The role of the second trimester genetic sonogram in screening for fetal Down syndrome

The Genetic Sonogram is an ultrasound examination done on second trimester fetuses that not only evaluates the fetus for structural malformations, but also searches for the sonographic markers of fetal Down syndrome. The main markers that comprise the genetic sonogram include the nuchal fold, short femur and humerus, pyelectasis, hyperechoic bowel, echogenic intracardiac focus, and any major abnormality. The absence of any marker on a second trimester scan conveys a 60-80% reduction in prior risk of Down syndrome based on advanced maternal age or serum screen risk. The presence of sonographic markers, either singly or in combination, will raise the baseline risk of Down syndrome using likelihood ratios calculated for each individual marker. Using this approach, approximately 75% of fetuses with Down syndrome can be identified by modifying the patient's baseline risk according to the results of the ultrasound. The second trimester scan will likely continue to play an important role in the future in the detection of aneuploidy.     

Second trimester prenatal ultrasound for the detection of pregnancies at increased risk of Down syndrome

OBJECTIVE: To determine the association between second trimester ultrasound findings (genetic sonogram) and the risk of Down syndrome. METHODS: Prospective population-based cohort study of women who were at increased risk of chromosome abnormality based on serum screening. RESULTS: Overall 9244 women with singleton pregnancies were included, including 245 whose fetuses had Down syndrome. Overall, 15.3% of the women had an abnormal genetic sonogram, including 14.2% of pregnancies with normal fetuses and 53.1% of those with Down syndrome. If the genetic sonogram were normal, the risk that a woman had a fetus with Down syndrome was reduced (likelihood ratio 0.55 [95% CI 0.49, 0.62]) However, if the normal genetic sonogram were used to counsel these high-risk women that they could avoid amniocentesis, approximately half of the cases of Down syndrome (115 of 245) would have been missed. The isolated ultrasound soft markers were the most commonly observed abnormality. These were seen in a high proportion of Down syndrome fetuses (13.9%) and normal fetuses (9.3%). In the absence of a structural anomaly, the isolated ultrasound soft markers of choroid plexus cyst, echogenic bowel, renal pyelectasis, clenched hands, clinodactyly, two-vessel umbilical cord, short femur, and short humerus were not associated with Down syndrome. Nuchal fold thickening was a notable exception, as a thick nuchal fold raised the risk of Down syndrome even when it was seen without an associated structural anomaly. LIMITATIONS: All women included in this study were at high risk of Down syndrome based on serum screening, and thus the results of this study cannot be used as a basis to modify maternal age-related risk. CONCLUSIONS: The accuracy of the genetic sonogram is less than previously reported. The genetic sonogram should not be used as a sequential test following serum biochemistry, as this would substantially reduce the prenatal diagnosis of Down syndrome cases. In contrast to prior reports, most isolated soft markers were not associated with Down syndrome.     

Trisomy 21. Second-trimester ultrasound

Not every aspect of sonographic examination reveals karyotypic abnormalities. Ultrasound examination of a fetus with trisomy 21 generally reveals normal amniotic fluid, normal placentation, and normal fetal growth. In addition, other chromosomal abnormalities have many of the same sonographic findings as Down syndrome, and many findings have a large overlap with phenotypically normal fetuses. The importance of second-trimester ultrasound screening for Down syndrome has remained great because of its ease of use and relative effectiveness. Trained sonographers can adjust the relative risk for trisomy 21 and alter the need for genetic amniocentesis. It is important that parents understand the limitations of a screening test and the risks and benefits of possible subsequent confirmatory testing. If a major structural abnormality is identified on ultrasound, karyotype determination should be considered. Nuchal thickness in the first or second trimester remains the most clinically useful marker for trisomy 21. The predictive value of all the markers depends on the population studied and can be modified by a host of biochemical markers and historical factors. If fetal karyotype analysis could be performed without sampling through the uterus, prenatal diagnosis could be offered to all pregnant women, and screening would be unnecessary. Despite its limitations, ultrasound will have an important role in prenatal diagnosis at least until isolating and testing fetal cells from maternal blood or other sources becomes practical and widely available. Whether used alone or in conjunction with additional biochemical or molecular serum markers, ultrasound is an important and powerful tool in prenatal genetic evaluation.     

Use of genetic sonography for adjusting the risk for fetal Down syndrome

Systematic evaluation of ultrasound findings known to be associated with trisomy 21, at an appropriate gestational age, has been referred to as a genetic sonogram. A number of high-risk centers performing genetic sonography have reported detection of ultrasound abnormalities in the majority of fetuses with fetal Down syndrome. However, nonspecific markers are more commonly observed than structural abnormalities, which are detected in less than 20% of cases in a nonselected population. Also, the actual sensitivity of a genetic sonogram will depend on various factors including the markers sought, gestational age, reasons for referral, and of course the quality of the ultrasound. Appropriate use of a genetic sonogram can help to modify the risk of fetal Down syndrome by decreasing the risk when the ultrasound is normal, or increasing the risk when specific ultrasound markers are detected. The postultrasound risk can be estimated by applying specific likelihood ratios, reflecting the strength of individual markers, with the a priori risk based on maternal age alone, or combined with biochemical markers when known. We review this approach of age-adjusted ultrasound risk assessment for fetal Down syndrome and illustrate how the risk can be estimated. Individual sonographic markers are also discussed.     

Role of ultrasound for Down syndrome screening in advanced maternal age.

OBJECTIVE: To determine the sensitivity and false-positive rate of Down syndrome screening by use of maternal serum screen and the genetic sonogram in women > or =35 years of age. STUDY DESIGN: We searched our perinatal databases retrospectively from January 1992 to January 2000 for the following criteria: known Down syndrome fetus or newborn, advanced maternal age, and genetic sonogram from 14-24 weeks' gestation. The a priori maternal age or maternal serum screen risk was modified by likelihood ratios for ultrasound markers. Without markers the risk was reduced by 50%. The cut-off was 1:270. RESULTS: Age and maternal serum screen had a sensitivity of 90.5% and a false-positive rate of 27.1%. Age and ultrasound had a 95.2% sensitivity and 43.5% false-positive rate, whereas the combination of age, maternal serum screen, and ultrasound had a 97.6% sensitivity and a 22.0% false-positive rate. CONCLUSION: The combination of age, maternal serum screen, and ultrasound improves the sensitivity for Down syndrome detection in the advanced maternal age population.     

The mid-trimester genetic sonogram

The genetic sonogram is a composite algorithm combining multiple individual markers to increase Down syndrome risk prediction. Transformation of sonographic information into a standard mathematical format represented an early challenge that has now been surmounted. Using increasingly sophisticated mathematical techniques, individual patient risk can be estimated. High diagnostic accuracy comparable to standard mid-trimester serum algorithms has been reported. Most recently, a few studies have reported the ability to combine serum and biochemical markers to achieve diagnostic accuracy comparable to first-trimester screen. Even fewer studies have reported combinations of ultrasound and maternal urine markers. While it is clear that consistently high sensitivity and specificity for Down syndrome can be achieved, almost all the studies are based on high-risk groups. Studies in low-risk populations have suffered from lack of standardization. The relevance of genetic sonogram in a low-risk population thus remains to be proven. The most significant challenge, however, remains the development of uniform and reproducible sonographic and measurement standards. This is likely to be the most important factor in optimizing the accuracy of the mid-trimester genetic sonogram.     

Pregnancy associated plasma protein-A2: A novel biomarker for down syndrome

IntroductionIn an effort to improve prenatal screening for Trisomy 21, we evaluated pregnancy associated plasma protein-A2 (PAPP-A2) as a potential novel second trimester biomarker for Trisomy 21.MethodsTrisomy 21 and normal control mid-trimester placental samples were subjected to quantitative rt PCR analysis of seven genes we had previously found to be differentially expressed in Trisomy 21 placentae. The localization and differential expression of PAPP-A2 in second trimester placentae from normal and Trisomy 21 pregnancies was determined by immunohistochemistry. PAPP-A2 maternal serum protein levels in ten Trisomy 21 and ten diploid pregnancies were compared by Western blotting. Maternal serum PAPP-A2 levels were measured in 30 Down syndrome cases and 142 normal controls, using ELISA. Regression analysis was used to determine the correlation of PAPP-A2 with other existing markers of Trisomy 21.ResultsPAPP-A2 (aka PLAC 3) mRNA and protein expression were both increased in Down syndrome placentae as compared to diploid placentae. PAPP-A2 was also increased in maternal serum from Down syndrome pregnancies as compared to diploid pregnancies. PAPP-A2 expression correlated weakly with established markers.DiscussionThis work takes advantage of our previously performed systematic approach to the discovery of novel maternal serum biomarkers for Trisomy 21, using cDNA microarray analysis. Beginning with the validation of the microarray results, we have tracked PAPP-A2 overexpression in Down syndrome from placental mRNA to maternal serum protein.ConclusionPAPP-A2 could serve as an additional maternal serum marker in prenatal screening for Trisomy 21.     

First trimester screening for aneuploidy: Nuchal translucency sonography

Prenatal diagnosis of fetal aneuploidy is a continuously and rapidly evolving area of research. Currently in the United States, the standard of care for screening pregnancies for aneuploidy involves assessment of maternal age together with the use of multiple second trimester maternal serum markers. This screening approach identifies approximately 60% of pregnancies with fetuses affected with Down syndrome and provides results in the second trimester of pregnancy. First trimester screening for aneuploidy by using nuchal translucency sonography is one of the most promising areas of research in the detection of Down syndrome. This screening method involves measuring the normal space located between the cervical spine and overlying fetal skin at 10 to 14 weeks' gestation. Studies from both high risk and unselected patient populations suggest significant advantages to this approach for Down syndrome detection compared with currently available second trimester screening methods. The combination of first trimester biochemical screening and nuchal translucency measurements may further improve the efficacy of prenatal screening for aneuploidy. The article reviews studies suggesting a role for nuchal-translucency-based aneuploidy screening and describes areas of ongoing research in this field.     

Down syndrome risk estimation after normal genetic sonography

OBJECTIVE: The objective of this study was to determine whether there are any indication-specific variations in risk reduction for fetal Down syndrome after a normal genetic sonogram. STUDY DESIGN: A second-trimester genetic sonogram was offered to all pregnant women who were at increased risk for fetal Down syndrome (>/=1:274) because of either advanced maternal age (>/=35 years), an abnormal triple screen, or both. Outcome information included the results of genetic amniocentesis (if performed), the results of pediatric assessment, and follow-up after birth. Normal genetic sonography was defined as the absence of all ultrasound aneuploidy markers. RESULTS: The overall prevalence of fetal Down syndrome in the tested population was 1.41% (53/3,753 pregnancies); however, in the presence of normal genetic sonography, the overall prevalence of fetal Down syndrome was 0.21% (7/3,291 pregnancies). The overall risk reduction for fetal Down syndrome in the presence of normal genetic sonography was 6.64-fold (95% CI, 3.01-14.62); the overall negative likelihood ratio was 0.15 (95% CI, 0.07-0.33). In the presence of normal genetic sonography, the risk for fetal Down syndrome was reduced by 83% in patients with advanced maternal age, 88% in patients with abnormal triple screen, 89% in patients with abnormal triple screen who were <35 years old, and 84% in patients who had both abnormal triple screen and advanced maternal age. CONCLUSION: There were no significant variations in the risk reduction for fetal Down syndrome in the presence of normal genetic sonography. Regardless of the indication for testing, the likelihood for fetal Down syndrome was reduced by 83% to 89%. This information will be useful in counseling pregnant women who are at high risk for fetal Down syndrome and who prefer to undergo genetic sonography before deciding about genetic amniocentesis.     

Expected performance of second trimester maternal serum testing followed by a 'genetic sonogram' in screening of fetal Down syndrome

OBJECTIVE: To assess the efficacy of screening for fetal Down syndrome through the sequential provision of second trimester maternal serum tests and fetal ultrasound (the 'genetic sonogram'). METHODS: Monte Carlo modeling was used to generate typical results for second trimester serum screening. Serum test likelihood ratios were then multiplied by likelihood ratios associated with the provision of a genetic sonogram. The impact of adding the genetic sonogram on the detection rate (DR) and false-positive rate (FPR) were assessed for typical protocols that are currently in use. The effect of expanding the number of women who receive the genetic sonogram, variation in the marker frequencies in the genetic sonogram, and a multivariate Gaussian model that incorporated both serum and ultrasound measurements as continuous variables were also considered. RESULTS: When the genetic sonogram is offered only to those women who are screen-positive by serum testing, there can be a substantial reduction in the number of women with an indication for amniocentesis but also some loss in detection. The extent of these changes will partially depend on the serum tests and cutoff used. Providing the genetic sonogram to more women can reduce loss in detection without resulting in high amniocentesis rates. As a sequential screening tool, the genetic sonogram can be improved by incorporating markers that have a high frequency in affected pregnancies and by using ultrasound measurements as continuous variables. CONCLUSION: It should be possible to provide highly effective multistep screening protocols that maximize the benefits of both maternal serum and ultrasound while minimizing the amount of testing offered.     

Identifying Smith-Lemli-Opitz syndrome in conjunction with prenatal screening for Down syndrome

BACKGROUND: Smith-Lemli-Opitz syndrome (SLOS) is a rare hereditary disorder of cholesterol metabolism. We examine the feasibility of identifying SLOS as a part of a routine prenatal screening and evaluate diagnostic testing in maternal urine (or serum), in addition to amniotic fluid. METHODS: Our SLOS risk algorithm utilized three Down syndrome screening markers (estimated 62% detection rate; 0.3% screen-positive rate). Fifteen North American prenatal screening programs implemented this algorithm. RESULTS: SLOS risk was assigned to 1 079 301 pregnancies; 3083 were screen-positive (0.29%). Explanations were found for 1174, including 914 existing fetal deaths. Among the remaining pregnancies, 739 were screen-positive only for SLOS; 1170 were also screen-positive for other fetal disorders. Five of six SLOS pregnancies (83%) were screen-positive. All six had sonographic findings, were biochemically confirmed, and were terminated. Maternal urine steroid measurements were confirmatory in four cases tested. Second-trimester prevalence among Caucasians was 1 in 101 000 (1 in 130 000 overall; no cases in other racial groups). Among 739 pregnancies screen-positive only for SLOS, two cases were identified; another 69 had major fetal abnormalities. CONCLUSIONS: Although SLOS occurred less often than previously reported, many other major abnormalities were detected. Implementing the algorithm as an adjunct to Down syndrome screening may be feasible.     

Follow-up of sonographically detected soft markers for fetal aneuploidy

Sonographic soft markers of fetal Down syndrome were first reported in the 1980s. With improvements in aneuploidy screening, detection rates of 90% and higher are possible, and such screening is offered to women of all ages. The utility of sonographic detection and reporting of soft markers, particularly to women at low risk of fetal aneuploidy, is controversial. Some soft markers have no additional significance beyond an association with aneuploidy, while some potentially indicate other pathology, and therefore require sonographic follow-up or other evaluation. The definitions of soft markers vary among reported series, and any practice using such markers to adjust the risk of aneuploidy should carefully determine the most appropriate definitions as well as likelihood ratios and how to apply these in practice.     

From Down syndrome screening to noninvasive prenatal testing: 20 years' experience in Taiwan

Down syndrome is the most common autosomal chromosome aneuploidy. The prenatal Down syndrome screening protocol has been known in Taiwan for the past 20 years. The maternal serum double markers required for the screening test was first implemented into the general prenatal check-up back in 1994, where it had around a 60% detection rate at a 5% false positive rate. The first trimester combined test was started in 2005, and the maternal serum quadruple test was introduced in 2008 to replace the previous double test. The overall detection rate for the current screening strategies (first trimester combined or second trimester quadruple test) in Taiwan ranges between 80% and 85% at a fixed 5% false positive rate. Noninvasive prenatal testing (NIPT) is the latest powerful fetal aneuploidy detection method and has become commercially available in Taiwan starting from 2013. The sensitivity and specificity for NIPT are very high (both over 99%) according to large worldwide studies. Our preliminary data for NIPT from 11 medical centers in Taiwan have also shown a 100% detection rate for Down syndrome and Edwards syndrome, respectively. Invasive chromosome studies such as amniocentesis or chorionic villus sampling cannot be replaced by NIPT, and all prenatal screening and NIPT results require confirmation using invasive testing. This review discusses the Down syndrome screening method assessments and the progress of NIPT in Taiwan.     

First-trimester screening for aneuploidy

Prenatal screening for aneuploidy in the first trimester using novel ultrasound and maternal serum markers represents a promising improvement over the currently available second-trimester screening methods. This article reviews the current status of first-trimester screening for Down syndrome and other aneuploidies and explores the issues related to implementing first-trimester screening into mainstream prenatal care in the United States.     

Second-trimester genetic sonography in patients with advanced maternal age and normal triple screen

OBJECTIVE: To estimate the value of second-trimester genetic sonography in detecting fetal Down syndrome in patients with advanced maternal age (at least 35 years) and normal triple screen. METHODS: Since July 1999, a prospective collection and recording of all individual triple screen risks for fetal Down syndrome was initiated for all patients with advanced maternal age presenting in our ultrasound unit for second-trimester genetic sonography. Genetic sonography evaluated the presence or absence of multiple aneuploidy markers. Outcome information included the results of genetic amniocentesis, if performed, and the results of pediatric assessment and follow-up after birth. RESULTS: By June 2001, 959 patients with advanced maternal age and normal triple screen were identified. Outcome information was obtained in 768 patients. The median risk for fetal Down syndrome based on maternal age was 1:213 (range 1:37-1:274). The median risk for fetal Down syndrome based on triple screen results was 1:1069 (range 1:275-1:40,000). A total of 673 patients had normal genetic sonography, and none (0%) had Down syndrome; 95 had one or more aneuploidy markers present, and four (4.2%) had fetuses with Down syndrome. The triple screen risks for these four fetuses ranged from 1:319 to 1:833. CONCLUSION: This study suggests that patients with advanced maternal age and normal genetic sonography carried very little risk for Down syndrome. The use of genetic sonography may increase the detection rate of fetal Down syndrome in this group of pregnant women.     

Acceptability of serum screening as an alternative to cytogenetic diagnosis of down syndrome among women 35 years or older in Hong Kong

The addition of second trimester serum markers to maternal age increases the efficacy of screening for Down syndrome by maternal age alone. Among women aged 35 years or older, serum screening makes a large proportion of amniocentesis unnecessary. However, there are ethical and medicolegal concerns about serum screening in 'old' women, largely because some of the pregnancies affected by Down syndrome and other chromosomal abnormalities may not be detected. We investigated the acceptability of serum screening in women aged 35 years or older when it was offered as an alternative to prenatal cytogenetic diagnosis after detailed counselling. Women referred for prenatal diagnosis of Down syndrome because of advanced maternal age were given the options of cytogenetic diagnosis by chorionic villus sampling (CVS) or amniocentesis. As an alternative, they could choose to undergo second trimester serum human chorionic gonadotrophin (hCG) and alpha-fetoprotein (AFP) screening first before deciding on whether to undergo amniocentesis. Between January 1997 and October 1999, 3419 subjects were recruited. 1807 women (52. 9%) chose to undergo serum screening, 1516 women (44.3%) chose to have amniocentesis and 96 women chose to have CVS (2.8%). The proportion of women who chose serum screening rose steadily from 38. 8% in the year of 1997 to 63.4% in 1999. Significantly fewer Chinese women chose serum screening than non-Chinese. The decision as to whether to undergo an invasive diagnostic procedure or to be content with the relatively safer but less accurate screening test varies, being affected by the women's background and culture.     

The use of genetic sonography to reduce the need for amniocentesis in women at high-risk for Down syndrome

Much information has been published regarding the use of second-trimester genetic sonography for the prenatal detection of Down syndrome by examining multiple aneuploidy markers. Among high-risk mothers (advanced maternal age, abnormal triple screen, or both), while many undoubtedly will choose to have invasive testing as a first option, others will instead use the information derived from genetic sonography to obtain an adjusted risk for Down syndrome to guide their decision about genetic amniocentesis. Accordingly, it is imperative that these patients have accurate and detailed counseling regarding their degree of risk reduction when the genetic sonogram is normal. This article reviews the use of second trimester genetic sonography in reducing the need for amniocentesis in the high-risk patient. At our institution, in high-risk patients when the genetic ultrasound is normal, the amniocentesis rate has been only 3%. We have found that genetic sonography is a patient-driven service, and that the information obtained at the time of ultrasound is an important component of the patient's decision of whether or not to proceed with invasive testing.     

Second trimester maternal serum screening using alpha fetoprotein, free beta human chorionic gonadotropin and maternal age specific risk: result of chromosomal abnormalities detected in screen positive for Down syndrome in an Asian population.

BACKGROUND: This study was to determine the incidence of chromosome abnormalities in Taiwanese women undergoing prenatal chromosome analysis after a second trimester Down syndrome screening by using maternal age and serum dual-marker testing (alpha-fetoprotein and free-beta unit human chorionic gonadotropin). METHODS: A total of 10,098 Taiwanese women with pregnancy between 15 and 23 weeks' gestation received second-trimester Down syndrome risk evaluation by dual-marker and maternal age specific risk testing in a single medical center. The study took 22 months. Ninety-seven percent of this study population was less than 34 years old. Ninety-six percent of our cases were screened between 15-20 weeks of gestation. This population was included only after a routine ultrasonography scan for correction of gestational age and exclusion of major structural anomalies. By using an algorithm to detect Down's syndrome, with a risk of 1:270 as a cut-off value, 816 patients were screen-positive for Down syndrome (screen-positive rate 8.0%). Karyotypes were reviewed for 670 (82.1%) mothers who received prenatal karyotype analysis. RESULTS: Twelve cases of Down syndrome were identified in the screen positive group with an estimated detection rate of 67% (false positive rate 8%). Three cases of Down syndrome were detected in late trimester among the screen-negative group. Seven other fetal chromosome abnormalities were also found among the screen-positive pregnancy. In addition, seven cases were screen-positive for trisomy 18; all of these patients received amniocentesis and only one case was confirmed. CONCLUSION: These findings indicate that this screening program combining alpha-fetoprotein (AFP), free beta human chorionic gonadotropin (free-hCG) and maternal age-specific would achieve a screening efficiency in Taiwanese populations as comparable to those obtained in Caucasian populations. Our results also suggest that approximately 3% of pregnancies with a positive dual marker and maternal age-specific screen results will have a chromosome abnormality despite having a normal routine ultrasound scan. Mothers with positive screening results should be made aware of the implications of a positive result.     

Ultrasound markers for Down syndrome

Trisomy 21 (Down syndrome) is the most common chromosomal abnormality. Sonographic findings in fetuses with Down syndrome include both structural abnormalities and nonstructural abnormalities or "markers." These markers are known as "soft markers" of aneuploidy. These markers are nonspecific, often transient. The most commonly studied soft markers of aneuploidy include a thickened nuchal fold, long bones shortening, mild fetal pyelectasis, echogenic bowel, echogenic intracardiac focus, FMF angle > 90 degrees, pathologic velocity of Ductus venosus and choroid plexus cyst.     

Second trimester trisomy 21 maternal serum marker screening. Results of a countrywide study of 854,902 patients

OBJECTIVES: In France, maternal serum marker screening is governed by specific legislation. We conducted a study of the countrywide trisomy 21 screening based on second trimester maternal serum markers. METHODS: We reviewed the medical records of 854,902 patients prospectively screened for second trimester maternal serum markers in the 60 authorized laboratories over the two-year period 1997-1998. All patients screened in France were included. The risk of trisomy 21 was calculated from the combination of maternal age and maternal serum markers. The same cut-off (1/250) was used in all laboratories. RESULTS: In 1998, 65% of pregnant women underwent maternal serum screening. In the 837,765 patients under 38 years of age who were screened, 54,321 (6.48%; 5% CI 6.42-6.53%) had a calculated risk >1/250. Of the 884 Down syndrome cases observed, 626 were detected by maternal serum markers (70.8%; 5% CI 67.8-73.8%). These good results can be explained by a strict quality control of all steps. For the 13,891 patients over 38 years of age, the Down syndrome detection rate was 98.9% for a 34% false-positive rate. CONCLUSIONS: Strict rules covering prenatal trisomy 21 screening are of benefit to patients, practitioners and laboratories alike, and ensure good quality control, a high trisomy 21 detection rate and a low amniocentesis rate.