This chapter should be cited as follows:
Ford H, Meagher S, et al., Glob Libr Women's Med
ISSN: 1756-2228; DOI 10.3843/GLOWM.419223

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editors: Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy

Chapter

Ultrasound Screening for Chromosomal and Genetic Abnormalities

First published: July 2025

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INTRODUCTION

Ultrasound examination at 11–13 weeks of gestation is a widely accepted tool for screening for fetal abnormalities. In addition to early detection of fetal abnormalities, advantages of screening at this gestation include accurate assessment of gestational age, diagnosis of multiple pregnancy and confirmation of chorionicity, as well as being an important part of aneuploidy screening.^1,2,3 A pictorial guide based on the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)’s recommendations for structures that should be included in the first-trimester assessment was published in 2018 and updated more recently in 2023.³ This assessment can be performed by a transabdominal ultrasound approach, however, given multiple factors such as fetal age, fetal lie, presence of fibroids or other uterine anomalies or increased maternal body habitus, a transvaginal assessment may increase the yield of detection significantly.^1,3 Anatomical examination at 11–13 weeks does not replace the need for a structural assessment of the fetus in the midtrimester, but rather provides an opportunity to assess the fetus for major structural abnormalities and risk of chromosomal and genetic abnormalities earlier in the pregnancy. The traditional role of the morphology ultrasound to assess for structural and potential chromosomal or genetic abnormalities will be discussed in this chapter, including the role of assessment of second-trimester markers of aneuploidy and assessment of structures that are not visible by the end of the first trimester.

DETECTION OF FETAL ANOMALIES

Whilst the mid-trimester examination is considered the official fetal anomaly screening assessment, due to its ability to detect a greater number of abnormalities as more structures are developed and amenable to visualization, an ultrasound performed at 11–13 weeks, when conducted using a standardized protocol, can identify many severe fetal abnormalities at a gestational age when termination of pregnancy may be technically easier and/or more acceptable to patients in some settings. ISUOG’s recommendations for structures that should be examined in the first trimester³ are listed in Table 1. In a multicenter, retrospective cohort study of over 100 000 women in the UK published in 2019, Syngelaki et al.² demonstrated that trained ultrasound operators following a standardized protocol could confirm all cases of acrania, alobar holoprosencephaly, tricuspid or pulmonary atresia, encephalocele (Figure 1), exomphalos, gastroschisis and body-stalk anomaly through an 11–13-week assessment. Additionally, they detected a significant number of cases of open neural tube defects, hypoplastic left heart syndrome (Figure 2), atrioventricular septal defect, lower urinary tract obstruction, lethal skeletal dysplasia (Figure 3) and major limb abnormalities,² as shown in Table 2.

Some fetal anomalies cannot be definitively detected in the first trimester due to inadequate visualization or incomplete development of relevant structures.^2,3,4 Examples include structures of the central nervous system (agenesis of the corpus callosum), chest and heart (pulmonary and aortic stenosis, ventricular septal defect, congenital airway pulmonary malformation), gastrointestinal tract (GIT) (esophageal or duodenal atresia, bowel obstruction) and genitourinary system abnormalities (multicystic kidney, horseshoe or duplex kidney, cloacal abnormality, ambiguous genitalia).²

Screening for aneuploidy has evolved from using a simple age-related risk cut-off to offer invasive diagnostic testing, to incorporating biochemical and ultrasound markers, and now includes cell-free DNA (cfDNA) screening technology, also known as non-invasive prenatal testing (NIPT). These screening options are discussed later in this chapter. The recommendation for an ultrasound examination to precede cfDNA testing at 8–11 weeks of gestation, primarily to exclude contraindications such as a non-viable pregnancy or ‘vanishing twin’, has improved ultrasound access in high-income countries and provided an opportunity for early assessment of the fetus from 10 weeks of gestation and subsequent population analysis at this stage. In an opinion piece by Rolnik et al.⁴ describing their own research in this area, the authors state that recent advances in ultrasound technology, such as high-frequency, high-resolution transvaginal probes and three-dimensional ultrasound modalities can facilitate a definitive diagnosis of some fetal defects, such as acrania and conjoined twins, at this early gestation, and additionally provide a high degree of suspicion for many other conditions, which can then be confirmed at the 11–14-week ultrasound assessment. Examples include nuchal edema with suspicion of major cardiac or other anomaly (Figure 4), major limb abnormalities such as sirenomelia and spinal defects.⁴ Early detection of major fetal anomalies prior to the 11-week stage in some cases may then guide the choice of screening or diagnostic modality, including moving straight to invasive testing rather than non-invasive screening options. Indeed, these authors concluded that, in their review of over 6000 ultrasound assessments performed prior to cfDNA testing, changes in management were observed in 1 in 10 pregnancies. These changes included delaying cfDNA testing (in cases of incorrect gestational dating), offering invasive testing instead of screening (in cases of fetal malformation or hydrops) and foregoing testing altogether (in cases of miscarriage or vanishing twin).⁴

However, regardless of the type of aneuploidy screening performed, current recommendations still advocate for a mid-trimester assessment for all pregnant women. With respect to the mid-trimester evaluation of fetal anatomy, clear guidelines from ISUOG outline the structures that should be assessed⁵, as detailed in Table 3. These guidelines are useful for the development and implementation of clinical practice protocols in individual settings.

ANEUPLOIDY SCREENING

Options for aneuploidy screening vary between countries, both in regard to differences between availability and accessibility of screening in low- vs middle- or high-income countries, as well as based on recommendations from national consensus groups. The detection rates of the different methods of fetal aneuploidy screening are presented in Table 4.

Age-based assessment of risk for trisomy 21

Before the development of the various non-invasive screening methods available today, age-based risk stratification was the only approach for prenatal screening for trisomy 21 (Down syndrome). Women above a certain age threshold were offered invasive testing based solely on their age-related risk. For example, in the UK, women aged ≥38 years were offered invasive testing, which resulted in a detection rate of approximately 30% at a screen-positive rate of about 5%.⁶

Markers of trisomy 21 in the second trimester

Mid-trimester ultrasound (at 19–24 weeks of gestation) was one of the first methods used in routine practice to assess for trisomy 21,⁷ with many studies reporting anomalies such as ventriculomegaly, absent or hypoplastic nasal bone, increased nuchal fold thickness, intracardiac echogenic foci, aberrant right subclavian artery, short femur or humerus, echogenic bowel, and hydronephrosis as markers of trisomy 21.⁸ This is because such ‘markers’ are more commonly seen in fetuses with trisomy 21 than in those with normal karyotype. However, the strengths of these associations are variable, with some of these markers remaining important in the absence of other findings, whilst others do not increase the risk of trisomy 21 when found in isolation. A large meta-analysis by Agathokleous et al.⁸ published in 2013 reviewed these markers and their association with trisomy 21, providing positive and negative likelihood ratios for each marker. These can be used to help determine those at increased risk, in combination with results from combined first-trimester screening, or in isolation with age-related risk when these results are not available. Of note, in the absence of any of the defined markers in this meta-analysis, there was a 7.7-fold reduction in the risk of trisomy 21.⁸ When using the markers in isolation, there is a 3- to 4-fold increase in risk in the presence of either ventriculomegaly, increased nuchal fold thickness or aberrant right subclavian artery, and a 6- to 7-fold increase with hypoplastic or absent nasal bone.⁸ However, when performing a detailed morphological ultrasound examination, these markers are best used along with their positive and negative likelihood ratios to determine the combined likelihood ratio and the risk for a patient rather than using the markers in isolation.⁸

Second-trimester serum screening for trisomy 21 (quadruple test)

In the 1980s, it was discovered that certain biochemical markers in maternal blood differed between pregnancies with trisomy 21 and those with normal karyotype. This led to the development of maternal serum screening. The screening panel measures four markers from 16 weeks of gestation: alpha-fetoprotein, unconjugated estriol, human chorionic gonadotropin and inhibin A. When these results are combined with maternal age, the test can identify approximately 70% of fetuses with trisomy 21 at a false-positive rate of approximately 5%.⁶

Nuchal translucency and combined first-trimester screening assessment for trisomy 21

From the 1990s, the measurement of nuchal translucency was standardized and measurement protocols developed as a non-invasive way of risk assessment. Nuchal translucency thickness (Figure 5) can be measured between 11+0 and 13+6 weeks, which is equivalent to a crown–rump length of 45–84 mm.⁶ When combined with maternal age, this was shown to identify approximately 75% of affected fetuses with a false-positive rate of 5%.⁶ Research into additional maternal first-trimester serum biomarkers, specifically the free beta subunit of human chorionic gonadotropin (free β-hCG) and pregnancy-associated plasma protein A (PAPP-A), demonstrated that, when combined with maternal age and nuchal translucency, detection rates increased to approximately 90% at a fixed false-positive rate of 5%. The inclusion of additional first-trimester ultrasound markers, such as the presence or absence of the nasal bone, further improved detection rates to over 95%.⁶ A predetermined risk cut-off is often used to define 'high risk,' such as 1 : 300 or 1 : 150. However, individual patient factors, biochemical results and the nuchal translucency measurement should all be considered alongside the calculated risk value. For example, a risk of 1 : 200 at maternal age of 42 is actually lower than the age-related risk alone but still corresponds to a 0.5% probability. Conversely, a markedly increased nuchal translucency may elevate the risk of trisomy 21, but should also prompt detailed assessment of fetal anatomy, particularly when biochemical markers are within normal limits, as there may be an underlying structural anomaly causing the increased nuchal translucency. In such cases, if a fetal abnormality is detected, counseling regarding invasive diagnostic testing is a reasonable next step.

Cell-free DNA screening

Trisomy 21 is not the only common aneuploidy amenable to screening. Trisomy 18 (Edward syndrome) can be detected with the second-trimester maternal serum screen, and risks for trisomies 13 (Patau syndrome) and 18 can be calculated for those undergoing a first-trimester combined screening assessment. A large meta-analysis published in 2014 by Metcalfe et al.⁹ found detection rates of 83.1% for trisomy 13 (with a false-positive rate of 4.4%) and 91.9% for trisomy 18 (with a false-positive rate of 5.4%) using the combined screening assessment. These common aneuploidies were among the first to be evaluated for detection through fetal cfDNA analysis. What was initially assumed to be fetal DNA, and therefore thought to be a potential diagnostic test, was then found to be trophoblastic DNA fragments from the placenta, and forms the basis of cfDNA testing as currently performed.

Given the improved detection rate, in higher-income countries there has been an increased uptake in cfDNA as a first-line aneuploidy screening tool. However, the significant cost of this test and the lack of consistent cost-effectiveness data on universal cfDNA screening compared to other screening modalities, has prevented universal uptake, even amongst higher-resource settings. At the time of writing, Belgium and the Netherlands are using cfDNA as their first-line test, and there have been calls in the UK to consider a risk-based (contingent) model to use cfDNA as a second-line screening test.¹⁰ In this scenario, if screening is offered, combined first-trimester screening is first undertaken and, if the risk of aneuploidy is between 1 : 150 and 1 : 1000 (so-called ‘intermediate risk’), cfDNA is offered as a second-line test.^11,12 For those with a risk <1 : 1000, no further testing is undertaken, and those with a risk of greater than or equal to 1 : 150 are offered invasive testing or cfDNA testing.¹² If structural abnormalities are detected, then invasive testing rather than cfDNA screening is recommended.^10,11,12 Many other countries leave the choice of screening up to the individual practitioner, which requires an important discussion regarding pretest probability, appropriate counseling in cases of abnormal results, and a possible recommendation for invasive testing based on the result.

cfDNA screening is a constantly evolving space. Whilst outside the scope of this chapter, cfDNA is increasingly being used to screen for sex chromosome abnormalities, rare autosomal trisomies, deletions and duplications, and microdeletion syndromes such as 22q.11 (formerly known as DiGeorge) syndrome. However, cfDNA testing for these rarer conditions has not been properly clinically validated, performance is lower than that for the common trisomies, and counseling is often challenging due to the variable and commonly unpredictable phenotype, all of which dramatically increase the need for practitioner education and pretest counseling on the limitations of the test. Positive predictive values (PPV) for sex chromosome abnormalities overall may be in the order of 50%¹³ (with PPV depending on the sex chromosome abnormality in question)¹³ and rare autosomal trisomies only approximately 4%,¹⁴ all of which should be clearly outlined to the patient prior to undertaking these tests. The decision to undergo this screening should be made with the knowledge that it may lead to higher rates of false-positive results that cannot be confirmed antenatally without the need for invasive testing.

FETAL ANOMALIES AND CHROMOSOMAL ABNORMALITY

Fetal anomalies highly suggestive of chromosomal abnormality

When multiple fetal abnormalities are seen, the suspicion of an underlying chromosomal or syndromic cause should be raised. However, there are some abnormalities that, even when seen in isolation, should raise the suspicion for an underlying chromosomal abnormality. Fetal abnormalities and markers commonly seen with the most prevalent chromosomal or genetic syndromes are listed in Table 5, and examples of fetal structural abnormalities are shown in Figures 6–12.

Micrognathia

Micrognathia refers to the arrested development of the fetal mandible, resulting in a small chin¹⁷ (Figure 9). Agnathia is the most severe form of mandible maldevelopment, with complete agenesis of the mandible,¹⁷ and retrognathia refers to a chin that is posteriorly positioned when compared in position to the maxilla,^17,23,24 and the term is commonly used synonymously with micrognathia.²³ The prevalence of micrognathia is approximately 1 : 1500²³ and is associated with over 300 genetic abnormalities, including trisomy 13, trisomy 18, Turner syndrome and DiGeorge Syndrome.²³ It may also be seen with primary mandibular disorders, such as Pierre Robin sequence (particularly if seen with cleft palate),^17,23 skeletal dysplasias²³ and other syndromes such as Meckel–Gruber syndrome,²³ Noonan’s syndrome²³ and Smith–Lemli–Opitz syndrome.²³ Aside from the association with chromosomal and syndromic causes, micrognathia is an important finding due to its potential for postnatal complications including airway obstruction at birth and feeding difficulties.²³

Micrognathia can be determined subjectively but, more recently, diagnostic indices such as the inferior facial angle (comparing the frontal bone to a line drawn at the angle of the mentum), frontonasomental angle (comparing the frontal bone to a line drawn at the tip of the nose and mentum) and jaw index (comparing the axial mandibular diameter to the anteroposterior distance of the mandible) have been developed.^23,24 Micrognathia may be less apparent with continued growth and development, particularly if diagnosed in the first or early second trimester, and therefore evaluation with these diagnostic indices may be particularly useful in these cases, along with careful follow-up.²³

Omphalocele

Herniation of bowel at the umbilical cord insertion site is a physiological process, with the bowel contents expected to return to the abdomen by 13 weeks of gestation.²⁵ Omphalocele (also known as exomphalos) is the failure of the contents to be relocated by this gestation; it may also contain liver along with bowel loops^17,26 but rarely stomach,²⁶ and its prevalence is approximately 1 in 5000.²⁷ It can be distinguished from gastroschisis by the sac seen encompassing the bowel contents through the midline herniation at the level of the umbilicus, as opposed to free loops of bowel through a paraumbilical defect.¹⁷ Up to 60% of omphalocele may be explained by an underlying chromosomal abnormality or syndrome,²⁷ including trisomy 18, trisomy 13, Beckwith–Wiedemann syndrome and pentalogy of Cantrell.¹⁷ This is in stark contrast to gastroschisis, in which associated chromosomal abnormality is rare.¹⁷ Aside from the association with underlying chromosomal abnormality, omphalocele is an important diagnosis to make when planning for delivery, particularly with respect to being in a location in which pediatric surgical services are available to plan for primary or delayed closure.²⁶

Rib abnormalities

An increased number of ribs is rarely a normal variant, and is usually associated with trisomy 21 or VACTERL sequence (vertebral anomalies, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies and limb defects).¹⁷ A reduced number of ribs may also be seen with trisomy 21, or skeletal dysplasias such as cleidocranial dysplasia and campomelic dysplasia.¹⁷ Short ribs are a common finding in many skeletal dysplasias, particularly those that are considered lethal due to the reduced thoracic circumference and associated pulmonary hypoplasia.¹⁷ These abnormalities may be seen in both the first and second trimesters; particularly in the case of skeletal dysplasias considered to be lethal, these abnormalities may be evident in the first trimester.

Fetal anomalies unlikely linked to chromosomal abnormality

When seen in isolation, the following fetal anomalies have a low risk of associated chromosomal abnormalities or genetic syndromes. As with all abnormalities that are seen, a careful fetal ultrasound examination should be performed to exclude additional abnormalities.

Muscular VSD

The classification of VSD depends on where within the septum the defect is seen, and may include the membranous septum (perimembranous VSD), muscular septum (muscular VSD, Figure 10) and atrioventricular valve inlet region (inlet VSD).²⁸ Muscular VSDs are seen in up to 5% of term fetuses.²⁹ The muscular portion of the septum can be divided into the mid-muscular region and apical region, with defects in the mid-muscular region approximately five times more common than those in the apical region.²⁸ Isolated muscular VSDs are unlikely to be seen in the first trimester, although membranous VSDs and those associated with complex cardiac anomalies can be seen. While VSD accounts for approximately 30% of cardiac abnormalities in fetuses affected by trisomy 21, when muscular VSD is seen in isolation, there is unlikely to be an additional increased risk above baseline for trisomy 21 or chromosomal abnormalities in general.^28,29 Whilst this is the most commonly seen congenital cardiac abnormality, when in isolation, spontaneous closure rates have been quoted as up to 90%.²⁹

Congenital pulmonary airway malformation (CPAM)

CPAM (Figure 11) and bronchopulmonary sequestration are congenital lung malformations that occur in approximately 1 in 8000 births.³⁰ CPAMs are cystic or solid lung masses, which may take on a macrocystic or microcystic appearance in the fetal lung, and are formed by an abnormal proliferation of immature respiratory bronchioles, for which the blood supply arises from the pulmonary vessels.³⁰ This is in comparison to bronchopulmonary sequestration, in which the blood supply arises from the systemic circulation. If seen in isolation, these echogenic lung lesions are not commonly associated with chromosomal abnormalites.¹⁷ Fetuses with very large CPAM are at risk of hydrops, however, outcomes are generally good for children born with isolated CPAM, with reported survival rates over 95%.³⁰ CPAM is a diagnosis predominantly made in the second trimester, although a very large CPAM may be seen in the first trimester if it causes mediastinal shift, and along with congenital diaphragmatic hernia, should be included in the differential diagnosis.

Gastroschisis

Gastroschisis (Figure 12) is an anterior abdominal wall defect, typically to the right side of the umbilical cord insertion site, which allows free loops of bowel to herniate outside the abdominal cavity and float freely in the amniotic fluid.³¹ Rarely, the stomach, bladder and internal genitalia can be exteriorized, and it can be differentiated from an omphalocele due to the lack of surrounding tissue membrane and a normal umbilical cord insertion site being identified.³¹ While risk factors such as young maternal age and recreational substance use have been attributed, it is not usually associated with chromosomal abnormalities when seen in isolation.³¹ However, the fetus should be carefully examined for both additional abnormalities, which increase suspicion for an underlying chromosomal or syndromic cause, and to exclude a rupture of an omphalocele membrane by carefully examining the cord insertion site.³¹ Fetal growth restriction is commonly associated with gastroschisis, however,³¹ and there is an increased risk of fetal distress, intrauterine fetal death and postnatal morbidity and mortality related to bowel complications.³¹ Diagnosis should prompt immediate referral to a center that can provide neonatal care and pediatric surgical services.³¹

CONCLUSION

Screening for fetal anomalies with ultrasound requires careful examination of the fetus, which is aided greatly by consensus and evidence-based standardized protocols to ensure that the fetus is adequately assessed at each gestation. It should always be remembered that ultrasound is an assessment of fetal structure, not of fetal genetics, and the diagnosis of fetal anomalies on ultrasound should always lead a clinician to consider the need for genetic testing. Even in the absence of fetal anomalies on ultrasound, screening for common chromosomal abnormalities such as trisomies 21, 18 and 13 should be considered. Whilst there are some ultrasound anomalies that, when seen in isolation, cause little concern for an underlying chromosomal or syndromic abnormality, such as muscular VSD or CPAM, other ultrasound anomalies, such as micrognathia, should always raise suspicion of an associated genetic abnormality, even when seen in apparent isolation.

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