Ultrasonographic detection of the second-trimester fetus with trisomy 18 and trisomy 21

Fetal nuchal skin-fold thickness during the 2nd trimester of pregnancy

A prospective study was conducted to determine the cut-off values of nuchal skin-fold thickness (NFT) with false-positive rates for each gestational week (GW) for chromosomal abnormalities during the 2nd trimester of pregnancy. A total of 2,313 women with normal singleton pregnancies were included in the study. Cases of multiple gestations, aneuploidy and major congenital malformations were excluded. The distribution of NFT between the 15th and 24th GW and the cut-off values of NFT with false-positive rates for chromosomal abnormalities were determined. A significant positive correlation was noted between NFT and GW. Statistically significant differences were observed in NFT for the each GW. The 95th percentile values of NFT between 15 and 24 weeks' gestation were 4.7, 4.77, 5.0, 5.5, 5.76, 5.9, 6.0, 6.1, 6.5 and 6.8 mm, respectively. In all fetuses, if the cut-off value of NFT was considered as 6 mm, the false-positive rate ranged from 1.8% to 37% in 15-24 weeks' gestation. Evaluation of NFT according to cut-off values determined by population-based percentiles for each GW might be a more appropriate screening method for chromosomal abnormalities than accepting NFT ≥ 6 mm for all fetuses as abnormal, regardless of gestational age.

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.

Literature Review and suggested protocol for managing ultrasound soft markers for Down syndrome: Thickened nuchal fold, echogenic bowel, shortened femur, shortened humerus, pyelectasis and absent or hypoplastic nasal bone

Mid-trimester soft markers have been linked with Down syndrome and other aneuploidies. There are many other prenatal screening tests available with better detection rates for Down syndrome than the mid-trimester ultrasound. Many patients confronted with the diagnosis of a soft marker become anxious and may request a diagnostic test (amniocentesis) despite the associated risk of miscarriage. This is also despite the fact that most fetuses with an isolated soft marker are chromosomally normal. The management of a pregnancy with a soft marker must therefore be planned in a manner designed to minimize patient anxiety. Likelihood ratios can be used to modify a patient's 'prior risk' (based on age or prior screening tests) and create a new risk. This calculation may help identify a subset of patients suitable for further investigation. It has been proposed that 'negative' likelihood ratios can be used to reduce a patient's risk if no soft marker is found at a mid-trimester ultrasound. There remain concerns about this approach and further research is required before this approach enters common practice. The published work surrounding the management of thickened nuchal fold, echogenic bowel, shortened femur, shortened humerus, pyelectasis (renal pelvis dilatation) and hypoplastic nasal bone is reviewed. Each soft marker has different associations and individual management plans for each of these soft markers are presented. Although isolated single umbilical artery is not usually considered a soft marker of aneuploidy, a management plan for this common finding is also included.

[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.

Fetal age assessment based on femur length at 10-25 weeks of gestation, and reference ranges for femur length to head circumference ratios

BACKGROUND

The aim of the present study is to establish new reference charts for gestational age assessment based on fetal femur length (FL), and new reference ranges for FL to head ratios at gestational weeks 10-25, and to determine the effect of maternal and fetal factors on these charts.

METHODS

Six hundred fifty low-risk women with regular menstrual periods and singleton pregnancies were recruited to a prospective cross-sectional study after obtaining written consent. FL, outer-outer biparietal diameter (BPD), and head circumference (HC) were measured at 10-25 weeks of gestation. We used regression analysis in order to construct mean curves and to assess the effect of maternal and fetal factors on age assessment.

RESULTS

The new chart for age assessment by means of FL was based on 636 measurements. The 95% CI of the mean corresponded to <1 day. The variation between the mean and the 90th percentile was 5, 6, and 7 days at 13, 18, and 23 weeks, respectively, similar to the results when using BPD or HC. Maternal age modestly influenced gestational age assessment (1.3 days/10 years, P = 0.005), whereas smoking, height, body mass index, multiparity, fetal sex, cephalic index, and breech presentation had no impact. Reference charts for FL to head ratios have been presented. Maternal age, fetal sex, and cephalic index influenced the FL/BPD ratio, whereas only fetal sex influenced FL/HC.

CONCLUSIONS

Fetal age assessment based on FL is an equally robust method as using HC. FL/HC is a more robust ratio to characterize fetal proportions than is FL/BPD.

Measurement of nuchal skin fold thickness in the second trimester: influence of imaging angle and fetal presentation

OBJECTIVE

To assess the effect of imaging angle and fetal presentation on the measurement of nuchal skin fold thickness (NFT) in the second trimester.

METHODS

Fetal NFT was prospectively measured in 921 women at 18-21 weeks' gestation. The population was divided into two groups according to fetal presentation. Group A comprised 643 fetuses in cephalic or transverse presentation and Group B comprised 278 fetuses in breech presentation. To determine whether there was a difference in NFT depending on imaging angle, in Group A the NFT was measured on the standard axial horizontal images and then on images obtained after moving the transducer about 30 degrees toward the occiput. In Group B an assessment was made as to whether NFT changed with fetal presentation. Initial measurements were made with the fetuses in breech presentation. Repeat NFT measurements were made after 15 min in 64 fetuses that had converted to cephalic or transverse presentation. Statistical analysis was performed using a paired t-test.

RESULTS

The mean NFT on the standard horizontal image was 5.2 mm, while that on the 30 degrees occiput image was 4.0 mm. The difference was significant (P < 0.001). One hundred and twenty-three (19.1%) fetuses had a NFT greater than 6 mm on the standard image, but only two (0.3%) had an NFT of > 6 mm on the 30 degrees occiput image. There were no chromosomal or structural abnormalities in the 123 fetuses in which the NFT was > 6 mm. The mean NFT of 64 fetuses in the breech presentation was 4.4 mm, while it was 3.9 mm when they had converted to other presentations. The difference was statistically significant (P = 0.001). In breech presentation, 13/64 (20.3%) fetuses had a NFT greater than 6 mm compared to only one (1.6%) fetus in the other presentations. There were no structural or chromosomal abnormalities in any fetus with a NFT > 6 mm in either group.

CONCLUSIONS

In this study the NFT measurement varied with scan direction and fetal presentation. To avoid falsely thickened NFT measurements the scan should be directed to image the posterior surface of the occipital bone. This can be achieved by scanning from a 30 degrees occipital direction. Also, NFT is greater in fetuses in a breech presentation. Attention to these details can reduce false-positive NFT measurements.

Second trimester total human chorionic gonadotropin, alpha-fetoprotein and unconjugated estriol in predicting pregnancy complications other than fetal aneuploidy

OBJECTIVE

To assess the value of alpha-fetoprotein (AFP), total human chorionic gonadotropin (ThCG) and unconjugated estriol in predicting certain complications of pregnancy other than fetal aneuploidy.

STUDY DESIGN

Among 2384 women that underwent biochemical screening between 15 and 22 weeks of gestation, pregnancy outcome was evaluated in 677 women under 35 years of age according to serum marker levels by using cut-off points discriminative for Down syndrome or neural tube defect (NTD).

RESULTS

High alpha-fetoprotein levels (MoM>/=2.0) were found to be significantly more frequent (P<0.05) in cases of fetal growth restriction (odds ratio=2.7), miscarriage (odds ratio=4.4) and intrauterine fetal death (odds ratio=5.8). High chorionic gonadotropin levels (MoM>/=2.02) were associated with intrauterine growth restriction (odds ratio=2.1; P<0.05), miscarriage (odds ratio=4; P<0.01), preterm birth (odds ratio=2.5; P<0.05), and intrauterine fetal death (odds ratio=4.2; P<0.01). Among pregnancies with intrauterine growth restriction and threatening preterm delivery, low unconjugated estriol levels (MoM</=0.74) were significantly more frequent (odds ratio=2.2; P<0.05 and odds ratio=2.6; P<0.01, respectively).

CONCLUSION

All three markers predictive for fetal trisomy 21 shown to be associated with various pregnancy complications in euploid pregnancies.

Prospective ranking of the sonographic markers for aneuploidy: data of 2143 prenatal cytogenetic diagnoses referred for abnormalities on ultrasound

OBJECTIVE

To design a scheme to rank sonographic anomalies as indicators of aneuploidy and record the distribution of data from 2143 prenatal amniotic fluid/chorionic villous sample diagnoses referred for karyotyping because of fetal anomalies detected with ultrasound.

METHODS

In all cases the records of sonographic anomalies were obtained prior to karyotyping. A cascade of seven prospective categories of ultrasound anomalies was chosen and the data were included in the highest compatible sonography category. The categories were in descending order: (I) combined central nervous system (CNS)/cranial shape and cardiac anomalies (excluding spina bifida and anencephaly); (II) key anomaly present (exomphalos/ intrauterine growth restriction/duodenal atresia/cystic hygroma/fetal hydrops/talipes--with other multiple anomalies); (III) CNS +/- other abnormality (excluding choroid plexus cyst, spina bifida, anencephaly); (IVa) increased nuchal translucency--first trimester +/- other abnormality; (IVb) increased nuchal thickening--second trimester +/- other abnormality; (V) cardiac anomaly +/- other abnormality; (VI) other markers of aneuploidy (pyelectasis/two vessel cord/echogenic bowel/short femur); and (VII) other (mostly isolated) malformations.

RESULTS

There were 412/2143 (19.2%) chromosome abnormalities detected in this sonographically abnormal group. Overall, the prevalence of aneuploidy significantly ranged from 51 to 3% according to the above I-VII ultrasound categories and from approximately 1-80% for individual ultrasound anomalies. Likelihood ratios were derived for many ultrasound anomalies for several aneuploidy groups: trisomies of 13; 18; and 21; 45,X and 45,X mosaics; triploidy; other autosomal duplications and/or deletions; and other (than 45,X) sex chromosomal aneuploidies.

CONCLUSION

It is suggested this data could be used to assist pre-procedural counselling of patients after the ultrasound scan in tertiary referral centres for prenatal cytogenetic diagnosis.

The variables affecting nuchal skin-fold thickness in mid-trimester

OBJECTIVES

To find out variables that have an effect on the measured values of nuchal skin-fold thickness (NT) and to formulate a regression equation on the basis of those variables.

METHODS

The data on gestational age (GA), cephalic index (CI), presentation (Pr, vertex or breech), and the presence or absence of nuchal cord (NC) were collected prospectively on 548 normal singleton fetuses between 16 and 24 weeks gestation, and independent correlation of those variables with NT was calculated by multiple regression analysis and a regression equation was produced.

RESULTS

GA had positive correlation and CI had negative correlation with NT. The nuchal skin-fold was thicker among fetuses with breech presentation rather than those with vertex presentation and increased in the presence of nuchal cord. We calculated the expected NT through these observations: for fetuses presenting vertex, NT = 5.608 + 0.243GA - 0.066CI + NC* and for breech, NT = 2.803 + 0.392GA - 0.066CI + NC*.

CONCLUSIONS

This is the first report, which takes GA, CI, Pr, and NC for correlation factors with NT as a whole. These equations and further studies on determining the cutoff values of the nuchal skin-fold thickness may improve the sensitivity and specificity for the detection of Down syndrome and enable to minimize unnecessary invasive procedures such as amniocentesis or cordocentesis.

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.

Evaluation of fetal femur length to detect Down syndrome in a Thai population

OBJECTIVE

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

METHOD

A prospective study was performed by experienced perinatologists on 3137 women undergoing second-trimester amniocentesis, between 16 and 24 weeks of gestation, for the indications of advanced maternal age and past history of chromosomal abnormality. Biparietal diameter and femur length measurements were obtained before the procedures. Regression equations relating biparietal diameter to femur length were used to calculate observed femur length/expected femur length ratio in the chromosomally normal and Down syndrome fetuses. Sensitivity, specificity, false-positive rate and likelihood ratio of a positive test result at various observed femur length/expected femur length ratios for detection of Down syndrome were calculated. A receiver-operator characteristic curve was used to determine threshold screening ratio.

RESULTS

There were 3084 chromosomally normal pregnancies, 26 fetuses with Down syndrome (1:118), and 27 other chromosomal abnormalities. The relationship between femur length and biparietal diameter (BPD) was: expected femur length=-7.631+0.814 BPD, R(2)=0.78, P<0.001). Femur length in Down syndrome fetuses was significantly shorter than in normal fetuses (P<0.001). A ratio of 0.91 for observed femur length/expected femur length yielded a sensitivity of 42.3%, specificity of 86.2%, false positive rate of 13.8% and likelihood ratio of a positive test result of 3.07 (95% CI 1.94-4.84) for detection of Down syndrome.

CONCLUSIONS

In this study, femur length shortening in the second trimester appears to be a useful screening parameter for fetal Down syndrome in a Thai 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.

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.

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.

Gestational age standardized nuchal thickness values for estimating mid-trimester Down's syndrome risk

OBJECTIVE

Our aim was to develop gestational age standardized indices of fetal nuchal thickening. In addition, we wanted to develop a method for combining nuchal thickness data with maternal age for calculating individual Down's syndrome risk.

METHODS

Nuchal thickness was measured prospectively in pregnancies undergoing genetic amniocentesis. A regression equation for expected median nuchal thickness based on the biparietal diameter (BPD) was developed. Nuchal thickness values were expressed as multiples of the median (MoM). Additionally, a new parameter, percentage increase in nuchal thickness (PIN) (measured minus expected nuchal thickness) X100/expected nuchal thickness, was used. Receiver operator characteristics curves for Down's syndrome detection based on nuchal thickness values expressed as MoM, PIN, and in mm were compared. Log10 transformation of MoM data resulted in a Gaussian distribution, and the Down's syndrome likelihood ratios were calculated based on the heights of the Gaussian curves. Likelihood ratios were also calculated based on PIN values. The screening efficiency of maternal age alone was compared to age plus MoM, and age plus PIN values by multiplying age-related risk by the likelihood ratio corresponding to the given nuchal thickness MoM or PIN values.

RESULTS

There were 3,574 chromosomally normal and 50 Down's syndrome fetuses in the study. Both PIN and MoM values for nuchal thickness were closely correlated (R = 1.00, P<0.001) and each was poorly correlated with gestational age (R = 0.018, P = 0.28). The Down's syndrome screening efficiency of PIN, MoM, and nuchal thickness values in mm were not significantly different. The addition of nuchal thickness data to maternal age-related risk significantly improved the Down's syndrome screening efficiency: Area under the ROC curve for maternal age risk = 0.58, maternal age + PIN area = 0.79 (P<0.001 compared to maternal age alone) and for maternal age + MoM = 0.77 (P<0.005 compared to maternal age alone).

CONCLUSIONS

The development of gestational age standardized nuchal thickness indices makes it possible to combine ultrasound and maternal age-related risk to derive individual Down's syndrome odds.

Trade-offs in prenatal detection of Down syndrome

OBJECTIVES

This paper presents the results of different screening policies for prenatal detection of Down syndrome that would allow decision makers to make informed choices.

METHODS

A decision analysis model was built to compare 8 screening policies with regard to a selected set of outcome measures. Probabilities used in the analysis were obtained from official administrative data reports in Spain and Catalonia and from data published in the medical literature. Sensitivity analyses were carried out to test the robustness of screening policies' results to changes in uptake rates, diagnostic accuracy, and resources consumed.

RESULTS

Selected screening policies posed major trades-offs regarding detection rates, false-positive results, fetal loss, and costs of the programs. All outcome measures considered were found quite robust to changes in uptake rates. Sensitivity and specificity rates of screening tests were shown to be the most influential factors in the outcome measures considered.

CONCLUSIONS

The disclosed trade-offs emphasize the need to comprehensively inform decision makers about both positive and negative consequences of adopting one screening policy or another.

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

The role of midtrimester targeted fetal organ screening combined with the "triple test" and maternal age in the diagnosis of trisomy 21: a retrospective study

OBJECTIVE

Our purpose was to evaluate the role of fetal ultrasonography in prenatal detection of trisomy 21.

STUDY DESIGN

A retrospective study was performed on all cases of trisomy 21 diagnosed prenatally or postnatally between January 1990 and December 1993 in the Jerusalem metropolitan area. Our program of prenatal detection of trisomy 21 is a three-tiered, chronologically progressive screening that includes maternal age, biochemical serum markers, and targeted fetal organ survey. Sixty-seven thousand ninety-two babies were born during the study period; 17,084 maternal biochemistry analyses (triple test) were performed; and 6315 fetuses were karyotyped.

RESULTS

One hundred eight cases of trisomy 21 were diagnosed prenatally and postnatally. The overall rate of detection of trisomy 21 was 92.2% when our recommendations were followed. Among those cases diagnosed prenatally, 66.6% of trisomy 21 cases had been referred for fetal karyotyping because of maternal age > or = 35 years, 18.5% because of abnormal triple test results, and 11.1% because of fetal ultrasonographic findings associated with trisomy 21. Among women < 35 years old, 80% of trisomy 21 cases were detected prenatally. Of these, 50% had been referred for karyotyping because of triple test results and 30% because of abnormal ultrasonographic findings.

CONCLUSION

Midtrimester ultrasonographic targeted fetal organ screening plays a critical role in prenatal diagnosis of trisomy 21 among women under the age of 35 years.

The role of reduced ear size in the prenatal detection of chromosomal abnormalities

A prospective ultrasound study was performed between 18 and 38 weeks' gestation on 29 fetuses in a high-risk population, defined by the presence of structural anomalies, in order to investigate the usefulness of fetal ear measurements in the prenatal prediction of chromosomal abnormality. The prevalence of abnormal chromosomes was 34 per cent. The sensitivity (SE), specificity (SC), positive predictive values (PPV), and negative predictive values (NPV) of ear length for the detection of chromosomal abnormality were 80, 84.2, 72.7 and 88.9 per cent. The SE, SC, PPV, and NPV of ear width were 40, 94.7, 80 and 75 per cent. Fetal ear measurements may be a useful adjunct to the various ultrasound parameters in the prenatal detection of chromosome abnormality in a high-risk population of fetuses with structural anomaly(ies).

Early midtrimester fetal nuchal thickness: effectiveness as a marker of Down syndrome

OBJECTIVE

Our purpose was to assess the validity of nuchal thickness in the prediction of Down syndrome in early midtrimester fetuses.

STUDY DESIGN

In 1543 consecutive pregnancies undergoing amniocentesis, primarily for advanced maternal age, nuchal thickness was prospectively measured between 13 and 18 weeks and then correlated with the karyotype obtained from amniotic fluid.

RESULTS

With the cutoff value suggested in the literature (> or = 6 mm), 33.3% (6/18) of the cases of Down syndrome would be detected for a 0.1% (2/1424) false-positive rate, with a positive predictive value adjusted to a prevalence of Down syndrome in the general population of 1:3. To increase the sensitivity of the method, the threshold was lowered to 5 mm, achieving a sensitivity of 77.8% (14/18) for a 2.1% (30/1424) false-positive rate and an adjusted positive value of 1:19.

CONCLUSIONS

In early midtrimester fetuses decreasing the nuchal thickness threshold to 5 mm substantially enhances the detection of Down syndrome with a reasonable false-positive rate.

Femur/foot length ratio for detection of Down syndrome: results of a multicenter prospective study. The Association Française pour le Dépistage et la Prévention des Handicaps de l'Enfant Study Group

OBJECTIVE

The aim was to determine the sensitivity and specificity of a short femur for detection of trisomy 21 in the second trimester.

STUDY DESIGN

Thirty-six investigators in 12 centers measured biparietal diameter and femur and foot lengths in 3582 14- to 24-week-old fetuses in mothers undergoing amniocentesis for age, history of genetic disorder, or laboratory signs.

RESULTS

Among the various ratios for evaluating femur shortening the femur/foot ratio appeared to be the most discriminatory. At an upper cutoff level of 0.88 a sensitivity of 35% was obtained for 4.6% false positives in normal infants. However, to only obtain 2.3% false positives, the cutoff limit had to be set at 0.85, giving a sensitivity of no more than 15%.

CONCLUSION

Determination of the femur/foot ratio improves ultrasonographic detection of trisomy 21 in the second trimester, although for systematic use it would lead to an unacceptable number of unnecessary amniocenteses.

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.

Optimal nuchal skin-fold thresholds based on gestational age for prenatal detection of Down syndrome

OBJECTIVE

We attempted to determine optimal nuchal skin-fold thresholds throughout the second trimester of pregnancy for the detection of Down syndrome.

STUDY DESIGN

Thresholds of 5, 6, and 7 mm were prospectively tested in 8138 consecutive midtrimester pregnancies. The pregnancies were divided into early (14 to 18 weeks) and late (19 to 24 weeks) second-trimester time frames. The pregnancies were also subdivided by maternal age, and the same thresholds were tested for efficacy in the women < 35 years old versus those > or = 35 years old.

RESULTS

There were a total of 32 fetuses with trisomy 21 in the study population. From 14 to 18 weeks' gestation, a nuchal fold thickness of > or = 5 mm was the optimal threshold. Measurements of this magnitude were found in 2.9% of pregnancies and produced a sensitivity of 42% (11/26) for the detection of Down syndrome. When adjusted for the incidence of Down syndrome in the general population, the 5 mm threshold had a positive predictive value of 1 in 48. In the 19 to 24 week gestational time frame, > or = 6 mm appeared to be the optimal threshold, yielding a positive screen rate of 3.7% with a sensitivity of 83% (5/6). The adjusted positive predictive value was 1 in 38. The sensitivity of nuchal skin-fold thickness for Down syndrome detection was similar in women < 35 and > or = 35 years old. Positive predictive value was better in the women > or = 35 years old because of the higher prevalence of affected fetuses in this group.

CONCLUSIONS

Whereas definitive diagnostic testing should still be offered to all women with risk factors for a fetus with trisomy 21, differential nuchal skin-fold thresholds based on gestational age are effective for the detection of Down syndrome in pregnancies at low risk.

Nuchal-fold thickening in Down syndrome fetuses: transient appearance and spontaneous resolution in the second trimester

To elucidate the cause of the wide variation of sensitivity of the nuchal-fold thickness (NFT) for a noninvasive prenatal screening test for fetal Down syndrome, we consecutively measured the NFT throughout pregnancy in eight fetuses with Down syndrome and 100 normal fetuses (negative controls) using video-recorded ultrasonography. When 6 mm was set as the cutoff value, 6/8 Down syndrome fetuses showed a NFT above this value at least once during pregnancy. However, the thickening was transient in 5 of them and resolved spontaneously during the second trimester. Persistent thickening of nuchal-fold was only observed in one fetus. From the results of the present study, we recommend that the NFT be measured repetitively during the first and second trimesters as a noninvasive prenatal screening test for Down syndrome.

Transvaginal sonography-detection of findings suggestive of fetal chromosomal anomalies in the first and early second trimesters

Over a 4-year period, 14 dyskaryotic fetuses were diagnosed by amniocentesis, performed after early detection of malformations using transvaginal sonography (TVS). These 14 dyskaryotic fetuses were detected out of 4878 sonographic screenings performed by TVS between 9 and 16 weeks' gestation. Twenty-eight per cent of the referrals were at high risk and 72 per cent were at low risk for fetal malformations. Two hundred and twenty-nine fetuses (4.7 per cent) of the screened population had 265 anomalies, 39 per cent of them being transient. In 7 of the 14 dyskaryotic fetuses (50 per cent), the sonographically detected anomalies were transient, being undetected by follow-up sonographic screenings at later gestational ages (greater than or equal to 18 weeks). Postponing the first sonographic scan aimed at malformation detection to a later gestational age may lead to transient anomalies and their associated dyskaryosis being missed.

Screening for Down syndrome with the femur length/biparietal diameter ratio: a new twist of the data

OBJECTIVE

The purpose of this study was to determine the value of discordant morphometric measurements as identifiers of Down syndrome by evaluating the relationship of biparietal diameter, femur length, biparietal diameter/femur length ratio, and cephalic index between a group of fetuses with trisomy 21 and a control population.

STUDY DESIGN

Biometric measurements from 48 fetuses with trisomy were reviewed and compared with 107 normal fetuses of similar gestational age. Data were analyzed in 2-week gestational age intervals to determine the effect of gestational age on ultrasonographic detection of Down syndrome. Outcome measures were subject to least-squares linear regression and the t test for analysis.

RESULTS

A positive relationship between abnormal morphometric measurements and fetuses with Down syndrome was detected but only during specific weeks of pregnancy.

CONCLUSION

Although it appears that biometric measurements may be useful for Down syndrome, further study is needed before its widespread introduction into clinical practice.