New Down syndrome screening algorithm: ultrasonographic biometry and multiple serum markers combined with maternal age

Second trimester serum tests for Down's Syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21 - or the specific area of chromosome 21 implicated in causing Down's syndrome - rather than two. It is the commonest congenital cause of mental retardation. Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.  

OBJECTIVES

To estimate and compare the accuracy of second trimester serum markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out a sensitive and comprehensive literature search of MEDLINE (1980 to May 2007), EMBASE (1980 to 18 May 2007), BIOSIS via EDINA (1985 to 18 May 2007), CINAHL via OVID (1982 to 18 May 2007), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 2007, Issue 1), MEDION (May 2007), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (May 2007), The National Research Register (May 2007), Health Services Research Projects in Progress database (May 2007). We studied reference lists and published review articles.  

SELECTION CRITERIA

Studies evaluating tests of maternal serum in women at 14-24 weeks of gestation for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

Fifty-nine studies involving 341,261 pregnancies (including 1,994 with Down's syndrome) were included. Studies were generally high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Seventeen studies made direct comparisons between tests. Fifty-four test combinations were evaluated formed from combinations of 12 different tests and maternal age; alpha-fetoprotein (AFP), unconjugated oestriol (uE3), total human chorionic gonadotrophin (hCG), free beta human chorionic gonadotrophin (βhCG), free alpha human chorionic gonadotrophin (αhCG), Inhibin A, SP2, CA125, troponin, pregnancy-associated plasma protein A (PAPP-A), placental growth factor (PGF) and proform of eosinophil major basic protein (ProMBP).Meta-analysis of 12 best performing or frequently evaluated test combinations showed double and triple tests (involving AFP, uE3, total hCG, free βhCG) significantly outperform individual markers, detecting six to seven out of every 10 Down's syndrome pregnancies at a 5% false positive rate. Tests additionally involving inhibin performed best (eight out of every 10 Down's syndrome pregnancies) but were not shown to be significantly better than standard triple tests in direct comparisons. Significantly lower sensitivity occurred in women over the age of 35 years. Women who miscarried in the over 35 group were more likely to have been offered an invasive test to verify a negative screening results, whereas those under 35 were usually not offered invasive testing for a negative screening result. Pregnancy loss in women under 35 therefore leads to under ascertainment of screening results, potentially missing a proportion of affected pregnancies and affecting the accuracy of the sensitivity.

AUTHORS' CONCLUSIONS

Tests involving two or more markers in combination with maternal age are significantly more sensitive than those involving one marker. The value of combining four or more tests or including inhibin have not been proven to show statistically significant improvement. Further study is required to investigate reduced test performance in women aged over 35 and the impact of differential pregnancy loss on study findings.

Reevaluating humeral length for the detection of fetal trisomy 21

OBJECTIVE

The purpose of this study was to analyze humeral length (HL) in a normal population and to compare that with HL in a population of fetuses with trisomy 21 to determine the most efficient discriminating parameters for diagnostic accuracy.

METHODS

A nested case-control study comparing HLs from a normal population and a population of fetuses with trisomy 21 was conducted. Humeral length was regressed against gestational age for a consecutive well-dated population of normal singleton gestations presenting to the Washington University School of Medicine prenatal diagnosis units over a 5-year period. A second population of well-dated pregnancies with trisomy 21 diagnosed either prenatally or postnatally was also selected on the basis of the same criteria, except that anomalous fetuses were included. Various discriminating thresholds for a short HL were compared for efficiency in the detection of trisomy 21. These included the following: observed/expected HL (<or= 0.89), biparietal diameter/HL greater than 1.3, 1.4, 1.5, 1.6, 1.7, 1.75, 1.8, and 1.85 SD above the mean for gestation, HL less than 0.8 or less than 0.9 multiple of the median, and HL less than the fifth percentile for gestation.

RESULTS

A total of 620 normal pregnancies and 32 with trisomy 21 were extracted from the database. A receiver operating characteristic curve revealed HL less than the fifth percentile as the optimal discriminator for trisomy 21 detection (area under the receiver operating characteristic curve = 0.80). The positive likelihood ratio (LR+) was greatest (25.0) with HL less than the fifth percentile. When HL was considered in isolation without other sonographic markers of trisomy 21, the LR+ was 6.3.

CONCLUSIONS

Humeral length less than the fifth percentile was the most effective discriminator among the many studied.

Screening for trisomy 21 during the routine second-trimester ultrasound examination in an unselected Chilean population

OBJECTIVE

To evaluate the performance of a detailed ultrasound examination during the second trimester as a screening test for Down syndrome in an unselected Chilean population.

METHODS

This was part of an ongoing longitudinal study. Included were 3071 women with singleton pregnancies who underwent routine ultrasound examination between 21 + 0 and 25 + 6 gestational weeks as a screening test for chromosomal abnormalities and major congenital structural defects, and who were diagnosed as having trisomy 21 or being chromosomally normal. Maternal age, and eight soft markers and cardiac defects associated with Down syndrome were evaluated as a screening test using logistic regression analysis.

RESULTS

The incidence of Down syndrome was 0.6%, and the mean maternal age was 29.4 +/- 6.2 years. At least one of four soft markers (absent nasal bone, nuchal edema, short femur, echogenic foci) and/or cardiac defects was present in 77.8% of Down syndrome fetuses and in 3.1% of normal fetuses. Furthermore, with a false-positive rate of 1%, the detection rate using the combined model of ultrasound markers and maternal age was 72.2%.

CONCLUSIONS

Second-trimester ultrasound markers are able to detect over 70% of Down syndrome fetuses with only a 1% false-positive rate.

The second trimester genetic sonogram

The genetic sonogram, a fetal anatomic survey targeted at identifying features associated with aneuploidy, is carried out between 15 and 20 weeks' gestation. It has evolved as an adjunctive screening tool capable of further refining the individualized risk-calculation for trisomy that is based on maternal age or serum screening markers. The significance of a range of major structural anomalies and so-called "soft-markers" for trisomy, detected both in isolation and in combination, has been widely investigated. This review serves to describe the key components of the second trimester genetic sonogram and to illustrate how these markers are integrated into risk assessment for aneuploidy.

Dépistage de la trisomie 21 en France: le consensus du pire

Down syndrome screening has been based on second trimester maternal serum markers assay for many years. Another late strategy was based on the "genetic sonogram" performed in early second trimester in high-risk populations selected on maternal age or second trimester maternal serum markers. New strategies for Down syndrome screening have emerged over the last 10 years, with higher sensitivity and lower false-positive rates. First trimester ultrasound examination is a successful screening test; the sensitivity of nuchal translucency measurement is of 60 to 77% for a 5% false-positive rate. Combining nuchal translucency measurement with PAPP-A and free beta-hCG assay (first trimester combined screening) increases the sensitivity up to 82%. The most specific strategy is based on the integrated test, i.e., the integration of the quadruple test performed in second trimester (inhibine dimeric A, total beta-hCG, AFP, and uE3 assay) to the first trimester combined screening: for a 85% detection rate, the false-positive rate is estimated to 0.9%. However, it is ethical only with the patient agreement because it prevents access to the results of first trimester combined screening, and deprives the patient of an early diagnosis by CVS. Therefore, alternative strategies were proposed: step-wise sequential screening and contingent sequential screening. In the step-wise screening, karyotype is offered when the result of the combined test is beyond a specified threshold. If the combined test result is below this threshold, quadruple test is offered, and the final risk is calculated in the second trimester by integrating the results of the quadruple test with those of the combined test. Contingent screening also begins with the first trimester-combined test. According to its results, the patients are considered in one of the 3 following risk groups: high, intermediate, or low risk. An early karyotype is proposed to the high-risk group after combined testing. The low risk group is reassured and thus the quadruple test is not performed. The quadruple test is proposed to the intermediate risk group and final risk is calculated by the integration of the combined test result into the quadruple test result. The global detection rate of the step-wise or contingent sequential screening is estimated to 84% for a false-positive rate of 2%.

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.

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.

Correlation of second-trimester sonographic and biochemical markers

OBJECTIVES

To investigate correlations between sonographic soft markers and biochemical markers (human chorionic gonadotrophin, alpha-fetoprotein, and estriol) for Down syndrome in the second trimester of pregnancy.

METHODS

A total of 2183 women with apparently normal singleton fetuses who underwent second-trimester sonography (14-22 weeks) and maternal serum biochemical testing (triple test) were identified. Seven sonographic markers were recorded: nuchal fold thickness, humerus length, femur length, renal pyelectasis, hyperechoic bowel, echogenic intracardiac focus, and choroid plexus cysts.

RESULTS

Weak negative but statistically significant correlations were observed between human chorionic gonadotropin (multiples of the median) and both femur length (multiples of the median; Spearman p = -0.073; P < .01) and humerus length (multiples of the median; Spearman p = -0.083; P < .01). No other correlations significant at the 1% level were observed between femur length (multiples of the median) or humerus length (multiples of the median) and the biochemical markers. There were no significant correlations between nuchal fold thickness and any of the 3 biochemical markers. At the 5% (P < .05) level, the median human chorionic gonadotropin level (multiples of the median) was lower when an echogenic intracardiac focus was detected. Hyperechoic bowel also tended to be associated with higher median human chorionic gonadotropin (multiples of the median) and alpha-fetoprotein (multiples of the median) levels (P < .05).

CONCLUSIONS

We found that sonographic and biochemical markers for trisomy 21 are largely independent in unaffected pregnancies. For accurate risk estimation, correlations in both affected and unaffected pregnancies need to be considered. No or minimal correlation between sonographic markers and serum screening tests indicates that they can be used as independent modifiers of the maternal age-specific risk for Down syndrome.

Combined sonographic and biochemical markers for Down syndrome screening

OBJECTIVE

To evaluate the efficacy of fetal nuchal fold thickness and proximal long bone biometric measurements in modifying Down syndrome serum screening risk in a population of women referred for second-trimester sonography.

METHODS

Sonographic biometric measurements and biochemical markers were combined retrospectively for 2533 women with known pregnancy outcomes. Four different screening methods were compared: (1) advanced maternal age; (2) biochemical serum screening markers; (3) modification of serum screening risks on the basis of categorical cutoffs for nuchal fold and femur and humerus length; and (4) a combined approach in which the sonographic measurements were treated as multiples of the medians and entered, together with the serum screening results, into a multivariable algorithm. The efficacy was compared at second-trimester risk cutoffs of 1:270 and 1:100.

RESULTS

Down syndrome was present in 30 of the 2533 pregnancies (1 in 84). With the use of the 1:270 cutoff, biochemical screening had 93% sensitivity and a 40% false-positive rate. With application of the categorical method of fixed cutoffs to incorporate fetal biometry, the false-positive rate was reduced to 33% with no loss of sensitivity. The combined model had 83% sensitivity and a 19% false-positive rate. The combined method had the highest positive predictive value (1 in 20). Similar gains in efficacy could be shown with the 1:100 cutoff.

CONCLUSIONS

For this high-risk group, the multivariate model that combines serum screening and sonography can result in a substantial reduction in the number of amniocenteses. Although the addition of the sonographic biometric measurements resulted in some Down syndrome cases being missed, the net effect was a large improvement in the overall positive predictive value of the screening.

Combined serum and ultrasound screening for detection of fetal aneuploidy

Experience gathered over the last decade from high-risk centers provide strong evidence that mid-trimester sonographic markers are sensitive for Down syndrome prediction. More recent data indicate that combining mid trimester sonography with traditional serum markers significantly improves diagnostic accuracy over either group of markers by themselves.

The role of fetal echocardiography in genetic sonography

Genetic sonography identifies between 60% and 93% of fetuses with trisomy 21. One of the reasons for the variation in sensitivity is because of the under-detection of congenital heart defects. Although congenital heart defects are present in 56% of second trimester fetuses and 44% of newborns with trisomy 21, most studies evaluating second-trimester fetuses at risk for trisomy 21 detect less than 10% of heart malformations. This review discusses an approach that allows the fetal sonographer to incorporate fetal echocardiography, based upon the examiner's level of skill and experience, when evaluating the fetus at risk for trisomy 21. The cardiovascular examination consists of three levels. In the Level I examination only noncardiac markers are evaluated for a detection rate of 60% and false-positive rate of 5.9%. The Level II examination incorporates the four-chamber view with non-cardiac markers. If the examiner can identify atrial and/or ventricular chamber disproportion, then the sensitivity is increased to 75%, with a false-positive rate of 6.4%. The Level III examination utilizes grayscale and color Doppler ultrasound to evaluate the fetal heart. If the examiner can identify ventricular septal defects, atrioventricular septal defects, pericardial effusion, tricuspid regurgitation, and chamber disproportion, then the sensitivity of genetic sonography increases to 91% with a false-positive rate of 14%. This review includes Likelihood Ratios for each of the ultrasound markers so that the examiner can compute the risk for trisomy 21 for an individual patient.

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.

The genetic sonogram: a method of risk assessment for Down syndrome in the second trimester

OBJECTIVE

To determine the risk of Down syndrome in fetuses with sonographic markers using the Bayes theorem and likelihood ratios.

METHODS

We prospectively evaluated the midtrimester sonographic features of fetuses with Down syndrome and compared them with euploid fetuses. Patients were referred for an increased risk of aneuploidy and evaluated for the presence of structural defects, a nuchal fold, short long bones, pyelectasis, an echogenic intracardiac focus, and hyperechoic bowel. All fetuses underwent amniocentesis at the time of sonographic assessment. The sensitivity, specificity, and likelihood ratios for markers were calculated both as nonisolated and isolated findings.

RESULT

There were 164 fetuses with Down syndrome and 656 euploid fetuses. The presence of any marker resulted in sensitivity for the detection of Down syndrome of 80.5% with a false-positive rate of 12.4%. The absence of any markers conferred a likelihood ratio of 0.2, decreasing the risk of Down syndrome by 80%. As an isolated marker, the nuchal fold had an "infinite" likelihood ratio for Down syndrome; a short humerus had a likelihood ratio of 5.8, whereas structural anomalies had a likelihood ratio of 3.3. Other isolated markers had low likelihood ratios because of the higher prevalence in the unaffected population. The likelihood ratios for the presence of 1, 2, and 3 of any of the markers were 1.9, 6.2, and 80, respectively.

CONCLUSIONS

Although an isolated marker with a low likelihood ratio may not increase a patient's risk of Down syndrome, the presence of such a marker precludes reducing the risk of aneuploidy. Clusters of markers appear to confer a higher risk.

Advances in prenatal screening for Down syndrome: I. general principles and second trimester testing

BACKGROUND

Down syndrome is one of the most important causes of mental retardation in the population. In the absence of prenatal screening and diagnosis, prevalence at birth in the United States would currently exceed 1:600. The purpose of prenatal screening is to identify those women at the increased risk for an affected pregnancy and to maximize the options available to these women.

TESTS AVAILABLE

Second trimester serum screening involves combining the maternal age-specific risk for an affected pregnancy with the risks associated with the concentrations of maternal serum alpha-fetoprotein (MSAFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG) (triple testing). A forth analyte, inhibin-A (INH-A), is increasingly being utilized (quadruple testing). Optimal second trimester screening requires the integration of a number of clinical variables, the most important of which is an accurate assessment of gestational age. In addition to Down syndrome, the triple and quadruple tests preferentially identify fetal trisomy 18, Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith-Lemli-Opitz syndrome, and steroid sulfatase deficiency. Some programs modify the Down syndrome risks generated through maternal serum screening tests with fetal biometric data obtained by ultrasound. Other second trimester tests have shown promise, including the analysis of maternal urine and fetal cells in the maternal circulation, but none are in routine clinical use.

CONCLUSION

The second trimester triple and quadruple tests provide benchmarks for evaluating new screening protocols. The combination of fetal biometry, new test development as well as clarification of the role of co-factors that affect the concentrations of analytes in existing tests should lead to greater efficacy in second trimester screening for Down syndrome.

Pregnancy and birth after assisted reproduction

Pregnancy and birth after IVF has attracted much critical attention. Overall, pregnancy rates are about 30-40% with three transferred embryos. Abortion rates are high before 19 weeks of gestation, at 8% of established pregnancies, and ectopic pregnancies are regular occurrences. Pregnancy rates are highest (35%) in women aged 20-24 years, declining to 10% at and above age 40. The use of intracytoplasmic sperm injection (ICSI) and blastocyst transfer has transformed the establishment of pregnancies for couples with differing indications. High rates of implantation are achieved with blastocyst transfers, although many embryos die in vitro before this stage. Twins and triplets can result in up to 50% of pregnancies. Heterotopic pregnancies are rare. Abortion afflicts fetuses before 12 weeks (vanishing fetuses). Rates of spontaneous abortion can be as high as 50% with quadruplets and quintuplets, and may be even higher in cases of fetal aneuploidy. At birth, prematurity is high, especially with twins or higher multiple pregnancies. Fetal reduction is used to reduce multiple pregnancies for singletons. Ovarian hyperstimulation is a risk associated with multiple pregnancy.

The comprehensive midtrimester test: high-sensitivity Down syndrome test

OBJECTIVE

The purpose of this study was to develop a highly sensitive algorithm for midtrimester Down syndrome detection.

STUDY DESIGN

Urine (hyperglycosylated human chorionic gonadotropin, beta-core fragment of human chorionic gonadotropin), serum (alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol [uE(3)]), and ultrasound biometry (nuchal thickness, humerus length, the presence of gross ultrasonographic anomalies), and maternal age were measured at genetic amniocentesis. Stepwise logistic regression analysis was used to identify the most significant markers. A multivariate Gaussian algorithm plus age was used to derive patient-specific Down syndrome risk. Sensitivity and false-positive rates at different risk thresholds and the area under the receiver-operating characteristic curve were determined. A probability value of <.05 was significant.

RESULTS

There were 568 study cases with 17 Down syndrome cases (3.0%). The mean (+/-SD) maternal and gestational ages for the study group were 36.9 (+/-3.5) years and 16.2 (+/-1.4) weeks, respectively. The significant markers were nuchal thickness (P =.0001), hyperglycosylated human chorionic gonadotropin(P <.001), and beta-core fragment (P <.002). Neither maternal age nor gross sonographic anomaly contributed significantly to Down syndrome detection. The comprehensive midtrimester test was extremely efficient for Down syndrome detection in advanced maternal age only cases with a sensitivity of 92.3% at a 0.8% false-positive rate. In women <35 years old, all the Down syndrome cases were detected at 2.2% false positive rate. For the overall population, the sensitivity was 93.7% at 5% false-positive rate.

CONCLUSION

In a preliminary study, the comprehensive midtrimester test appeared highly sensitive in different age groups. Gross anomaly detection was not required for high performance, which makes the comprehensive midtrimester test potentially suitable for low-risk screening and as an alternative to amniocentesis in women who wish to avoid the procedure. This was a small study; thus, the clinical value of this test can only be established in large trials.

Second trimester ultrasound screening for chromosomal abnormalities

The use of prenatal ultrasound has proven efficacious for the prenatal diagnosis of chromosomal abnormalities. The first sonographic sign of Down syndrome, the thickened nuchal fold, was first described in 1985. Since that time, multiple sonographically-identified markers have been described as associated with Down syndrome. The genetic sonogram, involving a detailed search for sonographic signs of aneuploidy, can be used to both identify fetuses at high risk for aneuploidy and, when normal, can be used to decrease the risk for aneuploidy for a pregnancy when no sonographic markers are identified. Combining the genetic sonogram with maternal serum screening may be the best method of assessing aneuploidy risk for women who desire such an assessment in the second trimester. Trisomy 18, Trisomy 13, and triploidy are typically associated with sonographically identified abnormalities and have a high prenatal detection rate. The use of the described sonographic signs in low-risk women requires further investigation, however, patients at increased risk for aneuploidy due to advanced maternal age or abnormal serum screening can benefit from a genetic sonogram screening for sonographic signs of aneuploidy to adjust their baseline risk of an affected fetus.

Value of humerus length shortening for prenatal detection of Down syndrome in a Thai population

OBJECTIVE

To assess the value of humerus length shortening for prenatal detection of Down syndrome in a Thai population.

METHODS

A prospective study was performed on 3053 women undergoing second-trimester amniocentesis, between 16 and 24 weeks gestation, for the indications of advanced maternal age and a past history of chromosomal abnormality. Biparietal diameter (BPD) and humerus length measurements were obtained before the procedures. Regression equations relating BPD to humerus length were used to calculate observed humerus length/expected humerus length ratio in chromosomally normal and Down syndrome fetuses. Sensitivity, specificity, false-positive rate and likelihood ratio of a positive test result at various observed humerus length/expected humerus length ratios for detection of Down syndrome were calculated. A receiver-operator characteristic curve was used to determine the threshold screening ratio.

RESULTS

There were 3003 chromosomally normal pregnancies and 24 fetuses with Down syndrome. The relationship between humerus length and BPD was: expected humerus length = 0.7403BPD - 5.1057, R2= 0.77, P < 0.001. Humerus length in Down syndrome fetuses was significantly shorter than in normal fetuses (P < 0.001). A ratio of 0.91 for observed humerus length/expected humerus length yielded a sensitivity of 41.7%, specificity of 88.3%, a false-positive rate of 11.7% and likelihood ratio of a positive test result of 3.63 (95% confidence interval 2.24-5.88) for detection of Down syndrome.

CONCLUSIONS

Humerus length shortening in the second trimester appears to be a useful adjunctive screening parameter for fetal Down syndrome in a Thai population.

[What's new in fetal medicine?]

One of the major progress in fetal medicine in recent years is the increased sensitivity of sonographic screening for foetal malformations, due to technical improvement but also to a better training of professionals. Screening for chromosomal abnormalities is no longer based on maternal age alone. Second trimester maternal serum screening (MSS) is increasingly used: thus in 1997, 376,798 MSS tests were performed in France, yielding to the prenatal diagnosis of 391 cases of Down's syndrome. First trimester sonographic nuchal translucency measurement (NTM) is an effective screening method when performed under stringent conditions. Quality control however, is more difficult to implement on a large scale for NTM than for MSS. Performing screening tests sequentially carries a danger of generating an unnecessarily high number of amniocentesis, which may be obviated by a rational calculation of an individual's risk to carry an aneuploid baby. First trimester MSS is expected to become standard practice in the next years, probably in combination with NTM. Cytogenetics underwent substantial innovations recently, due to the ever-increasing use of molecular cytogenetics. FISH techniques allow: 1) precise analysis of unexpected structural chromosomal abnormalities diagnosed by routine amniocentesis, 2) rapid screening of the most common aneuploidies by amniocentesis when a fetal structural anomaly is detected by 3rd trimester ultrasound, 3) diagnosis of micro-deletions suspected by fetal ultrasound or post-mortem. Prenatal diagnosis by maternal blood sampling and fetal cells or DNA analysis is now part of routine clinical practice in selected cases, such as fetal sexing in families affected by an X linked disease. Thus one can select those pregnancies eligible to invasive prenatal diagnosis. Pre implantation diagnosis, which has not been legal in France until 1999 is now increasingly used as an alternative to first trimester diagnosis. As for fetal therapy, a major recent breakthrough is the prenatal management of twin to twin transfusion syndrome by either amnioreduction or laser coagulation of inter-twin vascular shunts. In addition, new pathophysiologic concepts involving the renin angiotestin system could lead to further therapeutic innovations. A European randomised trial is now being completed to establish the respective indications of drainage and Laser. All this underscores that fetal medicine is no longer solely a succession of dramatic technical breakthroughs, but is entered an era of large-scale diffusion that requires evidence based evaluation.

[Diagnosis of trisomy 21: nuchal translucency and/or serum markers?]

OBJECTIVES

To compare alternative methods of antenatal screening for Down's syndrome (nuchal translucency measurement and second trimester maternal serum screening) in a low-risk population and to evaluate the consequences of a sequential estimation of risk.

METHODS

In a consecutive series of 4308 women aged less than 38 with a singleton pregnancy, we examined the detection rate of nuchal translucency (NT) measurement at 10-14 weeks and maternal serum screening (MSS) by human chorionic gonadotropin and alpha-feto-protein at 14-18 weeks. Women with a NT measurement = 3 mm and women with a MSS derived risk = 1/250 were recommended to have an amniocentesis. A second trimester detailed ultrasound scan was also performed in all women. The outcome of all pregnancies was entered in a computerized database and the detection rate and false-positive rate of different screening strategies were analysed.

RESULTS

Of the 4308 pregnancies that were followed (mean maternal age 30.1 years), there were 12 cases of Down's syndrome (0.28%), all detected prenatally. Seven of twelve cases had a NT measurement above 3 mm (58%), and 6 out of ten cases with available MSS had a calculated risk = 1/250 (60%). Four of the five cases with NT measurement below 3 mm were detected by subsequent MSS. At a threshold giving 5% of positive tests, the sensitivity of NT screening and MSS were 75% and 60%, respectively.

DISCUSSION AND CONCLUSION

Sequential screening for Down's syndrome by nuchal translucency and second trimester biochemistry is effective and appears to increase the detection rate compared to the use of any single test. However, this strategy is likely to raise the false-positive rate and the interpretation of MSS derived risk should be combined to the first trimester NT measurement.

Screening for Down syndrome using first-trimester ultrasound and second-trimester maternal serum markers in a low-risk population: a prospective longitudinal study

OBJECTIVES

To compare nuchal translucency and second-trimester maternal serum measurements as alternative methods of antenatal screening for Down syndrome in a low-risk population and to evaluate the consequence of combining the results in the estimation of risk.

DESIGN

In a consecutive series of 4130 women aged less than 38 years with a singleton pregnancy, we examined both the detection rate of Down syndrome by nuchal translucency measurement at 10-14 weeks and maternal serum screening by human chorionic gonadotrophin and alpha-fetoprotein at 14-18 weeks. Women with a nuchal translucency measurement of > or = 3 mm and women with a maternal serum screening-derived risk > or = 1/250 were recommended to have amniocentesis. A second-trimester detailed ultrasound scan was also performed in all women. The outcome of all pregnancies was recorded prospectively and the detection rate and false-positive rate of different screening strategies were retrospectively analyzed.

RESULTS

Out of the 4130 pregnancies that were followed (mean maternal age, 30.1 years), 12 cases of Down syndrome were observed (0.28%), all detected prenatally. Seven of 12 cases had a nuchal translucency measurement of > or = 3 mm (58%), and six out of 10 cases with available maternal serum screening had a calculated risk of > or = 1/250 (60%). Four of the five Down syndrome cases with a nuchal translucency measurement of < 3 mm were detected by subsequent maternal serum screening. At a threshold giving 5% of positive tests, the sensitivity of nuchal translucency, maternal serum screening and combined risk screening were 75%, 60% and 90%, respectively.

CONCLUSIONS

In screening for Down syndrome, an approach which combines the results from first-trimester nuchal translucency and second-trimester biochemistry is effective and increases the detection rate compared to the use of any single test. However, this strategy is likely to raise the false-positive rate and the interpretation of maternal serum screening-derived risk should be combined with the first-trimester nuchal translucency measurement.

Maternal serum screening for Down's syndrome, open neural tube defects and trisomy 18

Maternal serum screening identifies women at an increased risk of a pregnancy with Down's syndrome or trisomy 18 or an open neural tube defect. The triple test, consisting of maternal serum alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin was carried out by a chemiluminescence immunoassay method in our laboratory. The study consisted of 373 pregnant women. The gestational range for the study group was 14-22 weeks. The mean maternal age for the study group was 28.53 +/- 5.46 years (range 17.4 to 43.5 years); 9.1% of the women were considered at high risk for Down's syndrome based on the test results. In our study the detection rate for Down's syndrome by prenatal karyotyping was 66.6%. Maternal serum screening allows reduction of the number of women requiring amniocentesis without a significant decrease in the detection rate.

Sonographic markers of fetal aneuploidy

A variety of ultrasound findings can be identified in fetuses with fetal aneuploidy. Typical findings vary with both the chromosome abnormality and gestational age at time of the ultrasound examination. Increased NT is the primary marker during the first trimester, whereas a variety of markers may be seen during the second trimester. The presence of ultrasound markers increases the risk for fetal aneuploidy, whereas a normal ultrasound reduces the risk. Optimal risk assessment includes consideration of other risk factors including maternal age, family history, and biochemical markers. It is expected that combined risks, incorporating ultrasound findings and biochemistry, will be available in the near future. How first-trimester screening is integrated with second-trimester screening remains to be determined.

Comparison of urinary hyperglycosylated human chorionic gonadotropin concentration with the serum triple screen for Down syndrome detection in high-risk pregnancies

OBJECTIVE

Both modest screening performance and declining patient and physician acceptance have stimulated interest in alternative markers to the triple screen for the detection of Down syndrome. Our purpose was to compare the concentration of a single urinary analyte, hyperglycosylated human chorionic gonadotropin, with the serum triple screen (alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol concentrations combined with age) for second-trimester Down syndrome detection.

STUDY DESIGN

Urine and blood were obtained from pregnant women in the second trimester undergoing genetic amniocentesis. Urinary hyperglycosylated human chorionic gonadotropin concentration and serum triple-screen values were measured. Individuals undergoing amniocentesis because of abnormal triple-screen results were excluded. Individual Down syndrome risks on the basis of urinary hyperglycosylated human chorionic gonadotropin concentration plus maternal age and on the basis of the triple-screen results were calculated. For each algorithm the sensitivity and false-positive rate for Down syndrome detection at different risk thresholds were determined. From these values receiver operating characteristic curves were constructed, and the area under the curve was determined for each algorithm. Finally, the performance of a new combination in which urinary hyperglycosylated human chorionic gonadotropin concentration replaced serum human chorionic gonadotropin concentration in the triple screen was ascertained.

RESULTS

We studied 24 pregnancies complicated by Down syndrome and 500 unaffected pregnancies between 14 and 22 weeks' gestation in a mostly white (93.5%) population undergoing amniocentesis primarily because of advanced maternal age. The sensitivity and false-positive rate for urinary hyperglycosylated human chorionic gonadotropin concentration were 75. 0% and 5.6%, respectively, whereas those for the triple screen were 75.0% and 33.2%, respectively. Urinary hyperglycosylated human chorionic gonadotropin concentration was superior to the triple screen (area under the curve, 0.9337 vs 0.7887; P =.02). The substitution of urinary hyperglycosylated human chorionic gonadotropin concentration for serum human chorionic gonadotropin concentration in the triple screen resulted in a 91.7% sensitivity at a 10.0% false-positive rate, versus a 54.2% sensitivity for the traditional triple screen at the same false-positive rate.

CONCLUSION

The performance of urinary hyperglycosylated human chorionic gonadotropin concentration was statistically superior to that of the serum triple screen in a high-risk population. The use of urinary hyperglycosylated human chorionic gonadotropin concentration as an alternative test or substitution of this measurement for serum human chorionic gonadotropin concentration in the triple screen would improve diagnostic accuracy and address many current concerns related to the triple screen.

Changing trends in patient decisions concerning genetic amniocentesis

OBJECTIVE

This study was undertaken to determine whether there was a change in patient decisions concerning genetic amniocentesis during the period 1995-1998.

STUDY DESIGN

All patients referred for genetic counseling because of advanced maternal age, abnormal serum triple-screen results, or ultrasonographic abnormalities between January and March 1995 and between January and March 1998 were evaluated through a retrospective chart review. Patient characteristics included age, race, and gestational age. Group 1 consisted of patients from 1995. Group 2 consisted of patients from 1998. Data on patient decisions concerning amniocentesis before and after genetic counseling and ultrasonographic examination were compared in each group. Groups 1 and 2 were then compared with respect to decisions before and after genetic counseling and ultrasonographic evaluation.

RESULTS

A total of 112 patients were studied. Group 1 consisted of 53 patients and group 2 consisted of 59 patients. When the groups were compared, no differences in age, race, or gestational age were noted. In group 1, before counseling, 18 of 53 patients desired genetic testing, compared with 44 of 53 after counseling (P =.02). In group 2, before counseling, 4 of 59 patients desired genetic testing, compared with 15 of 59 after counseling (P =.01). A significantly greater number of patients in group 1 than in group 2 desired genetic testing both before counseling (n = 18/53 vs n = 4/59; P =.01) and after counseling (n = 44/53 vs n = 15/59; P =.01).

CONCLUSION

Fewer patients at risk for Down syndrome in 1998 than in 1995 desired amniocentesis both before and after genetic counseling and ultrasonographic examination.

Prenatal ultrasound findings in affected and unaffected pregnancies that are screen-positive for trisomy 18: the California experience

This study investigated whether significant differences in ultrasound findings exist between trisomy 18 affected and unaffected pregnancies positive by serum screening. Ultrasound reports were reviewed for 335 screen-positive women. This represented 65% of all trisomy 18 screen-positive patients who had follow-up services at any of 117 Californian state-approved Prenatal Diagnosis Centers during a six-month period from November 1, 1995 to April 30, 1996. Ultrasound reports were available for 100% of trisomy 18 fetuses diagnosed during the six month period (n=23). Ultrasound findings were reported as normal in 35% of the fetuses affected with trisomy 18. The number and type of abnormalities observed in the affected and unaffected groups are described. When compared to unaffected cases, the trisomy 18 affected fetuses had a greater re-dating discrepancy on follow-up ultrasound evaluation and significantly lower femur length to biparietal diameter (FL/BPD) ratio measurements. We recommend that all women who are screen positive for trisomy 18 be encouraged to have amniocentesis, regardless of ultrasound findings, since affected fetuses may not be detected otherwise.

Combined ultrasound biometry, serum markers and age for Down syndrome risk estimation

OBJECTIVE

To compare Down syndrome screening efficiency of the standard serum triple analyte screen to that of a four-component screen consisting of ultrasound biometry and serum markers in the second trimester.

METHODS

The Down syndrome screening efficiency of the triple screen, i.e. alpha-fetoprotein (AFP), unconjugated estriol (E3), hCG and maternal age, was compared with the four-marker algorithm, i.e. humerus length, nuchal thickness, AFP and hCG plus maternal age. A quadrivariate Gaussian algorithm was used to calculate individual Down syndrome odds. Receiver operating characteristic (ROC) curves plotting sensitivity against false-positive rate were constructed for each algorithm and the areas under the curves were compared to determine which was superior. Sensitivity and false-positive rates at different Down syndrome risk thresholds were also compared.

RESULTS

There were 46 cases of Down syndrome (1.9%) with 2391 normal singleton pregnancies in a referral population in which triple screen, fetal biometry and karyotype had been done. The gestational age range for the study was 14-24 completed weeks. The median maternal age for the study group was 35.0 years (14.0-46.0 years). The areas (SE) under the ROC curves were 0.75(0.04) and 0.93(0.02) for the standard triple and the four-marker screen, respectively (P < 0.001). At a 10% false-positive rate, detection was 45.7% for the triple and 80.4% for the four-marker screen.

CONCLUSIONS

A new algorithm combining humerus length and nuchal thickness measurement with serum AFP, hCG and maternal age substantially improved Down syndrome screening efficiency compared with the traditional triple screen. The model appears promising and should be evaluated in an independent data set.

Critical appraisal of the use of nuchal fold thickness measurements for the prediction of Down syndrome

OBJECTIVE

Nuchal fold thickness is the best ultrasonographic predictor of fetal trisomy 21. However, the risk assigned on the basis of the commonly used threshold of nuchal fold thickness >/=6 mm does not take into consideration the significant associations between nuchal fold thickness and gestational age and between maternal age and Down syndrome. We propose a new method of calculating Down syndrome probability that takes into account both gestational age at examination and previously assessed probability of Down syndrome.

STUDY DESIGN

Nuchal fold thickness was measured at ultrasonographic examination at 14 to 22 weeks' gestation without previous knowledge of the fetal karyotype. Nuchal cystic hygromas were excluded from analysis. Statistical analyses included correlation, logistic regression to control for other ultrasonographic predictors of trisomy 21 and for maternal age, receiver operating characteristic curve, and likelihood ratios (defined as the ratio of the sensitivity to the false-positive rate). P <.05 was considered significant.

RESULTS

Mean gestational age at ultrasonography was 16.9 weeks' gestation (range, 14-22 weeks' gestation). Mean (+/-SD) nuchal fold thickness in fetuses with trisomy 21 (4.7 +/- 1.6 mm; n = 29) was greater than in euploid fetuses (3.2 +/- 0.9; n = 780; P <.001). Logistic regression analysis established that nuchal fold thickness was a significant predictor of trisomy 21 independent both of the other ultrasonographic markers and of maternal age (P <.001). Regression analysis showed that nuchal fold thickness was significantly correlated with gestational age among both fetuses with trisomy 21 and euploid fetuses and that the regression line of fetuses with trisomy 21 had a slope similar to that of euploid fetuses. The difference between observed and expected nuchal fold thicknesses on the basis of the biparietal diameter (as a function of gestational age) was used to obviate the confounding effect of gestational age. Differences between observed and expected nuchal fold thicknesses were then used to calculate likelihood ratios. These likelihood ratios could then be multiplied by the individual prior probability to obtain a patient-specific Down syndrome probability.

CONCLUSION

Nuchal fold thickness is correlated with gestational age in both euploid fetuses and fetuses with Down syndrome. Use of the difference between observed and expected nuchal fold thicknesses to determine likelihood ratios allows the calculation of individual posterior probabilities of Down syndrome that take into consideration both gestational age and maternal age.