Comparison of humerus length with femur length in fetuses with Down syndrome

Role of Sonographic Second Trimester Soft Markers in the Era of Cell-Free DNA Screening Options: A Review

Soft markers are sonographic structural, nonspecific signs with little pathological significance, often transient, usually considered as normal variants. However, they may also be associated with chromosomal abnormalities. The most widely examined soft markers include absent or hypoplastic nasal bone (NB), intracardiac echogenic focus (IEF), ventriculomegaly (VM), thickened nuchal fold (NF), choroid plexus cyst (CPC), echogenic bowel, short long bones, and urinary tract dilation (UTD). Although the use of noninvasive prenatal testing (NIPT) has been spreading quickly in maternal–fetal medicine, it is not a diagnostic test and it still remains unavailable or cost-prohibitive for most of the population in many countries. After normal screening test results in the first trimester, there is no uniform consensus regarding the clinical significance of isolated soft markers for aneuploidy. Nowadays, the search for soft markers in an ultrasound is still part of clinical evaluation, and the interpretation of these findings is often a matter of debate. In the present review, we summarize the recent literature about the role of soft markers in the era of NIPT and propose an overview of the different clinical guidelines.

Ultrasound screening: Status of markers and efficacy of screening for structural abnormalities

Aneuploidy is a major cause of perinatal morbidity and mortality and can have a significant impact on expecting parents and their families. With early screening and diagnosis it is important to be able to educate parents regarding the potential impact of the diagnosis. This knowledge allows parents the opportunity to consider management options early in the pregnancy, permitting more time to mentally and emotionally prepare both for the course of the pregnancy, and after the birth of the child should the pregnancy continue. Prenatal screening provides pregnant women a non-invasive risk assessment for the most common aneuploidies. Those who are considered "high-risk" then have the option for additional diagnostic (invasive) testing. Prior to the 1980s, prenatal screening consisted of risk assessment through maternal age; however, with the advent of maternal serum biochemical analysis and ultrasound, the field of prenatal screening developed significantly. As biochemical and sonographic advances continued into the 1990s, the emphasis shifted to risk assessment in the first trimester, with the combination of maternal serum analytes and sonographic evaluation of the nuchal translucency.(1) Within the last decade, the introduction of non-invasive screening (NIPT/S) has shown great impact on the expansion and evolving practice of prenatal screening. Although in many places the standard for prenatal testing continues to include maternal serum analytes and sonographic evaluation, the role of each marker alone and in combination remains important. In the era of increasingly available screening tests, especially with NIPT/(NIPS), this article attempts to review the current role of ultrasound in prenatal care and elucidate the role of ultrasound markers in prenatal screening.

Methods for the joint meta-analysis of multiple tests

Existing methods for meta-analysis of diagnostic test accuracy focus primarily on a single index test. We propose models for the joint meta-analysis of studies comparing multiple index tests on the same participants in paired designs. These models respect the grouping of data by studies, account for the within-study correlation between the tests' true-positive rates (TPRs) and between their false-positive rates (FPRs) (induced because tests are applied to the same participants), and allow for between-study correlations between TPRs and FPRs (such as those induced by threshold effects). We estimate models in the Bayesian setting. We demonstrate using a meta-analysis of screening for Down syndrome with two tests: shortened humerus (arm bone), and shortened femur (thigh bone). Separate and joint meta-analyses yielded similar TPR and FPR estimates. For example, the summary TPR for a shortened humerus was 35.3% (95% credible interval (CrI): 26.9, 41.8%) versus 37.9% (27.7, 50.3%) with joint versus separate meta-analysis. Joint meta-analysis is more efficient when calculating comparative accuracy: the difference in the summary TPRs was 0.0% (-8.9, 9.5%; TPR higher for shortened humerus) with joint versus 2.6% (-14.7, 19.8%) with separate meta-analyses. Simulation and empirical analyses are needed to refine the role of the proposed methodology.

Do race-specific definitions of short long bones improve the detection of down syndrome on second-trimester genetic sonograms?

OBJECTIVE

The purpose of this study was to determine whether the use of race-specific definitions of short femur and humerus lengths improves Down syndrome detection.

METHODS

This was a retrospective cohort study over 16 years. For each self-reported maternal race (white, African American, Hispanic, and Asian), we evaluated the efficiency of Down syndrome detection using published race-specific formulas compared with a standard formula for short femur and humerus lengths (observed versus expected lengths < or =0.91 and < or =0.89, respectively). The sensitivity, specificity, and 95% confidence intervals for each parameter were compared. Screening performance was compared by areas under the receiver operating characteristic curves.

RESULTS

Of 58,710 women, 209 (0.3%) had a diagnosis of a fetus with Down syndrome. Although the race-based formula increased sensitivity in each population, the increase was statistically significant only in the white population, whereas a decrease in specificity was statistically significant in all 4 populations, as denoted by nonoverlapping confidence intervals. The area under the receiver operating characteristic curve for the model using the race-specific definition of short femur length was 0.67 versus 0.65 compared with the standard definition, and for humerus length it was 0.70 versus 0.71.

CONCLUSIONS

The use of race-based formulas for the determination of short femur and humerus lengths did not significantly improve the detection rates for Down syndrome.

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.

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.

[Role of "subtle" ultrasonographic signs during antenatal screening for trisomy 21 during the second trimester of pregnancy: meta-analysis and CPDPN protocol of the Grenoble University Hospital]

OBJECTIVE

A meta-analysis about subtle ultrasonographic signs in second trimester of pregnancy.

MATERIALS AND METHODS

196 articles dealing with the subject--from 1985 to July 2002--were studied. Data on the 11 reported signs were collected from 92 theoretically and/or statistically valid studies. Then, the studies were selected according to several criteria: isolated characteristic, defined thresholds, calculable sensitivity and specificity. After checking for homogeneity, a likelihood ratio was calculated for some of the signs.

RESULTS

This meta-analysis of the second trimester ultrasonographic signs of Down's syndrome enabled us to estimate the likelihood ratio (LHR) of six signs. At 22 weeks'gestation (WG) these signs are: pyelectasis equal to or greater than 5 mm; nuchal fold thickness equal to or greater than 6 mm; persistence of choroid plexus cysts; shortness of the femur and humerus below the tenth percentile; hyperechogenic bowe; and nasal bone length less than 2.5 mm.

CONCLUSION

These validated ultrasonographic signs are independent of nuchal translucency thickness at 12 WG and of maternal serum biochemistry. This allows to calculate a combinate risk for nuchal translucency, maternal serum biochemistry and second trimester ultrasonographic signs when they are validated.

Femur and humerus length in trisomy 21 fetuses at 11-14 weeks of gestation

OBJECTIVE

To determine the value of measuring fetal femur and humerus length at 11-14 weeks of gestation in screening for chromosomal defects.

METHODS

Femur and humerus lengths were measured using transabdominal ultrasound in 1018 fetuses immediately before chorionic villus sampling for karyotyping at 11-14 weeks of gestation. In the group of chromosomally normal fetuses, regression analysis was used to determine the association between long bone length and crown-rump length (CRL). Femur and humerus lengths in fetuses with trisomy 21 were compared with those of normal fetuses.

RESULTS

The median gestation was 12 (range, 11-14) weeks. The karyotype was normal in 920 fetuses and abnormal in 98, including 65 cases of trisomy 21. In the chromosomally normal group the fetal femur and humerus lengths increased significantly with CRL (femur length = - 6.330 + 0.215 x CRL in mm, r = 0.874, P < 0.0001; humerus length = - 6.240 + 0.220 x CRL in mm, r = 0.871, P < 0.0001). In the Bland-Altman plot the mean difference between paired measurements of femur length was 0.21 mm (95% limits of agreement - 0.52 to 0.48 mm) and of humerus length was 0.23 mm (95% limits of agreement - 0.57 to 0.55 mm). In the trisomy 21 fetuses the median femur and humerus lengths were significantly below the appropriate normal mean for CRL by 0.4 and 0.3 mm, respectively (P = 0.002), but they were below the respective 5th centile of the normal range in only six (9.2%) and three (4.6%) of the cases, respectively.

CONCLUSION

At 11-14 weeks of gestation the femur and humerus lengths in trisomy 21 fetuses are significantly reduced but the degree of deviation from normal is too small for these measurements to be useful in screening for trisomy 21.

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.

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.

Second trimester markers of aneuploidy

The risks of aneuploidy associated with identification of a sonographic marker in the low risk population is controversial. Prior risk estimates have been derived usually from high risk populations. Screening programmes in the first trimester, second trimester and combined first and second trimester will undoubtedly alter the second trimester scan as a screening tool for aneuploidy. This chapter reviews the current sonographic markers and the difficulties in their application to the general population.

Femur length and trisomy 21: impact of gestational age on screening efficiency

OBJECTIVE

This study assesses two methods used to define relatively short femur in screening for trisomy 21 and examines changes in performance of screening with gestational age.

DESIGN

Retrospective analysis of data on menstrual age, femur length (FL) and biparietal diameter (BPD) in 49 trisomy 21 pregnancies and 6069 normal controls. Reference ranges were derived for BPD/FL versus menstrual age and for FL versus BPD. Two methods of defining short femur (BPD/FL and observed-to-expected FL ratio) were examined for false-positive rates and detection rates for trisomy 21 at different gestational ages.

RESULTS

In the control group the BPD/FL ratio and its standard deviation decreased with menstrual age. Trisomy 21 was associated with a significantly higher BPD/FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). Eleven percent of 28 fetuses examined at 15-17 weeks had a BPD/FL above the 95th centile compared with 24% of 21 fetuses examined at 18-20 weeks (P = 0.40). The median observed-to-expected FL ratio in the control group was 1.0 throughout the gestational age range but the standard deviation decreased significantly with menstrual age (P < 0.01). Trisomy 21 was associated with a significantly reduced observed-to-expected FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). A fixed cut-off of 0.91 for observed-to-expected FL ratio provided a false-positive rate of 12% at 15-17 weeks compared with 6% at 18-20 weeks of gestation (P < 0.001) with detection rates of 29 and 38%, respectively (P = 0.73).

CONCLUSION

Irrespective of the definition used to define the condition, relatively short femur is a poor marker for trisomy 21 particularly when the assessment takes place before 18 weeks of gestation.

Noninvasive means of identifying fetuses with possible Down syndrome: a review

Women who are 35 years or older are offered invasive prenatal testing because of the increased risk of chromosomal abnormalities, especially Down syndrome. In an attempt to increase the number of Down syndrome fetuses being detected and decrease the number of invasive procedures being performed on pregnancies not affected with a chromosome abnormality, both biochemical and ultrasound screening methods are being studied and are summarized in this article. The ultrasound markers reviewed include increased nuchal thickness, increased nuchal lucency, shortened femur, shortened humerus, pyelectasis, hypoplastic ears, echogenic intracardiac focus, hypoplasia of the fifth middle phalanx, and echogenic bowel.

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

OBJECTIVE

We compared the Down syndrome screening efficiency of a new algorithm that combines humerus length measurement and serum analytes versus that of the traditional triple-analyte serum screen.

STUDY DESIGN

Humerus length measurements were obtained prospectively in 1743 midtrimester (14 to 24 weeks) singleton fetuses before genetic amniocentesis. All patients had triple-marker serum screening before amniocentesis. Data on humerus length were expressed as multiples of the median, and were normalized by log transformation. Backward multiple stepwise logistic regression analysis was performed to determine which combination of biometry and serum markers best predicted fetal Down syndrome. The screening efficiency of the traditional triple-analyte algorithm was compared with that of a new multivariate gaussian algorithm that combined biometry and serum markers.

RESULTS

There were 31 (1.8%) fetuses with Down syndrome in the study population. In the regression analysis humerus length, human chorionic gonadotropin, alpha-fetoprotein, and maternal age were significant predictors of Down syndrome, but unconjugated estriol was not. The combined algorithm (humerus length, human chorionic gonadotropin, and alpha-fetoprotein and age) was superior to the traditional triple screen for Down syndrome detection. The sensitivities at fixed false-positive rates were consistently higher in the combination than in the triple-screen protocol. For example, at a 10% false-positive rate the sensitivities were 65.0% and 52.3%, respectively. Similarly, at a 15% false-positive rate the sensitivities were 73.5% and 55.0%, respectively.

CONCLUSION

A new screening algorithm combining humerus length and serum analytes was superior to the traditional triple screen. Although we used a high-risk population in this study, it is expected that the observed superiority of the combination screen would persist in a population of younger women. The development of a combined biometric and serum analyte screening algorithm for estimating individual odds could represent an advance in prenatal Down syndrome screening.

Screening for the aneuploid fetus

Advances in ultrasound technology have dramatically improved the detection of fetal defects. Although only an invasive test can provide a diagnosis, the incorporation of sonography into current biochemically based screening programs should significantly improve the detection of a host of other physically based fetal abnormalities. This article provides an overview and discussion of the prenatal sonographic features that may suggest the presence of a significant chromosomal abnormality.

Down's syndrome: altered chondrogenesis in fetal rib

Down's syndrome (DS), a human genetic abnormality usually caused by an extra chromosome 21, presents a wide range of major and minor anomalies, the most significant of which are mental retardation and congenital heart defects. The anomalous phenotype also includes short stature and neck, thin calvaria, and cartilage hypoplasia. The genesis of these skeletal features is unknown. Histopathologic sections of fetal cartilage from skull, vertebra, rib, and femur were studied in 16 fetuses with DS (17-22 wk old) and 13 control non-DS fetuses (19-22 wk old) with other pathologies not directly affecting skeletal growth. Rib growth cartilage morphology showed a previously unreported structural anomaly in DS, an increase in the hypertrophic portion with a concomitant decrease in proliferative and resting zones. The hypertrophic chondrocytic zone was markedly increased in DS compared with non-DS (149 +/- 68 microm versus 36 +/- 20 microm, and 26 +/- 12 versus 7 +/- 3 expressed in percent of the total length; p < 0.0001), whereas the proliferative zone (114 +/- 58 microm versus 165 +/- 43 microm, 20 +/- 10 versus 33 +/- 4 in percent of the total length; p < 0.001) and the resting zone (53 +/- 4 versus 59 +/- 6 in %, p < 0.009) were decreased. These features were not found in the femoral epiphyseal growth plate or in cartilage from vertebra and skull. Our results demonstrate an imbalance toward the hypertrophic phenotype. This abnormality, found in DS fetuses 17-22 wk old, may represent an early manifestation of an abnormal growth cartilage maturation pattern, which manifests postnatally in long bones, leading to diminished growth rates.

Antenatal screening for Down's syndrome

BACKGROUND

In 1968 the first antenatal diagnosis of Down's syndrome was made and screening on the basis of selecting women of advanced maternal age for amniocentesis was gradually introduced into medical practice. In 1983 it was shown that low maternal serum alpha fetoprotein (AFP) was associated with Down's syndrome. Later, raised maternal serum human chorionic gonadotrophin (hCG), and low unconjugated oestriol (uE3) were found to be markers of Down's syndrome. In 1988 the three biochemical markers were used together with maternal age as a method of screening, and this has been widely adopted. PRINCIPLES OF ANTENATAL SCREENING FOR DOWN'S SYNDROME: Methods of screening need to be fully evaluated before being introduced into routine clinical practice. This included choosing markers for which there is sufficient scientific evidence of efficacy, quantifying performance in terms of detection and false positive rates, and establishing methods of monitoring performance. Screening needs to be provided as an integrated service, coordinating and managing the separate aspects of the screening process. SERUM MARKERS AT 15-22 WEEKS OF PREGNANCY: A large number of serum markers have been found to be associated with Down's syndrome between 15 and 22 weeks of pregnancy. The principal markers are AFP, hCG or its individual subunits (free alpha- and free beta-hCG), uE3, and inhibin A. Screening performance varies according to the choice of markers used and whether ultrasound is used to estimate gestational age (table 1). When an ultrasound scan is used to estimate gestational age the detection rate for a 5% false positive rate is estimated to be 59% using the double test (AFP and hCG), 69% using the triple test (AFP, hCG, uE3), and 76% using the quadruple test (AFP, hCG, uE3, inhibin A), all in combination with maternal age. Other factors that can usefully be taken into account in screening are maternal weight, the presence of insulin dependent diabetes mellitus, multiple pregnancy, ethnic origin, previous Down's syndrome pregnancy, and whether the test is the first one in a pregnancy or a repeat. Factors such as parity and smoking are associated with one or more of the serum markers, but the effect is too small to justify adjusting for these factors in interpreting a screening test.

URINARY MARKERS AND FETAL CELLS IN MATERNAL BLOOD

Urinary beta-core hCG has been investigated in a number of studies and shown to be raised in pregnancies with Down's syndrome. This area is currently the subject of active research and the use of urine in future screening programmes may be a practical possibility. Other urinary markers, such as total oestriol and free beta-hCG may also be of value. Fetal cells can be identified in the maternal circulation and techniques such as fluorescent in situ hybridisation can be used to identify aneuploidies, including Down's syndrome and trisomy 18. This approach may, in the future, be of value in screening or diagnosis. Currently, the techniques available do not have the performance, simplicity, or economy needed to replace existing methods.

DEMONSTRATION PROJECTS

Demonstration projects are valuable in determining the feasibility of screening and in refining the practical application of screening. They are of less value in determining the performance of different screening methods. Several demonstration projects have been conducted using the triple and double tests. In general, the uptake of screening was about 80%. The screen positive rates were about 5-6%. About 80% of women with positive screening results had an invasive diagnostic test, and of those found to have a pregnancy with Down's syndrome, about 90% chose to have a termination of pregnancy. ULTRASOUND MARKERS AT 15-22 WEEKS OF PREGNANCY: There are a number of ultrasound markers of Down's syndrome at 15-22 weeks, including nuchal fold thickness, cardiac abnormalities, duodenal atresia, femur length, humerus length, pyelectasis, and hyperechogenic bowel. (ABSTRA

Combining humerus and femur length for improved ultrasonographic identification of pregnancies at increased risk for trisomy 21

OBJECTIVE

Our purpose was to evaluate the value of the combination of femur and humerus length measurements in ultrasonographic screening for trisomy 21.

STUDY DESIGN

Direct necropsy measurements were analyzed on 703 midgestational fetuses (641 normal, 62 with trisomy 21). The (leg+arm length)/foot length ratio was found to be significantly shortened for fetuses with trisomy 21. On the basis of necropsy data 576 midgestational pregnancies were evaluated ultrasonographically for (femur+humerus length)/foot length ratio to identify fetuses at increased risk for trisomy 21.

RESULTS

An ultrasonographic (femur+humerus length)/foot length ratio < or = 1.75 gave a 15.3 odds ratio risk for trisomy 21 in our high-risk population and correctly identified 53% of fetuses with trisomy 21, with a false-positive rate of 7%. In addition, the use of this ratio eliminates the need for gestational age-corrected nomograms and complicated calculations in ultrasonographic screening.

CONCLUSION

The (femur+humerus length)/foot length ratio may be an additional effective ultrasonographic marker for identification of fetuses at increased risk for trisomy 21.

Adjusting the risk for trisomy 21 on the basis of second-trimester ultrasonography

OBJECTIVE

Our purpose was to establish the sensitivity and specificity of various ultrasonographic markers of trisomy 21 in the second trimester of pregnancy on the basis of literature review and to generate tables that would allow adjusting the risk for trisomy 21, and therefore the need for genetic amniocentesis, depending on the presence or absence of these markers.

STUDY DESIGN

A computer search was performed of the English literature, including the years 1983 through 1993, of studies that used second-trimester ultrasonography to detect fetuses with trisomy 21. After statistical analysis of the reported studies was performed, the average sensitivity and specificity of the following ultrasonographic markers were determined: structural malformations, short femur, short humerus, combination of short femur and short humerus, pyelectasis, nuchal fold thickening, echogenic bowel, and short ear length. After the average sensitivity and specificity of these ultrasonographic markers were established, appropriate tables were generated by Bayes' theorem to adjust the risk for trisomy 21 in the second trimester depending on the presence or absence of these markers. Statistical analyses were performed with the statistical package Excel on a personal computer.

RESULTS

The average detection rate (sensitivity) of structural fetal malformations was 28%, short femur 31%, short humerus 33%, short femur and humerus 32%, nuchal fold thickening 32%, echogenic bowel 7%, and short ear length 71%. The nuchal fold thickening had the highest specificity (99.5%). Isolated pyelectasis was not associated with an increased risk for trisomy 21. However the risk was increased when pyelectasis was associated with other markers. In the presence of normal ultrasonographic results, the negative prediction can be combined with maternal age-related or biochemical prediction of trisomy 21 to help in the informed consent process in counseling women about the benefits and harms of genetic amniocentesis. Genetic amniocentesis should be considered in women of any age when second-trimester ultrasonography reveals the presence of one or more of the following: fetal structural malformations, short femur (determined by biparietal diameter-to-femur length ratio), combination of short femur and humerus, abnormal (> or = 6 mm) nuchal fold thickening, echogenic bowel, or short ear length.

CONCLUSION

In experienced hands second-trimester ultrasonography may be used to adjust the priori risk of both high and low-risk women for trisomy 21 and therefore the need for genetic amniocentesis.

Growth profiles of human autosomal trisomies at midgestation

Somatic and visceral growth profiles of midgestation human fetuses with trisomy 21, 18, or 13 demonstrate that each disorder has a characteristic pattern of growth aberration. The most striking deviations are short limbs in trisomy 21, subnormal adrenal and lung weights in trisomy 18, and supranormal spleen and kidney weights in trisomy 13.

The utility of fetal biometry as an adjunct to the multiple-marker screening test for Down syndrome

OBJECTIVE

Our purpose was to investigate fetal biometry as an adjunct to the multiple-marker screen (maternal age, serum alpha-fetoprotein, estriol, and human chorionic gonadotropin) for Down syndrome.

STUDY DESIGN

Fifty-two cases of Down syndrome were compared with 7514 normal fetuses. The measured/predicted femur length ratio had the best discriminant value (1.0 +/- 0.11 vs 0.93 +/- 0.13, p < 0.0001). Multivariate gaussian algorithms were developed and each computed a likelihood ratio for Down syndrome. The trivariate algorithm incorporated the three maternal analytes, whereas the quadrivariate version also included the femur length ratio. The study population included 38 cases of Down syndrome and 1098 euploid controls. The midtrimester risk was the product of the age-related risk and the likelihood ratio.

RESULTS

The relative difference in the femur length ratio between normal and affected fetuses was small in comparison to that of the maternal serum analytes. At a risk cutoff of > or = 1:190 the detection rates were similar and actually favored the trivariate algorithm but differed only by one case of Down syndrome.

CONCLUSION

The addition of the measured/predicted femur length ratio had a negligible effect on the performance of the multiple-marker screening test.

Humerus and femur length in fetuses with Down syndrome

The aim of the study was to assess the value of sonographic measurement of fetal humerus and femur lengths in the second trimester as a screening tool for Down syndrome (DS). We reviewed retrospectively fetal sonographic biometry made at the time of amniocentesis between 15 and 19 weeks. The study group consisted of 27 DS fetuses. The control group comprised 500 normal fetuses chosen randomly. The expected humeral and femoral lengths for a given biparietal diameter were estimated by linear regression equations from the 500 normal fetuses. Receiver operating characteristic curve analysis was performed to evaluate both the detection rate and the false-positive rate of different cut-off values of measured to expected lengths ratios. The median femur and humeral lengths in DS fetuses were 0.91 times the expected values. No significant differences in the detection rate and false-positive rate were found between the humerus and femur lengths. When the humeral and femoral lengths were combined, we observed a remarkable reduction in the false-positive rate. A measured to expected length ratio of 0.91 detected 44.4 per cent of DS fetuses with 7.6 per cent false positives. These results suggest that the combination of femoral and humeral lengths may permit a more efficient use of ultrasound in screening for Down syndrome than the use of either alone.

Humerus and femur length shortening in the detection of Down's syndrome

OBJECTIVE

Our purpose was to determine the utility of screening for humerus and femur length shortening in prenatal detection of Down's syndrome (trisomy 21).

STUDY DESIGN

On the basis of the biparietal diameter, predicted humerus and femur lengths were derived from a sample control group of 350 karyotypically normal fetuses. The regression formulas were prospectively applied to 45 consecutive fetuses with Down's syndrome and 942 fetuses with normal karyotypes. Ratios of measured humerus length/predicted humerus length and measured femur length/predicted femur length were calculated for each fetus.

RESULTS

Eleven of 45 (24.4%) fetuses with Down's syndrome had short humerus (measured humerus length/predicted humerus length ratio of < or = 0.89), and 11 (24.4%) had short femur (measured femur length/predicted femur length ratio of < or = 0.91. In comparison, among 942 karyotypically normal fetuses only 43 (4.5%) showed short humerus and 44 (4.7%) showed short femur (p < 0.05). Fetuses with both short humerus and short femur carried an 11-fold greater risk of Down's syndrome (risk ratio = 11.1; 95% confidence interval = 5.0 to 25.0).

CONCLUSION

Prenatal ultrasonographic detection of short humerus to short femur, or both significantly increase the risk of Down's syndrome; this information may be useful in screening programs.