US Evaluation of Twin Pregnancies: Importance of Chorionicity and Amnionicity

Pearls and Pitfalls of First-Trimester US Screening and Prenatal Testing: A Pictorial Review

First-trimester US is imperative in evaluation of early pregnancy to confirm pregnancy location and number and gestational age. The 2024 Society of Radiologists in Ultrasound consensus conference established a first-trimester lexicon to highlight the importance of clear and concise language, which is incorporated and featured by the authors. With improved technologies and understanding of fetal development, first-trimester anatomic studies, between 11 weeks and 13 weeks 6 days gestation, are becoming more frequently used. While not a replacement for the second-trimester anatomic study, systematic evaluation of fetal anatomy at this early gestational age allows detection of 40%-70% of anomalies, whether structural or related to aneuploidy. All patients, regardless of age or baseline risk, should be offered screening and diagnostic testing for chromosomal abnormalities. A variety of prenatal screening approaches are available, each with strengths and limitations. Noninvasive prenatal testing with detection of fetal cell-free DNA can be performed in the first trimester and is the most sensitive and specific screening for the common fetal aneuploidies, but is not equivalent to diagnostic testing. Alternatively, serum analytes for maternal biomarkers in conjunction with nuchal translucency (NT) measurement can be used to calculate a risk estimate for common trisomies. Increased NT is the most common abnormality seen in the first trimester. Positive screening results, increased NT, or other anomaly at US should prompt genetic counseling and be confirmed with diagnostic testing (chorionic villus sampling or amniocentesis). Early detection of aneuploidy and structural anomalies allows counseling and informs decisions for pregnancy management. ©RSNA, 2025 Supplemental material is available for this article.

Selective termination: a life-saving procedure for complicated monochorionic gestations

Monochorionic twin pregnancies are a subset of twin pregnancies that face potential complications related to a shared circulation between the fetuses. These complications are related to anastomotic placental vessels connecting the cardiovascular systems of the two fetuses, which can result in significant sequela if one twin experiences intrauterine death. The sudden cardiovascular collapse in this scenario leads to a massive blood shift away from the healthy co-twin, significantly jeopardizing its life and long-term neurodevelopmental outcome. Such conditions include selective fetal growth restriction with abnormal Doppler findings, twin-twin transfusion with impending death in one twin and discordant fetal anomalies, for which fetal interventions are ineffective in improving outcomes or preventing the imminent death of the abnormal twin. Obstetricians have a professional obligation to respect the autonomy of pregnant patients and to maximize beneficence-based obligations to both pregnant and fetal patients. The goal of a selective termination is to maximize the health and life of the surviving fetal patient. It is recommended that policymakers consider including selective termination as an exemption to abortion ban laws.

A Lexicon for First-Trimester US: Society of Radiologists in Ultrasound Consensus Conference Recommendations

The Society of Radiologists in Ultrasound convened a multisociety panel to develop a first-trimester US lexicon based on scientific evidence, societal guidelines, and expert consensus that would be appropriate for imagers, clinicians, and patients. Through a modified Delphi process with consensus of at least 80%, agreement was reached for preferred terms, synonyms, and terms to avoid. An intrauterine pregnancy (IUP) is defined as a pregnancy implanted in a normal location within the uterus. In contrast, an ectopic pregnancy (EP) is any pregnancy implanted in an abnormal location, whether extrauterine or intrauterine, thus categorizing cesarean scar implantations as EPs. The term pregnancy of unknown location is used in the setting of a pregnant patient without evidence of a definite or probable IUP or EP at transvaginal US. Since cardiac development is a gradual process and cardiac chambers are not fully formed in the first trimester, the term cardiac activity is recommended in lieu of 'heart motion' or 'heartbeat.' The terms 'living' and 'viable' should also be avoided in the first trimester. 'Pregnancy failure' is replaced by early pregnancy loss (EPL). When paired with various modifiers, EPL is used to describe a pregnancy in the first trimester that may or will not progress, is in the process of expulsion, or has either incompletely or completely passed.

ACR Appropriateness Criteria® Multiple Gestations: 2024 Update

The incidence of twin pregnancies has been rising, largely attributable to increasing use of artificial reproductive techniques. Ultrasound plays a critical role in establishing the chorionicity and amnionicity of multiple gestations, a key predictor of the expected risk and complications, along with guiding future clinical and imaging follow-up examinations and intervals. People carrying multiple gestations will typically undergo more ultrasound examinations (and occasionally fetal MRI) than those carrying singletons, at minimum including a first trimester dating scan, nuchal translucency scan at 11 to 14 weeks, an anatomy scan at 18 to 22 weeks, and other scans in the second and third trimesters for growth and surveillance. This document clarifies the most appropriate imaging guidelines for multiple gestations for seven clinical scenarios/variants, which range from initial imaging, follow-up imaging, growth and surveillance for uncomplicated multiple gestations, and those complicated by a known abnormality or discordance between fetuses. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision process support the systematic analysis of the medical literature from peer reviewed journals. Established methodology principles such as Grading of Recommendations Assessment, Development, and Evaluation or GRADE are adapted to evaluate the evidence. The RAND/UCLA Appropriateness Method User Manual provides the methodology to determine the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where peer reviewed literature is lacking or equivocal, experts may be the primary evidentiary source available to formulate a recommendation.

A Lexicon for First-Trimester US: Society of Radiologists in Ultrasound Consensus Conference Recommendations

The Society of Radiologists in Ultrasound convened a multisociety panel to develop a first-trimester US lexicon based on scientific evidence, societal guidelines, and expert consensus that would be appropriate for imagers, clinicians, and patients. Through a modified Delphi process with consensus of at least 80%, agreement was reached for preferred terms, synonyms, and terms to avoid. An intrauterine pregnancy (IUP) is defined as a pregnancy implanted in a normal location within the uterus. In contrast, an ectopic pregnancy (EP) is any pregnancy implanted in an abnormal location, whether extrauterine or intrauterine, thus categorizing cesarean scar implantations as EPs. The term pregnancy of unknown location is used in the setting of a pregnant patient without evidence of a definite or probable IUP or EP at transvaginal US. Since cardiac development is a gradual process and cardiac chambers are not fully formed in the first trimester, the term cardiac activity is recommended in lieu of 'heart motion' or 'heartbeat.' The terms 'living' and 'viable' should also be avoided in the first trimester. 'Pregnancy failure' is replaced by early pregnancy loss (EPL). When paired with various modifiers, EPL is used to describe a pregnancy in the first trimester that may or will not progress, is in the process of expulsion, or has either incompletely or completely passed. © RSNA and Elsevier, 2024 Supplemental material is available for this article. This article is a simultaneous joint publication in Radiology and American Journal of Obstetrics & Gynecology. All rights reserved. The articles are identical except for minor stylistic and spelling differences in keeping with each journal's style. Either version may be used in citing this article. See also the editorial by Scoutt and Norton in this issue.

Delivery Methods in Twin Gestations: Evaluating Outcomes, Risk Factors, and the Paradigm Shift Towards Elective Cesarean Deliveries

During the past decade, there has been a consistent rise in the number of twin births and the number of overall cesarean sections (CS) worldwide. This is owed to a variety of social, economic, educational, and scientific factors. More women are opting to advance their professional careers and gain financial stability before having children. Although this approach is understandable, a new set of challenges are faced as a result, the most important of which has been infertility due to advanced maternal age and the subsequent use of assisted fertility treatments, which have been noted to cause multiple gestations. Twin gestations are considered high-risk pregnancies and are associated with an amplitude of potential complications. Arguably, the biggest decision an obstetrician must make when dealing with this population is choosing the most appropriate mode of delivery. Given the lack of clear guidance pertaining to twin deliveries, CSs may often be perceived as safer and are often preferred over vaginal deliveries (VD). In this narrative review, we aimed to compare the outcomes of different delivery methods (CS versus VD) to investigate whether CS is truly superior to VD. Data were collected from the past two decades and analyzed based on the neonatal and maternal outcomes for each delivery mode. Our results indicate that planned VD is just as safe as CS, if not superior, in most uncomplicated twin pregnancies. Thus, it is best to advise and encourage healthy expecting twin carriers to undergo VD and explore any hesitations or fears they might have. Furthermore, a detailed guideline regarding twin delivery is essential to establish and better navigate twin deliveries, lower the rate of unnecessary CSs, and reduce overall twin gestation morbidity and mortality.

Ankyloglossia in Monochorionic Diamniotic and Dichorionic Diamniotic Twins: A Cross-Sectional Study

Objective: To determine the prevalence of ankyloglossia in diamniotic monochorionic and diamniotic dichorionic twins, as well as to verify the relationship between gender and type of pregnancy. Study Design: A cross-sectional observational study, carried out with 52 pairs of dichorionic/diamniotic twins and 49 monochorionic/diamniotic pairs. The data collection was carried out through the analysis of medical records and the results of the Neonatal Screening of the Tongue Frenulum Assessment Protocol in Babies, and corresponded to the period of 2 years (2020-2022). Statistical analysis of data was performed, considering the significance value of 5%. The study was approved by the Human Research Ethics Committee of the institution. Results: The statistical analysis of multiple logistic regression between the two groups of twins (Mono/Di and Di/Di) according to the socioeconomic, demographic, and clinical-epidemiological profile was statistically significant for some variables. The prevalence of ankyloglossia, according to the type of twin pregnancy, showed a statistically significant difference. There was no statistical difference in relation to sex and ankyloglossia, or between couples diagnosed with ankyloglossia according to the type of pregnancy. Conclusion: Monochorionic/diamniotic twins had a higher prevalence of ankyloglossia, regardless of gender.

Assessment of Amniotic Fluid Volume in Pregnancy

Amniotic fluid (AF) is an integral part of the fetal environment and is essential for fetal growth and development. Pathways of AF recirculation include the fetal lungs, swallowing, absorption through the fetal gastrointestinal tract, excretion through fetal urine production, and movement. In addition to being a marker for fetal health, adequate AF is necessary for fetal lung development, growth, and movement. The role of diagnostic imaging is to provide a detailed fetal survey, placental evaluation, and clinical correlation with maternal conditions to help identify causes of AF abnormalities and thereby enable specific therapy. Oligohydramnios prompts evaluation for fetal growth restriction as well as genitourinary issues, including renal agenesis, multicystic dysplastic kidneys, ureteropelvic junction obstruction, and bladder outlet obstruction. Premature preterm rupture of membranes should also be clinically excluded as a cause of oligohydramnios. Clinical trials evaluating amnioinfusion are underway as a potential intervention for renal causes of oligohydramnios. Most cases of polyhydramnios are idiopathic, with maternal diabetes being a common cause. Polyhydramnios prompts evaluation for fetal gastrointestinal obstruction and oropharyngeal or thoracic masses, as well as neurologic or musculoskeletal anomalies. Amnioreduction is performed only for maternal indications such as symptomatic polyhydramnios causing maternal respiratory distress. Polyhydramnios with fetal growth restriction is paradoxical and can occur with maternal diabetes and hypertension. When these maternal conditions are absent, this raises concern for aneuploidy. The authors describe the pathways of AF production and circulation, US and MRI assessment of AF, disease-specific disruption of AF pathways, and an algorithmic approach to AF abnormalities. ^©RSNA, 2023 Online supplemental material is available for this article. Quiz questions for this article are available through the Online Learning Center.

Use and misuse of ultrasound in obstetrics with reference to developing countries

Maternal and neonatal health is one of the main global health challenges. Every day, approximately 800 women and 7,000 newborns die due to complications during pregnancy, delivery, and neonatal period. The leading causes of maternal death in sub-Saharan Africa are obstetric hemorrhage (28.8%), hypertensive disorders in pregnancy (22.1%), non-obstetric complications (18.8%), and pregnancy-related infections (11.5%). Diagnostic ultrasound examinations can be used in a variety of specific circumstances during pregnancy. Because adverse outcomes may also arise in low-risk pregnancies, it is assumed that routine ultrasound in all pregnancies will enable earlier detection and improved management of pregnancy complications. The World Health Organization (WHO) estimated in 1997 that 50% of developing countries had no access to ultrasound imaging, and available equipment was outdated or broken. Unfortunately, besides all the exceptional benefits of ultrasound in obstetrics, its inappropriate use and abuse are reported. Using ultrasound to view, take a picture, or determine the sex of a fetus without a medical indication can be considered ethically unjustifiable. Ultrasound assessment when indicated should be every woman's right in the new era. However, it is still only a privilege in some parts of the world. Investment in both equipment and human resources has been clearly shown to be cost-effective and should be an obligatory step in the improvement of health care. Well-developed health systems should guide developing countries, creating principles for the organization of the health system with an accent on the correct, legal, and ethical use of diagnostic ultrasound in pregnancy to avoid its misuse. The aim of the article is to present the importance of correct and appropriate use of ultrasound in obstetrics and gynecology with reference to developing countries.

Chorionicity and neurodevelopmental outcomes in twin pregnancy: a systematic review and meta-analysis

OBJECTIVE

Through a systematic review and meta-analyses, we aimed to analyze the impact of chorionicity on neurodevelopment outcomes.

STUDY DESIGN

We conducted a comprehensive search strategy through Medline, Embase, Web of Science, and reference lists of the retrieved studies until August 2022. Studies that examined the association between chorionicity and children's neurodevelopment outcomes were included.

RESULTS

Twelve studies were included. Monochorionic (MC) twins increased the odds of neurodevelopment impairment, cerebral palsy compared to dichorionic (DC) twins. The differences in neurodevelopmental impairment and cerebral palsy between the two groups disappeared after excluding infants with twin-twin transfusion (TTTS). After fetoscopic laser surgery (FLS) for MC twins, there were no differences too.

CONCLUSIONS

Compared to DC twins, MC twins were associated with an increased risk of neurodevelopment impairment. MC twins complicated by TTTS were at high risk of neurologic disability, and FLS was an acceptable treatment modality for them.

Gestational hypertensive disease and small for gestational age infants in twin pregnancy: A systematic review and meta-analysis

AIM

The review is to explore the connection between gestational hypertension diseases (GHD) and small for gestational age (SGA) in twin pregnancies.

METHODS

According to the recommendations of PRISMA, relevant studies were systematically searched through PubMed, Web of Science, Cochrane Library, Embase from inception until January 16, 2022. Subgroup analysis was performed according to chorionicity and diagnostic criteria of SGA. Odds ratios (OR) were assessed to judge the link between GHD and SGA in twin pregnant women. A random-effect model was used to estimate the pooled hazard ratio when there was significant heterogeneity (I²  > 50%); otherwise, a fixed-effect model was conducted.

RESULTS

Seven articles containing 470 589 twin pregnant women were included. The increased risk of SGA was connected to the twin pregnancies complicated with GHD (OR = 1.57, 95% confidence interval [CI] = 1.10-2.24, p = 0.01). After subgroup analysis, the connection between SGA and GHD had no statistical significance (OR = 1.17, 95% CI = 0.95-1.44, p = 0.14) when the enrolled studies using the SGA diagnosis referred to singleton birth weight, but significant (OR = 2.14, 95% CI = 1.77-2.60, p<0.001) in the group using the SGA diagnosis referred to twin birth weight. Stratified by chorionicity, SGA was relevant to GHD in the dichorionic (DC) group (OR = 1.68, 95% CI = 1.17-2.42, p = 0.005), while not in the monochorionic (MC) group (OR = 1.68, 95% CI = 0.93-3.03, p = 0.09). More future articles are warranted to confirm these outcomes.

CONCLUSIONS

Our review demonstrated that GHD in DC twin pregnancies was related to an enlarged risk of SGA. Two SGA diagnosis references led to different results. Twin pregnancies complicated with GHD were at significantly higher risk of SGA when twin birth weight reference was used.

Excellent perinatal outcome of monoamniotic twin pregnancy with timely diagnosis and optimal management - a retrospective cohort study

OBJECTIVES

Monoamniotic twins represent a high-risk pregnancy requiring intense follow-up, elective birth and careful consideration of the mode and timing of delivery. We conducted this study to evaluate the perinatal and neonatal outcomes of monoamniotic twin pregnancies in the largest tertiary hospital in Finland.

METHODS

This was a retrospective cohort study including all monoamniotic twin pregnancies during a 17-year period (2002-2018) managed in Helsinki University Hospital. Data on mothers and children were collected from patient files. Chorionicity and amnionicity were defined in first-trimester ultrasound screening.

RESULTS

There were altogether 31 monoamniotic twin pregnancies during the study period, including four cases of conjoined twins which all underwent termination of pregnancy, and three miscarriages. In the remaining 24 pregnancies that continued past 24 weeks of gestation there was 97.9% survival (one intrauterine death). Three pregnancies were complicated with twin-to-twin transfusion syndrome. All children were delivered by cesarean section with a mean gestational age of 32 + 5 weeks (27 + 1-34 + 2 weeks). Respiratory distress syndrome (RDS) was observed in 57% (27/47) of neonates and grade I-II intraventricular haemorrhage (IVH) in 6.3% (3/47) of neonates. There were no neonatal deaths and no maternal complications.

CONCLUSIONS

Monoamniotic twinning is a rare form of pregnancy and carries risks for perinatal and neonatal complications. With timely diagnosis, close monitoring in specialized feto-maternal unit and elective delivery at 32-34 weeks the outcome is usually excellent.

Introducing a New Predictor of Chorionicity in the Second and Third Trimesters: The “Cleft Sign”

Objective:

The aim of this study was to suggest a new sonographic sign, which could be helpful in the determination of chorionicity in twin pregnancies, during the second and third trimesters.

Materials and Methods:

This was a retrospective study evaluating the sonographic images of 50 women with twin pregnancy. Out of 50 twin pregnancy case reviewed, only 27 were included. Those patient cases that were included were scanned by a single radiologist. The presence of a hypoechoic cleft, in the thickness of placental bulk, could be detected in all of the dichorionic pregnancy cases. This sonographic appearance was referred to as the “cleft sign.”

Results:

The “cleft sign” was present in all the dichorionic diamniotic (DC-DA) twin pregnancy cases, but no similar appearance was seen in the monochorionic diamniotic (MC-DA) twins. Considering this sign, the radiologist was able to correctly diagnose the chorionicity of all 24 patients in the second and third trimester.

Conclusion:

The findings of the present study suggest a possible role for this newly identified “cleft sign.” This sonographic appearance is a hypoechoic cleft in the thickness of placenta, of dichorionic pregnancies, as an indicator of DC twin pregnancy. In this retrospective case analysis, the “cleft sign” was diagnostically significant in the second and third trimesters.

On the origin of zygosity and chorionicity in twinning: evidence from human in vitro fertilization

Assisted reproduction is presumed to increase monozygotic twin rates, with the possible contribution of laboratory and medical interventions. Monozygotic dichorionic gestations are supposed to originate from the splitting of an embryo during the first four days of development, before blastocyst formation. Single embryo transfers could result in dichorionic pregnancies, currently explained by embryo splitting as described in the worldwide used medical textbooks, or concomitant conception. However, such splitting has never been observed in human in vitro fertilization, and downregulated frozen cycles could also produce multiple gestations. Several models of the possible origins of dichorionicity have been suggested. However, some possible underlying mechanisms observed from assisted reproduction seem to have been overlooked. In this review, we aimed to document the current knowledge, criticize the accepted dogma, and propose new insights into the origin of zygosity and chorionicity.

Perinatal Outcomes and Risk Factors for Preterm Birth in Twin Pregnancies in a Chinese Population: A Multi-center Retrospective Study

Background: Twin pregnancies are associated with an increased risk of adverse maternal and neonatal outcomes, mainly owing to prematurity. Few studies have evaluated the risk factors for preterm birth (PTB) in Chinese population. The objective of this study is to present the short-term maternal-neonatal outcomes, investigating the potential risk factors associated with preterm birth in Chinese twin pregnancies.

Methods: A multi-center retrospective study of women pregnant with twins ≥28 weeks of gestation was conducted. Maternal and neonatal outcomes were analyzed. Logistic regression was used to identify potential risk factors for PTB before 37, 34, and 32 weeks, respectively.

Results: A total of 3,288 twin pregnancies and 6,576 neonates were included in 99,585 pregnancies. The rate of twin pregnancy was 3.3%, while the PTB rate before 37, 34, and 32 weeks among this population were 62.1, 18.8, and 10.4%, respectively. Logistic regression revealed that monochorionicity [Odds ratio (OR) 3.028, 95% confident interval (CI) 2.489–3.683, P < 0.001], gestational weight gain (GWG) <10 kg (OR 2.285, 95% CI 1.563–3.339, P < 0.001) and GWG between 10 and 15 kg (OR 1.478, 95% CI 1.188–1.839, P < 0.001), preeclampsia (PE) (OR 3.067, 95% CI 2.142–4.390, P < 0.001), and intrahepatic cholestasis of pregnancy (ICP) (OR 3.122, 95% CI 2.121–4.596, P < 0.001) were the risk factors for PTB before 37 weeks. Monochorionicity (OR 2.865, 95% CI 2.344–3.501, P < 0.001), age < 25 years (OR 1.888, 95% CI 1.307–2.728, P = 0.001), and GWG <10 kg (OR 3.100, 95% CI 2.198–4.372, P < 0.001) were risk factors for PTB before 34 weeks. Monochorionicity (OR 2.566, 95% CI 1.991–3.307, P < 0.001), age younger than 25 years (OR 1.964, 95% CI 1.265–3.048, P = 0.003), and GWG <10 kg (OR 4.319, 95% CI 2.931–6.364, P < 0.001) were the risk factors for PTB before 32 weeks.

Conclusions: Monochorionicity and GWG <10 kg were two major risk factors for PTB before 32, 34, and 37 weeks, whereas maternal age, PE, and ICP were also risk factors for PTB in specific gestational age.

Sonographic Assessment of Fetal Growth Abnormalities

Fetal growth abnormalities have significant consequences for pregnancy management and maternal and fetal well-being. The accurate diagnosis of fetal growth abnormalities contributes to optimal antenatal management, which may minimize the sequelae of inadequate or excessive fetal growth. An accurate diagnosis of abnormal fetal growth depends on accurate pregnancy dating and serial growth measurements. The fetal size at any given stage of pregnancy is either appropriate or inappropriate for the given gestational age (GA). Pregnancy dating is most accurate in the first trimester, as biologic variability does not come into play until the second and third trimesters. The authors describe the determination of GA with use of standard US measurements and how additional parameters can be used to confirm dating. Once dates are established, serial measurements are used to identity abnormal growth patterns. The sometimes confusing definitions of abnormal growth are clarified, the differentiation of a constitutionally small but healthy fetus from a growth-restricted at-risk fetus is described, and the roles of Doppler US and other adjunctive examinations in the management of growth restriction are discussed. In addition, the definition of selective growth restriction in twin pregnancy is briefly discussed, as is the role of Doppler US in the classification of subtypes of selective growth restriction in monochorionic twinning. The criteria for diagnosing macrosomia and the management of affected pregnancies also are reviewed. The importance of correct pregnancy dating in the detection and surveillance of abnormal fetal growth and for prevention of perinatal maternal and fetal morbidity and mortality cannot be overstated. The online slide presentation from the RSNA Annual Meeting is available for this article. ^©RSNA, 2020.