provided the first-time characterization of the key players of human placental origin with molecular changes resulting from chromatin modifications, which could drive human labor.

Anandamide down-regulates placental transporter expression through CB2 receptor-mediated inhibition of cAMP synthesis

The BCRP/ABCG2 efflux transporter is expressed on the membrane of placental syncytiotrophoblasts and protects the fetus from toxicant exposure. Syncytiotrophoblasts arise from the fusion of cytotrophoblasts, a process negatively regulated by the endocannabinoid, anandamide (AEA). It is unknown whether AEA can influence fetal concentrations of xenobiotics by modulating the expression of transporters in syncytiotrophoblasts. Here, we sought to characterize and identify the mechanism(s) responsible for AEA-mediated down-regulation of the BCRP transporter in human placental explants and BeWo trophoblasts. Treatment of human placental explants with AEA (1 μM, 24 h) reduced hCGα, syncytin-1, and BCRP mRNAs by ˜30%. Similarly, treatment of BeWo trophoblasts with AEA (0-10 μM, 3-24 h) coordinately down-regulated mRNAs for hCGß, syncytin-2, and BCRP. In turn, AEA increased the sensitivity of trophoblasts to the cytotoxicity of mitoxantrone, a known BCRP substrate, and environmental and dietary contaminants including mycoestrogens and perfluorinated chemicals. AEA-treated trophoblasts also demonstrated reduced BCRP transport of the mycoestrogen zearalenone and the diabetes drug glyburide, labeled with BODIPY. The AEA-mediated reduction of BCRP mRNA was abrogated when placental cells were co-treated with AM630, a CB2 receptor inhibitor, or 8-Br-cAMP, a cAMP analog. AEA reduced intracellular cAMP levels in trophoblasts by 75% at 1 h, and completely inhibited forskolin-induced phosphorylation of the cAMP response element binding protein (CREB). AEA also decreased p-CREB binding to the BCRP promoter. Taken together, our data indicate that AEA down-regulates placental transporter expression and activity via CB2-cAMP signaling. This novel mechanism may explain the repression of placental BCRP expression observed during diseases of pregnancy.

Vitamin D stimulates multiple microRNAs to inhibit CRH and other pro-labor genes in human placenta

Maternal vitamin D deficiency is linked to adverse pregnancy outcomes including spontaneous preterm birth (SPB). Placental corticotropin-releasing hormone (CRH) has been proposed to be part of a clock that governs the length of gestation in humans, with elevated maternal serum levels predicting early delivery. In this study, we test the hypothesis that vitamin D could contribute to the prevention of preterm labor by inhibiting CRH and other pro-labor mediators. The biological activity of vitamin D occurs via two pathways: non-genomic and genomic responses, both of which involve binding of 1,25-dihydroxyvitamin D (1,25(OH)2D), the active metabolite of vitamin D binding to the vitamin D receptor (VDR). By using chromatin immunoprecipitation followed by sequencing (ChIP-seq), we found that 1,25(OH)2D stimulates association of VDR with a number of miRNA genes including MIR181B2 and MIR26B, and their mature products miR-181b-5p and miR-26b-5p are predicted to target CRH and cyclooxygenase-2 (COX-2) mRNA at 3'-untranslated region (UTR), respectively. We performed RT-qPCR analysis to validate that expression of mature miR-181b-5p and miR-26b-5p in term human syncytiotrophoblast increased in response to treatment with 1,25(OH)2D. miR-181b-5p- or miR-26b-5p-mediated inhibition of CRH or COX-2 was further assessed by the use of miRNA mimics/inhibitors and a luciferase reporter assay. Taken together, this study has identified novel mechanisms by which vitamin D downregulates pro-labor genes and could lower the risk of preterm delivery.

Trends in invasive prenatal diagnostic testing at a single institution

OBJECTIVE

As diagnostic methodologies evolve, we sought to determine whether invasive testing rates would decline, whether there would be a shift in indications for invasive testing, and whether the diagnostic yield would increase.

METHODS

We conducted a retrospective, observational study from 2006 through 2015. We quantified the number of invasive procedures per year and examined what percentage of these procedures yielded abnormal results. We also examined the indications for testing and determined the trend of these indications during the study period.

RESULTS

The number of amniocenteses showed a steady decline (P < .05). The number of CVS procedures has increased and was recently equivalent to amniocentesis. The percentage of abnormal results steadily increased from 11.4% to 27.0% (P < .001). The abnormal aneuploidy screening indication remained constant over time. Advanced maternal age (AMA) as the sole indication substantially declined from 42.3% to 15.52% (P < .001). Testing for a known single gene disorder steadily increased from 3.0% to 9.20% (P = .018).

CONCLUSION

Our study showed a significant decline in the number of amniocenteses, a steady increase in the percentage of abnormal results from invasive testing, and a decline in AMA as the sole indication for invasive testing.

Screening a small molecule library to identify inhibitors of NF-κB inducing kinase and pro-labor genes in human placenta

The non-canonical NF-κB signaling (RelB/p52) pathway drives pro-labor genes in the human placenta, including corticotropin-releasing hormone (CRH) and cyclooxygenase-2 (COX-2), making this a potential therapeutic target to delay onset of labor. Here we sought to identify small molecule compounds from a pre-existing chemical library of orally active drugs that can inhibit this NF-κB signaling, and in turn, human placental CRH and COX-2 production. We used a cell-based assay coupled with a dual-luciferase reporter system to perform an in vitro screening of a small molecule library of 1,120 compounds for inhibition of the non-canonical NF-κB pathway. Cell toxicity studies and drug efflux transport MRP1 assays were used to further characterize the lead compounds. We have found that 14 drugs have selective inhibitory activity against lymphotoxin beta complex-induced activation of RelB/p52 in HEK293T cells, several of which also inhibited expression of CRH and COX-2 in human term trophoblast. We identified sulfapyridine and propranolol with activity against CRH and COX-2 that deserve further study. These drugs could serve as the basis for development of orally active drugs to affect length of gestation, first in an animal model, and then in clinical trials to prevent preterm birth during human pregnancy.

Cerclage in twin pregnancy with dilated cervix between 16 to 24 weeks of gestation: retrospective cohort study

BACKGROUND

Cervical dilation in the second trimester is associated with a greater than 90% rate of spontaneous preterm birth and a poor perinatal prognosis.

OBJECTIVE

To compare the perinatal outcomes of twin pregnancies with dilated cervix in women who underwent either cerclage or expectant management.

STUDY DESIGN

Retrospective cohort study of asymptomatic twin pregnancies identified with cervical dilation of ≥1 cm at 16-24 weeks (1997-2014) at 7 institutions. Exclusion criteria were genetic or major fetal anomaly, multifetal reduction at >14 weeks, prior cerclage placement, monochorionic-monoamniotic placentation, active vaginal bleeding, labor, chorioamnionitis, elective termination of pregnancy, or medically indicated preterm birth. The primary outcome was incidence of spontaneous preterm birth at <34 weeks. Secondary outcomes were incidence of spontaneous preterm birth at <32 weeks, <28 weeks, and <24 weeks; perinatal mortality; and composite adverse neonatal outcome (respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, and sepsis).

RESULTS

A total of 76 women with twin pregnancy with dilated cervix of 1.0-4.5 cm were managed with either cerclage (n = 38) or expectant management (n = 38). Demographic characteristics were not significantly different. Analysis was adjusted for amniocentesis and vaginal progesterone use. In the cerclage group, 29 women (76%) received prophylactic indomethacin and 36 (94%) received prophylactic antibiotics, whereas the expectant management group did not. Interval from time at diagnosis of open cervix to delivery in the cerclage group was 10.46 ± 5.6 weeks vs 3.7 ± 3.2 weeks in the expectant management group, with a mean difference of 6.76 weeks (95% confidence interval [CI], 4.71-8.81). There were significant decreases in spontaneous preterm birth at <34 weeks (52.6% vs 94.7%; adjusted odds ratio [aOR], 0.06; 95% CI, 0.03-0.34), at <32 weeks (44.7% vs 89.4%; aOR, 0.08; 95% CI, 0.03-0.34); at <28 weeks (31.6% vs 89.4%; aOR, 0.05; 95% CI, 0.01-0.2); and at <24 weeks (13.1% vs 47.3%; aOR, 0.17; 95% CI, 0.05-0.54). There were also significant reductions in perinatal mortality (27.6% vs 59.2%; aOR, 0.24; 95% CI, 0.11-0.5), neonatal intensive care unit admission (75.9% vs 97.6%; aOR, 0.07; 95% CI, 0.01-0.66), and composite adverse neonatal outcome (33.9% vs 90.5%; aOR, 0.05; 95% CI, 0.01-0.21).

CONCLUSION

Cerclage, indomethacin, and antibiotics in twin pregnancies with dilated cervix ≥1 cm before 24 weeks were associated with significant longer latency period from diagnosis to delivery (6.7 weeks), decreased incidence of spontaneous preterm birth at any given gestational age, and improved perinatal outcome when compared with expectant management.

Mono-(2-Ethylhexyl) Phthalate Promotes Pro-Labor Gene Expression in the Human Placenta

Women exposed to phthalates during pregnancy are at increased risk for delivering preterm, but the mechanism behind this relationship is unknown. Placental corticotropin-releasing hormone (CRH) and cyclooxygenase-2 (COX-2) are key mediators of parturition and are regulated by the non-canonical NF-kB (RelB/p52) signaling pathway. In this study, we demonstrate that one of the major phthalate metabolites, mono-(2-ethylhexyl)-phthalate (MEHP), increased CRH and COX-2 mRNA and protein abundance in a dose-dependent manner in primary cultures of cytotrophoblast. This was coupled with an increase in nuclear import of RelB/p52 and its association with the CRH and COX-2 promoters. Silencing of NF-kB inducing kinase, a central signaling component of the non-canonical NF-kB pathway, blocked MEHP-induced upregulation of CRH and COX-2. These results suggest a potential mechanism mediated by RelB/p52 by which phthalates could prematurely induce pro-labor gene activity and lead to preterm birth.