Genetic Screening and Diagnosis

Trisomy 21

Trisomy 21, or Down syndrome, is the most frequent autosomal chromosome syndrome with characteristic craniofacial features and congenital anomalies ( Fig. 10.1 ; see also Table 10.1 ). The relationship of Down syndrome to advanced maternal age is well known (see Table 10.2 ). Approximately 95% of cases arise in maternal meiosis, usually meiosis I, and have 47 chromosomes (47,XX,+21 or 47,XY,+21). Mosaicism for chromosome 21 occurs in 2% to 4% of cases of Down syndrome and usually results in a higher IQ (70 to 80). Women with Down syndrome are usually fertile, and although relatively few trisomic mothers have reproduced, approximately 30% of their offspring are also trisomic. Men are invariably infertile.

Translocations (sporadic or familial) most commonly associated with Down syndrome involve chromosomes 14 and 21. One parent may have the same translocation, [45t(14q;21q)], referred to as a Robertsonian translocation. Empiric risks for having an offspring with Down syndrome are approximately 10% for female Robertsonian translocation carriers and 2% for male translocation carriers. A potential concern is that offspring who are diploid (46,XX or 46,XY) actually have uniparental disomy (UPD), a condition in which both of a given pair of chromosomes originate from the same parent. In a study of 65 Robertsonian translocation carriers, only one UPD case was observed (0.6%). Based on reports of UPD testing in 357 inherited and 102 de novo Robertsonian translocation cases the authors concluded that the overall risk for UPD is approximately 3%.

Other structural rearrangements that result in Down syndrome include t(21q;21q) and translocations that involve chromosome 21 and other acrocentric chromosomes (13,15 or 22). In t(21q;21q) carriers, only trisomic or monosomic zygotes are produced, and the latter presumably appear as preclinical embryonic losses. Parents who have other translocations have a low empiric risk of having offspring with Down syndrome.

Trisomy 13

Trisomy 13 occurs in approximately 1 in 20,000 live births. The clinical features of trisomy 13 are summarized in Table 10.1 . Most cases are caused by nondisjunction (47, +13) and are maternal in origin. Robertsonian translocations are responsible for fewer than 20% of cases and are invariably associated with two group D (13 to 15) chromosomes joining at their centromeric regions. If neither parent has a rearrangement, the risk for subsequent affected progeny is not increased. If either parent has a balanced 13q;14q translocation, the recurrence risk for an affected offspring is increased but only to 1% to 2%. The exception is a 13q;13q parental translocation in which no normal gametes are formed. For live births with trisomy 13, survival beyond 3 years is rare.

Trisomy 18

Trisomy 18 occurs in 1 per 8000 live births (see Table 10.1 ). Stillbirth is not uncommon. Fetal movement is feeble, and approximately 50% develop nonreassuring fetal status during labor. For live births, mean survival is measured in months, and pronounced developmental and growth retardation is apparent. Approximately 80% of trisomy 18 cases are caused by primary nondisjunction (47,+18). Errors usually arise in maternal meiosis, frequently meiosis II. Recurrence risk is approximately 1%.

Other Autosomal Trisomies

All autosomes show trisomies, but usually trisomies other than those described previously end in abortuses. In addition to trisomies 13, 18, and 21, only a few other trisomies are detected in liveborns (8, 9, 14, 16, and 22), often as mosaics in conjunction with a normal cell line (46 chromosomes). All exhibit some degree of intellectual disability, various structural anomalies, and intrauterine growth restriction (IUGR).

Monosomy X (45,X)

The incidence of 45,X in liveborn girls is approximately 1 in 10,000. Monosomy X, or Turner syndrome, accounts for 10% of all first-trimester abortions; therefore it can be calculated that more than 99% of 45,X conceptuses are lost early in pregnancy. The error usually (80%) involves loss of a paternal sex chromosome. Mosaicism (i.e., 45,X/46,XX) is frequent.

Common features include primary ovarian failure, absent pubertal development due to gonadal dysgenesis (streak gonads), and short stature (<150 cm). Structural abnormalities of the X chromosome may also result in premature ovarian failure. Both the long arm and the short arm of the X chromosome contain determinants necessary for ovarian differentiation and for normal stature. Various somatic anomalies include renal and cardiac defects, skeletal abnormalities (cubitus valgus and clinodactyly), vertebral anomalies, pigmented nevi, nail hypoplasia, and a low posterior hairline. Performance IQ is lower than verbal IQ, but overall IQ is considered normal. Adult-onset diseases include hypertension, coronary artery disease, hypothyroidism, and type 2 diabetes mellitus.

Low-dose estrogen therapy is needed to induce puberty, and long-term hormone replacement is needed in adulthood. Pregnancy may be achieved with the use of donor eggs but requires careful monitoring of cardiovascular status before, during, and after pregnancy. Growth hormone treatment increases the final adult height 6 to 8 cm. Comprehensive guidelines for evaluation and clinical management of Turner syndrome are available.

Klinefelter Syndrome

Approximately 1 in 1000 males are born with Klinefelter syndrome, the result of two or more X chromosomes (47,XXY; 48,XXXY; and 49,XXXXY). Characteristic features include small testes, azoospermia, elevated follicle-stimulating hormone and luteinizing hormone levels, and decreased testosterone. The most common chromosome complement associated with this phenotype is 47,XXY.

Intellectual disability is uncommon in 47,XXY males, but behavioral problems and receptive language difficulties are common. Intellectual disability is almost invariably associated with 48,XXXY and 49,XXXXY. Skeletal, trunk, and craniofacial anomalies occur infrequently in 47,XXY but are commonly observed in 48,XXXY and 49,XXXXY. Regardless of the specific chromosome complement, patients with Klinefelter syndrome all have male phenotypes. The penis may be hypoplastic, but hypospadias is uncommon. With intracytoplasmic sperm injection (ICSI) and other assisted reproductive technology (ART), siring a pregnancy is now possible. Guidelines on evaluation and clinical management are available.

Polysomy X in Girls (47,XXX; 48,XXXX; 49,XXXX)

Approximately 1 in 800 liveborn girls has more than two X chromosomes. Most of these individuals have a normal reproductive system. The theoretic risk of delivering an infant who also has an abnormal chromosome complement is 50%, given half of the maternal gametes carry 24 chromosomes (24,XX). Empiric risks are much less. Somatic anomalies are uncommon but have been observed in some prenatally detected cases. The IQ of such individuals is 10 to 15 points lower than that of their siblings. The absolute risk for intellectual disability with 47,XXX does not exceed 5% to 10%, and even then, IQ is usually 60 to 80. However, 48,XXXX and 49,XXXXX individuals are invariably intellectually disabled and are more likely to have somatic malformations than individuals with a 47,XXX complement.

Polysomy Y in Boys (47,XYY and 48,XXYY)

Presence of more than one Y chromosome is another frequent chromosome abnormality in liveborn boys (1 in 1000). Males with 47,XYY are more likely than 46,XY boys to be tall and are at increased risk for learning disabilities, speech and language delay, and behavioral and emotional difficulties. These individuals have normal male phenotype and sexual development.

Cell-Free DNA Analysis

The newest screening test for aneuploidy is cfDNA analysis, sometimes referred to as noninvasive prenatal screening (NIPS). This method uses massive parallel sequencing analysis of cfDNA and was introduced into clinical practice in 2011 . Such testing can be performed as early as 10 weeks' gestation. Numerous studies have validated the ability of cfDNA screening for common trisomies (13, 18 and 21) and sex chromosome abnormalities with high sensitivity and false-positive rates less than 1%. However, patients should be counseled about the limitations of cfDNA analysis, including the possibility of a false-positive or false-negative test result and the necessity of confirmatory diagnostic testing to confirm screening results.

Maternal plasma contains small fragments of cfDNA (50 to 200 base pairs) derived from the breakdown of both maternal and fetal cells, primarily derived from the placenta. The concept of using cfDNA for prenatal diagnosis is not new; cfDNA has been used successfully to determine fetal sex in pregnancies at risk for X-linked disorders by identifying the Y-chromosome signal. In Europe, noninvasive testing is commonly used to determine fetal rhesus factor (Rh) status in RhD-negative women using real-time polymerase chain reaction (PCR) amplification. A similar approach can be adapted for the detection of some single-gene disorders. However, to screen for aneuploidy requires a different approach—the use of massively parallel DNA shotgun sequencing (MPSS).

Detection of aneuploidy is more difficult than for single-gene disorders because detecting fetal trisomy must reflect quantitative differences between affected and unaffected pregnancies. With MPSS technology, millions of fragments of maternal and fetal DNA are sequenced simultaneously in a single sample of maternal plasma, which is assigned to a given chromosome region and counted in comparison with a reference standard expected of a normal individual. A woman carrying a trisomy 21 fetus will have relatively more chromosome 21 counts (transcripts) than a woman carrying a normal fetus. Alternatively, some laboratories use a targeted approach that sequences specific chromosomes of interest, such as 18 and 21, and adjusts for the proportion of fetal DNA (fetal fraction) to provide a patient-specific risk assessment that takes into account maternal age. An alternative approach is to use single nucleotide polymorphism (SNP)-based sequencing, which allows for the detection of triploidy and some of the common deletion syndromes.

A number of studies have demonstrated the ability to detect fetal trisomy 21, 18, 13 and sex chromosome abnormalities using MPSS. A blinded, nested, case-control study of 4664 pregnancies at increased risk for trisomy 21 from 27 prenatal diagnostic centers worldwide validated the use of cfDNA analysis as a screening test for trisomy 21. In this study, 209 of 221 cases of trisomy 21 were detected; sensitivity was 98.6%, with a false-positive rate of 0.2%. Subsequently, Palomaki and colleagues reported that all cases of trisomy 18 in this cohort were detected, with a false-positive rate of 0.28%; however, only 91.7% of the cases of trisomy 13 were detected, with a false-positive rate of 0.97%.

Norton and colleagues conducted a multicenter, prospective cohort study—the Noninvasive Chromosomal Evaluation (NICE) study—of more than 3200 primarily high-risk women undergoing invasive diagnostic testing. For trisomy 21, the sensitivity was 100%, with a false-positive rate of 0.03%, whereas the sensitivity for trisomy 18 was 97.4% and the false-positive rate 0.07%. Furthermore, 29% of the chromosome abnormalities in this cohort were abnormalities other than trisomy 18 and 21, and hence it is important to counsel patients about the limitations of cfDNA testing.

In a cohort study of more than 2000 women who presented for routine first-trimester screening (11 to 14 weeks) with a mean maternal age of 31.8 years, Nicolaides and colleagues demonstrated that cfDNA analysis using targeted MPSS is feasible in a lower-risk population. The detection rate for trisomy 21 was 100%, with a false-positive rate of 0.1%. Norton and colleagues reported similar results in the international, blinded, prospective multicenter Noninvasive Examination of Trisomy (NEXT) study, which compared cfDNA to first-trimester screening at 10 to 14 weeks’ gestation in 15,841 women (mean age 30.7) with a singleton gestation. The positive predictive value (PPV) for trisomy 21 was 80.9% (95% confidence interval [CI], 66.7 to 90.9). Although all cases of trisomy 21 and 13 were detected, it is important to note that cfDNA testing failed to detect one in 10 cases of trisomy 18, nor would it have detected the other forms of aneuploidy found in this study population. This highlights the need to counsel patients about the limitations of screening tests to detect all forms of aneuploidy.

Another limitation of cfDNA screening is the reported assay failure rate of up to 5%. One of the reasons is a low fetal fraction; a minimum fetal fraction of 4% is required. The ability to detect the small differences between euploid and triploid fetuses depends on the relative proportion of fetal to maternal cfDNA. The average fetal fraction at 10 to 22 weeks’ gestation is 10% independent of gestational age, maternal age, race/ethnicity, or fetal karyotype. Fetal fraction decreases with maternal weight, and if cfDNA screening fails in an obese patient, it may be necessary to repeat the test on a second sample or to offer serum and/or ultrasound screening as an alternative. Further consideration should be given to offering invasive testing because recent studies have reported a higher frequency of aneuploidy when cfDNA testing fails. In the NEXT study the rate of aneuploidy among patients with no cfDNA results was 2.7%, a prevalence of 1 in 38 ; hence careful consideration should be given to offering diagnostic testing to this group of patients.

Although the false-positive and false-negative rates for cfDNA screening are lower than sequential and second-trimester screening, discordancies between cfDNA results and the fetus have been reported. A systematic review of the literature from 1997 to 2016 identified 182 cases of false-positives and 24 false-negative results. False-negative cases were reported for trisomy 21, 18, and 13 and mosaicism for these chromosomes. Approximately a third of the false-positive cases were trisomy 21 and a third trisomy 18; 26% were trisomy 13. Other aneuploidies were found in three cases and multiple aneuploidies in eight cases. This study excluded discordant sex chromosome results. A biologic or technical explanation for the discordancy between the cfDNA result and the fetus could be identified in only a third of these cases. Of these, 32% has placental mosaicism and 48% had a maternal copy number variation. Other explanations included a vanishing twin with trisomy 21, maternal chromosome abnormalities of chromosome 21 and 18, and nine cases with maternal cancer in which six had multiple aneuploidies. Maternal and fetal mosaicism for 45,X/XX can also lead to false-positive cfDNA results. These findings underscore the importance of offering confirmatory diagnostic testing prior to pregnancy termination when cfDNA indicates a high risk for a chromosome abnormality. Patients will need to be counseled that a negative cfDNA test does not ensure an unaffected pregnancy and cannot provide the diagnostic accuracy of an invasive prenatal diagnostic test, especially if fetal structural anomalies exist or with a family history of a genetic disorder.

The American College of Obstetricians and Gynecologists (ACOG) and Society of Maternal Fetal Medicine (SMFM) recommend cfDNA as one of the screening options for women with a singleton gestation at increased risk for aneuploidy, including women 35 years or older and those with fetal ultrasound markers and structural anomalies associated with aneuploidy, prior pregnancy with a trisomic offspring, positive maternal serum screening test, or a parental Robertsonian translocation carrying risk for trisomy 21. The American College of Medical Genetics and Genomics (ACMG) acknowledges that all women regardless of risk status should have the option to select cfDNA analysis.

Experience using cfDNA analysis in multiple gestations is limited, but meta-analysis suggests that cfDNA has similar sensitivity and specificity rates as in singleton pregnancies. For dizygotic twins, a higher fetal fraction (minimum of 8%) may be necessary to detect a quantitative difference. cfDNA results should be interpreted cautiously in pregnancies with a vanishing twin; results may be discordant because the vanishing twin may continue to release cfDNA into the maternal circulation. SNP-based testing may be helpful in identifying this phenomenon.

Some laboratories offer screening for select microdeletion syndromes, although this has not been endorsed by most professional societies due to limited experience and insufficient information on test performance. Individually these are rare disorders (1 in 3000 to 1 in 50,000), so the PPV is expected to be low. Petersen et al. reported a combined 13% PPV for select microdeletion syndromes and a 21% PPV for the 22q11.2 deletion, with higher false-positive rates than for common aneuploidies. A SNP-based cfDNA test for the 22q11.2 deletion demonstrated a sensitivity of 97.8% and specificity of 99.75% on a sample of 10 affected fetuses and 390 controls. That the mean gestational age was higher (21.7 weeks) and the fetal fraction higher (16.6%) may have affected the test performance. Expanded cfDNA screening is likely in the future but will require pretest and posttest counseling to ensure patients understand the limitations of screening.

First-Trimester Screening

First-trimester screening is an excellent screening option for women at low risk for aneuploidy and can be performed between 11 and 14 weeks using a combination of biochemical markers, pregnancy-associated plasma protein A (PAPP-A) and free β–human chorionic gonadotropin (β-hCG), and ultrasound measurement of the nuchal translucency (NT), a sonolucent space present in all fetuses behind the fetal neck. The detection rate for trisomy 21 is greater than 80%, with a false-positive rate of 5% compared with a 70% detection rate based on NT measurement alone . In trisomy 21, PAPP-A levels are typically reduced, whereas hCG levels and the NT measurement are increased. First-trimester screening is comparable or superior to second-trimester screening alone, and, most importantly, it provides parents with the option of earlier diagnostic testing in the event the screen indicates that the fetus is at high risk for aneuploidy. However, mandatory training and quality assurance for the NT measurement is critical. The incorporation of the other sonographic markers such as the presence of a nasal bone (NB), reverse ductus venosus flow, and tricuspid regurgitation has been proposed to increase the detection rates further. In general, these markers are not used except in specialized centers.

Although NT alone has a lower detection rate for trisomy 21 (64% to 70%) and is not recommended as a primary screening test, it is important to note that an NT greater than 4 mm always was associated with an abnormal noninvasive screen in the National Institute of Child Health and Human Development (NICHD) First and Second Trimester Evaluation of Risk (FASTER) trial. Therefore women with an NT exceeding this threshold should be offered diagnostic testing and forego serum analyte testing. Only 8% of pregnancies with NT greater than 3 mm had a screen-negative test. When an increased NT measurement is associated with a normal karyotype, fetal loss rates are increased and other fetal anomalies—in particular congenital cardiac defects—and genetic syndromes are observed. A targeted ultrasound examination during the second trimester and fetal echocardiography are recommended when the NT measurement is 3.5 mm or greater and the fetal karyotype is normal.

Second-Trimester Serum Screening

The most widely used second-trimester aneuploidy screening test is the so-called quad screen, which uses four biochemical analytes—α-fetoprotein (AFP), hCG, unconjugated estriol (uE ₃ ), and dimeric inhibin A (INHA). Performed between 15 and 22 weeks’ gestation, the detection rate for trisomy 21 is approximately 75% in women who are younger than 35 years of age and more than 80% in women 35 years or older, with a false-positive rate of 5%. For trisomy 18, using only the first three markers provides a detection rate of approximately 70%. Serum screening does not detect other age-related forms of aneuploidy such as Klinefelter syndrome (47,XXY).

Serum hCG and INHA levels are increased in women carrying fetuses with Down syndrome. Levels of AFP and uE ₃ in maternal serum are lower in pregnancies affected with Down syndrome compared with unaffected pregnancies. Typically, levels of AFP, uE ₃ , and hCG are reduced in trisomy 18. A simple approach to detect trisomy 18 is to offer invasive prenatal diagnostic testing whenever serum screening for each of these three markers falls below certain thresholds (maternal serum -fetoprotein [MSAFP], 0.6 multiples of the median [MoM]; hCG, 0.55 MoM; uE ₃ , 0.5 MoM). Using these thresholds would detect 60% to 80% of trisomy 18 fetuses with a 0.4% amniocentesis rate. Calculating individual risk estimation on the basis of three markers and maternal age, Palomaki and colleagues reported that 60% of trisomy 18 pregnancies can be detected with a low false-positive rate of 0.2%. The value of individual risk estimates is that one in nine pregnancies identified as being at increased risk for trisomy 18 by serum screening would actually be affected.

Confounding factors influence serum screening, and adjustments for gestational age, maternal weight, ethnicity, diabetes, and number of fetuses is necessary. Weight adjustment is needed because without adjustment, dilutional effects would result in heavier women having a spuriously low value, whereas thinner women would have a spuriously elevated value. In women with type 1 diabetes mellitus, a population at increased risk for NTDs, the median levels of MSAFP, uE ₃ and hCG are lower than in nondiabetic women. In black women, who have a lower risk for a fetal NTD, the median MSAFP is higher than in other ethnic groups. Maternal smoking increases MSAFP by 3% but decreases maternal serum uE ₃ and hCG levels by 3% and 23%, respectively. Maternal serum hCG is higher and MSAFP is lower in pregnancies conceived in vitro compared with pregnancies conceived spontaneously. Algorithms typically take these confounders into account prior to the report being conveyed.

First- and Second-Trimester Screening

Several approaches have been proposed using the combination of both first- and second-trimester screening to increase the detection rate over that achieved by screening in either trimester alone, with detection rates of 88% to 96% with false-positive rates of 5% reported. A caveat is that independent screening (i.e., using both first- and second-trimester screening tests to assess the risk separately and independently) is not recommended because of unacceptably high false-positive rates.

Sequential screening begins with first-trimester screening. A woman is informed of the adjusted risk for aneuploidy based on the first-trimester results. If her risk is high (greater than 1 in 50), she is offered genetic counseling and diagnostic testing. If the risk is low or moderate, a second-trimester screening test is performed with results of both the first- and second-trimester screening tests used to generate a final adjusted risk for trisomies 21 and 18. This is called the stepwise approach . With contingency screening, not all women will proceed to second-trimester screening because this occurs only with an intermediate risk; if the risk is low after the first-trimester screening, no further testing is indicated. Detection rates of the contingency approach are approximately 90%, with low positive screening rates (2% to 3%). Malone and colleagues compared several different first- plus second-trimester contingent sequential approaches and concluded that the optimal method was contingency screening, in which patients were divided into three groups: (1) women whose calculated (NT, PAPP-A, hCG) first-trimester risk was greater than 1 in 30 would undergo chorionic villus sampling (CVS); (2) women whose risk was less than 1 in 1500 would undergo no further testing; and (3) all other women would undergo second-trimester serum testing. Using this approach, only 21.8% of the cohort would need second-trimester testing to detect 93% of trisomy 21 cases with a 4.3% false-positive rate; 65% would be detected in the first trimester, with only 1.5% of patients having CVS procedures.

Integrated screening has the highest theoretic (modeled) detection rate (93% to 96%) among this group of serum-analyte based tests, but with this approach first-trimester screening, results are withheld until the second-trimester screen is completed. The individual receives only a single adjusted risk for trisomy 21 and trisomy 18 based on the results of both the first- and second-trimester screen. The obvious disadvantage with this approach is that the individual does not have an option of a diagnostic procedure in the first trimester even if screening would have indicated a high risk for trisomy 21 or 18. In addition, theoretic detection is not always reached because some patients fail to return for their second-trimester screening.

Serum integrated screening is an acceptable alternative when an NT measurement cannot be obtained and is not available. With this approach, first-trimester PAPP-A and second-trimester serum analytes are used to determine the risk for trisomy 21. The patient receives one adjusted risk after the second-trimester screen is completed. In the FASTER trial, the sensitivity of this approach was 88%.

Aneuploidy Screening in Multiple Gestation

In dizygotic pregnancies, each twin has an individual risk for trisomy 21; analysis of the European National Down Syndrome Cytogenetic Registry found that dizygotic pregnancies were one-third more likely to have at least one fetus with trisomy 21 than age-adjusted singleton pregnancies. The pregnancy-specific and fetus-specific risks should be the same for monozygotic twins as for singleton pregnancies, although the study found the actual risk to be approximately a third that of a singleton pregnancy. Down syndrome screening using multiple serum markers is less sensitive in twin pregnancies than in singleton pregnancies. Using singleton cutoffs, one study showed that 73% of monozygotic twin pregnancies but only 43% of dizygotic twin pregnancies with Down syndrome were detected, given a 5% false-positive rate. Decreased sensitivity in detecting trisomy 21 in dizygotic twins reflects the blunting effect of the concomitant presence of one normal and one aneuploid fetus. Thus patients with twins should be informed that the detection rate by serum screening is less than that in singleton pregnancies. First-trimester screening identifies approximately 70% of Down syndrome pregnancies; NT measurement alone has been shown to be as effective a screening test for higher-order multiple gestation as it is for twin gestations. Addition of NB assessment to the NT measurement increased the detection rate to 87% at a screen-positive rate of 5% and to 89% when serum analytes were included in a retrospective study of 2094 twin pregnancies. Although the experience is limited, cfDNA analysis may prove to be a good alternative for screening multiple gestation. Meta-analysis suggests similar sensitivity and specificity to those of singleton gestations.

Ultrasound Screening for Aneuploidy

Second-trimester ultrasonography may detect anomalies associated with aneuploidy, such as cardiac defects or duodenal atresia (see Chapter 9 ). In 1985, Benacerraf and colleagues showed a significant association between the thickness of the fetal nuchal skin fold and the presence of trisomy 21. Other markers that have been used in the genetic sonogram include the NB length, short femur or humerus, echogenic intracardiac focus, echogenic bowel, and pyelectasis. As markers were studied in larger numbers of women, it became possible to assign likelihood ratios to each marker and to do a formal risk adjustment from the a priori risk to an ultrasound-adjusted risk. However, most of the markers perform poorly as individual predictors of Down syndrome, having a very low sensitivity and a high rate of false-positive results. Furthermore, the test performance depends on the skill of the examiner and the subjective assessment required for several markers, particularly echogenic intracardiac focus and echogenic bowel. Most women who undergo a genetic sonogram have no markers of aneuploidy, but women should be informed that the absence of markers does not rule out the possibility of Down syndrome or other chromosome abnormalities.

The incidental finding of “soft markers” such as echogenic intracardiac focus or pyelectasis in otherwise low-risk pregnancies can cause a great deal of anxiety in the expectant parents. Opinions vary as to what to do when these markers are noted in a low-risk patient. A reasonable approach is to refer such a patient for consultation with an expert to evaluate the presence of other markers in the context of other screening tests (i.e., multiple marker screening or cfDNA analysis).

Indications for Prenatal Cytogenetic Testing

ACOG recommends that all women, regardless of age, should be offered screening or diagnostic testing for aneuploidy including the option of a CMA. Prenatal cytogenetic testing can be as basic as a routine G-banded karyotype, or in some cases a more targeted approach such as fluorescence in situ hybridization (FISH) may be recommended.