The imprecision in rates of Down syndrome by 1-year maternal age intervals: a critical analysis of rates used in biochemical screening

Maternal age in the epidemiology of common autosomal trisomies

The birth prevalence rate of each common autosomal trisomy generally increases with advancing maternal age and there is a substantial fetal loss rate between late first trimester and term. The literature is reviewed in order to provide the best estimates of these rates, taking account where possible of biases due to prenatal diagnosis and selective termination of pregnancy. There is an almost exponential increase in Down syndrome birth prevalence between ages 15 and 45 but at older ages the curve flattens. There is no evidence of the claimed relatively high birth prevalence at extremely low ages. Gestation-specific intra-uterine fetal loss rates are estimated by follow-up of women declining termination of pregnancy after prenatal diagnosis, comparison of observed rates with those expected from birth prevalence and comparison of age-specific curves developed for prenatal diagnosis and birth. Down syndrome fetal loss rates reduce with gestation and increase with maternal age. Edwards and Patau syndrome birth prevalence is approximately 1/8 and 1/13 that of Down syndrome overall, although the ratio differs according to maternal age, particularly for Patau syndrome where it reduces steadily from 1/9 to 1/19. Fetal loss rates are higher for Edwards and Patau syndromes than for Down syndrome.

Assessment of laboratories offering cell-free (cf) DNA screening for Down syndrome: results of the 2018 College of American Pathology External Educational Exercises

PURPOSE

Summarize and interpret results from exercises distributed to laboratories offering cell-free (cf) DNA screening for Down syndrome.

METHODS

The College of American Pathologists distributed three patient-derived plasma specimens twice in 2018. Sequencing platforms, test methods, results, and responses to supplemental questions were collected. Results were not graded but discrepancies were identified.

RESULTS

Sixty-five laboratories from six continents enrolled; six provided no results. The most common methodology was shotgun/genome sequencing (39/56, 70%). Overall, 40% of the gestational or maternal age responses were incorrect but 45% of the errors were corrected by the next distribution. Fetal fractions from 54 responding laboratories generally agreed with the intended response. No genotyping errors occurred (40/40 for trisomy 21 and 226/226 for euploid challenges) but 10 additional tests failed (3.6%). All 213 fetal sex calls were correct. Participants reported their clinical text for a Down syndrome screen positive test; 39% were classified as inadequate or misleading.

CONCLUSION

Patient-derived materials are suitable for all enrolled technologies/methodologies, but collecting material is challenging. Suggested clinical text includes the terms "screen positive" and "screen negative." Overall, laboratories performed well. Future efforts will focus on potential manufactured samples, clarifying results reporting and including additional chromosome abnormalities.

Estimation of single-year-of-age counts of live births, fetal losses, abortions, and pregnant women for counties of Texas

Objectives

We provide a methodology for estimating counts of single-year-of-age live-births, fetal-losses, abortions, and pregnant women from aggregated age-group counts. As a case study, we estimate counts for the 254 counties of Texas for the year 2010.

Results

We use interpolation to estimate counts of live-births, fetal-losses, and abortions by women of each single-year-of-age for all Texas counties. We then use these counts to estimate the numbers of pregnant women for each single-year-of-age, which were previously available only in aggregate. To support public health policy and planning, we provide single-year-of-age estimates of live-births, fetal-losses, abortions, and pregnant women for all Texas counties in the year 2010, as well as the estimation method source code.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-017-2496-x) contains supplementary material, which is available to authorized users.

Frequency of fetal karyotype abnormalities in women undergoing invasive testing in the absence of ultrasound and other high-risk indications

OBJECTIVES

No previous studies have reported the frequencies of individual chromosomal anomalies in normal-appearing fetuses stratified by maternal age (MA) and gestational age (GA). We therefore sought to (1) characterize the frequency of all fetal karyotype anomalies in sonographically normal appearing fetuses without pretest risk factors, and (2) assess MA and GA impact on the proportion of anomalies targeted by screening and consequent impact on residual risk following a negative result.

METHODS

Fetal karyotypes from samples without prior risk assessment or ultrasound anomalies were analyzed. We calculated, per single-year MA and in two GA intervals, the predicted frequency of each cytogenetic defect.

RESULTS

A total of 129 263 karyotypes were analyzed. The risk for significant, cytogenetically visible chromosomal anomalies, at 15 to 20 weeks GA, varies between 1/301 at MA of 18 years, and 1/9 at MA of 48 years. The proportion of clinically significant anomalies not addressed by current screening methods is 47% at MA of 18 years and 5% at MA of 48 years.

CONCLUSIONS

By determining frequencies for individual karyotype anomalies stratified by MA and GA, in the setting of normal-appearing fetuses, a more personalized risk assessment, including the residual risk after a normal fetal aneuploidy screening result, can be provided. © 2016 John Wiley & Sons, Ltd.

A retrospective analysis of amniocenteses performed for advanced maternal age and various other indications in Turkish women

OBJECTIVE

Prenatal cytogenetic diagnostic methods for the diagnosis of fetal chromosomal anomalies have been used reliably over the last 40 years. Advanced maternal age has become a basic indication for amniocentesis.

METHODS

We examined the results of the chromosome analyses of 3485 women that had amniocentesis for any reason during their antenatal care in our perinatology clinic in 2007-2009. Amniocentesis was performed for advanced maternal age in 1456 women (41.8%) and for other reasons in the remaining 2029 women (58.2%). Chromosomal anomalies were examined numerically and structurally.

RESULTS

When the amniocentesis results of the patients were reviewed as numerically normal or abnormal; 40 (2.7%) of 1456 amniocentesis procedures performed for advanced maternal age, 5 (0.9%) of 531 procedures performed for an increased double-test risk and 14 (1.3%) of 1095 procedures performed for an increased triple test risk were found to have chromosomal aneuploidy.

CONCLUSIONS

Maternal age is still the most prevalent indication for genetic amniocentesis other than positive prenatal screening tests. Among women with advanced maternal age, prenatal ultrasonography for soft markers of chromosomal aneuploidy accompanied with maternal serum biochemical screening tests should be evaluated during the decision making process of genetic amniocentesis.

Validation of first trimester screening for trisomy 21 in Singapore with reference to performance of nasal bone

INTRODUCTION

The aim of this study is to describe the performance of first trimester screening (FTS) for trisomy 21 using maternal age, serum biochemistry and fetal nuchal translucency (NT) in a single center and to evaluate the effect of nasal bone on screening performance.

MATERIAL AND METHODS

In 12,585 singleton pregnancies, the NT and nasal bone were examined. The majority of these mothers also had their serum biochemical markers analyzed. Risk was computed using different combinations of maternal age, biochemistry, NT and nasal bone. Down syndrome cases were confirmed by karyotyping.

RESULTS

There were 12,519 normal pregnancies, 31 with trisomy 21 and 35 with other chromosomal abnormalities. Without considering the nasal bone, the combined FTS detected 87.1% of trisomy 21 fetuses (false positive rate 5.1%), using 1:300 as the risk threshold, and this was further improved to 96.8% with the policy that classifies all fetuses with an absent nasal bone as high risk. Subgroup analysis showed that the detection rate would be 90.9%, with a false positive rate of 3.7%, if nasal bone was incorporated in the risk algorithm, compared to 81.8% and a false positive rate of 5.4% if it was not used.

DISCUSSION

FTS is very effective in early detection of trisomy 21 in Singapore. The nasal bone is a useful marker that can substantially improve the screening performance.

How valid are the rates of Down syndrome internationally? Findings from the International Clearinghouse for Birth Defects Surveillance and Research

Rates of Down syndrome (DS) show considerable international variation, but a systematic assessment of this variation is lacking. The goal of this study was to develop and test a method to assess the validity of DS rates in surveillance programs, as an indicator of quality of ascertainment. The proposed method compares the observed number of cases with DS (livebirths plus elective pregnancy terminations, adjusted for spontaneous fetal losses that would have occurred if the pregnancy had been allowed to continue) in each single year of maternal age, with the expected number of cases based on the best-published data on rates by year of maternal age. To test this method we used data from birth years 2000 to 2005 from 32 surveillance programs of the International Clearinghouse for Birth Defects Surveillance and Research. We computed the adjusted observed versus expected ratio (aOE) of DS birth prevalence among women 25-44 years old. The aOE ratio was close to unity in 13 programs (the 95% confidence interval included 1), above 1 in 2 programs and below 1 in 18 programs (P < 0.05). These findings suggest that DS rates internationally can be evaluated simply and systematically, and underscores how adjusting for spontaneous fetal loss is crucial and feasible. The aOE ratio can help better interpret and compare the reported rates, measure the degree of under- or over-registration, and promote quality improvement in surveillance programs that will ultimately provide better data for research, service planning, and public health programs.

[Impact of maternal age distribution on the expected live birth prevalence of Down's syndrome in the metropolitan France between 1965 and 2008]

OBJECTIVES

To evaluate the impact of demographic factors such as maternal age on the expected prevalence of Down's syndrome at birth.

PATIENTS AND METHODS

We used data from the French National Institute of Statistics (INSEE) concerning live births and maternal age. Expected prevalence at birth is derived from a mathematical regression model commonly used in biochemical screening.

RESULTS

We present continuous expected prevalence at birth over a long period, from 1965 to 2008. Over the last three decades, mean maternal age has increased by 4 years, leading to a two-fold increased in the expected live-birth prevalence of Down's syndrome infants, from 12.1 per 10,000 in 1977 to 21.7 per 10,000 in 2008.

CONCLUSION

It is important to continue to consider the impact modifications in demographic factors, such as maternal age, particularly in the evaluation of screening practices and policies for Down's syndrome.

Risk assessment of trisomy 21 by maternal age and fetal nuchal translucency thickness in 7,096 unselected pregnancies in Slovenia

AIM

To evaluate the screening for trisomy 21 by maternal age and nuchal translucency in a low-risk population.

METHODS

Screening was performed in 7,096 singleton pregnancies. The estimated risk for trisomy 21, the detection rate (DR), false positive rate (FPR) and the cut-off nuchal translucency thickness to obtain a 5% FPR were calculated.

RESULTS

The median maternal age was 28.6 years. The estimated risk for trisomy 21 was 1 in 300 or greater in 2.4% (171 of 7,096) of all pregnancies and in 75% (9 of 12) of trisomy 21 pregnancies. The DR for all aneuploidies was 83.3%, and 75% for trisomy 21. The estimated FPR at risk 1 in 300 for the whole population in 2004 was 3.8%. It is predicted to remain below 4% at least until 2007; to achieve a 5% FPR in 2007 the risk limit 1 in 400 is proposed.

CONCLUSIONS

Screening for trisomy 21 in a low-risk population in Slovenia gives comparable results to those in other countries. The only result that varies is the percentage of screen positive patients at the risk limit 1 in 300. We believe the risk limit should be specifically estimated for each country based on its population distribution of maternal age.

Fitting observed and theoretical choices - women's choices about prenatal diagnosis of Down syndrome

Choices regarding prenatal diagnosis of Down syndrome - the most frequent chromosomal defect - are particularly relevant to decision analysis, since women's decisions are based on the assessment of their risk of carrying a child with Down syndrome, and involve tradeoffs (giving birth to an affected child vs procedure-related miscarriage). The aim of this study, based on face-to-face interviews with 78 women aged 25-35 with prior experience of pregnancy, was to compare the women' expressed choices towards prenatal diagnosis with those derived from theoretical models of choice (expected utility theory, rank-dependent theory, and cumulative prospect theory). The main finding obtained in this study was that the cumulative prospect model fitted the observed choices best: both subjective transformation of probabilities and loss aversion, which are basic features of the cumulative prospect model, have to be taken into account to make the observed choices consistent with the theoretical ones.

Screening for Trisomy 21 with Maternal Age, Fetal Nuchal Translucency and Maternal Serum Biochemistry at 11–14 Weeks: A Regional Experience from Germany

OBJECTIVE

To examine the efficacy of first trimester screening for trisomy 21 using a combination of maternal age, fetal nuchal translucency (NT), maternal serum free beta-human chorionic gonadotropin (free beta-hCG) and pregnancy-associated plasma protein A (PAPP-A) in a regional setting [maternity unit of the Women's University Hospital, Hannover Medical School (study center); two regional private centers for prenatal diagnosis and human genetics; laboratory for prenatal diagnosis and human genetics].

METHODS

Fetal NT, crown-rump length, maternal serum free beta-hCG and PAPP-A were measured at 11-14 weeks of gestation. Risk calculation was carried out using the FMF computer algorithm. The patients were informed and counseled about possible invasive test options if the risk was 1 in 300 or greater. Fetal outcome was obtained by questionnaires given to the patients or sent to their gynecologists. The detection and false-positive rates for the different screening strategies were calculated.

RESULTS

Pregnancy outcome was obtained in 2,497 cases, of which 2,196 cases had completed first trimester screening with NT and maternal serum biochemistry and 301 additional cases had NT measurement only. The median age was 32.5 years. In our population 11 affected fetuses were found. The estimated risk for trisomy 21 was 1 in 300 or greater in 64, 82, 88 and 88% of affected fetuses using maternal age alone, in combination with nuchal translucency, with maternal serum biochemical markers or with both NT and biochemical markers for a false-positive rate of 28.2, 5.1, 15.3 and 4.0%.

CONCLUSIONS

First trimester screening using maternal age, NT, free beta-hCG and PAPP-A is highly effective for the detection of trisomy 21 and is associated with a sensitivity of about 90% for 5% false-positive patients.

Prospective first-trimester screening for trisomy 21 in 30,564 pregnancies

OBJECTIVE

This study was undertaken to evaluate the performance of a 1-stop clinic for first-trimester assessment of risk (OSCAR) for trisomy 21 by a combination of maternal age, fetal nuchal translucency (NT) thickness, and maternal serum-free ss- human chorionic gonadotrophin (hCG) and pregnancy-associated plasma protein-A (PAPP-A).

STUDY DESIGN

OSCAR was carried out in 30,564 pregnancies at 11 to 13 + 6 weeks. Patient-specific risks for trisomy 21 and detection and false-positive rates were calculated.

RESULTS

The median maternal age was 34 (range 15-49) years. Chromosomal abnormalities were identified in 330 pregnancies, including 196 cases of trisomy 21. The estimated risk for trisomy 21 was 1 in 300 or greater in 7.5% of the normal pregnancies, in 93.4% of those with trisomy 21 and in 88.8% of those with other chromosomal defects.

CONCLUSION

The most effective method of screening for chromosomal defects is by first-trimester fetal NT and maternal serum biochemistry.

Primary prevention of Down's syndrome

BACKGROUND

Antenatal screening has the capacity to detect more than 90% of Down's syndrome pregnancies leading to therapeutic abortion. Successes in recent years with such so-called 'secondary' prevention have not been matched with progress in primary prevention. Despite considerable research over many decades the principle cause of the disorder is unknown.

METHODS

This paper considers three potential primary prevention strategies, (1) avoiding reproduction at advanced maternal age, (2) pre-implantation genetic diagnosis for couples who are at high risk of Down's syndrome, and (3) folic acid supplementation. The principle aetiological hypotheses are also reviewed.

INTERPRETATION

A strategy of completing the family before a maternal age of 30 could more than halve the birth prevalence of this disorder. Women with a high a priori risk should have access to pre-implantation genetic diagnosis, which can lead to a reasonably high pregnancy rate with an extremely low risk of a Down's syndrome. The evidence suggesting an aetiological role for defective folate and methyl metabolism is not sufficient to justify an active preventative strategy of folic acid supplementation without performing a large clinical trial. Current supplementation policies designed to prevent neural tube defects may incidentally prevent Down's syndrome, provided a sufficiently high dose of folic acid is used. Further progress in primary prevention is hampered by limited aetiological knowledge and there is an urgent need to refocus research in that direction.

Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities

There is extensive evidence that effective screening for major chromosomal abnormalities can be provided in the first trimester of pregnancy. Prospective studies in a total of 200,868 pregnancies, including 871 fetuses with trisomy 21, have demonstrated that increased nuchal translucency can identify 76.8% of fetuses with trisomy 21, which represents a false-positive rate of 4.2%. When fetal nuchal translucency was combined with maternal serum free-beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A in prospective studies in a total of 44,613 pregnancies, including 215 fetuses with trisomy 21, the detection rate was 87.0% for a false-positive rate of 5.0%. Studies from specialist centers with 15,822 pregnancies, which included 397 fetuses with trisomy 21, have demonstrated that the absence of the nasal bone can identify 69.0% of trisomy 21 fetuses, which represents a false-positive rate of 1.4%. It has been estimated that first-trimester screening by a combination of sonography and maternal serum testing can identify 97% of trisomy 21 fetuses, which represents a false-positive rate of 5%, or that the detection rate can be 91%, which represents a false-positive rate of 0.5%. In addition to increased nuchal translucency, important sonographic markers for chromosomal abnormalities, include fetal growth restriction, tachycardia, abnormal flow in the ductus venosus, megacystis, exomphalos and single umbilical artery. Most pregnant women prefer screening in the first, rather than in the second, trimester. As with all aspects of good clinical practice, those care givers who perform first-trimester screening should be trained appropriately, and their results should be subjected to external quality assurance.

Combined first-trimester versus second-trimester serum screening for Down syndrome: a cost analysis

OBJECTIVE

The purpose of this study was to compare the cost-effectiveness of combined first-trimester screening for fetal Down syndrome with second-trimester maternal serum triple screening.

STUDY DESIGN

A first-trimester screening approach that used nuchal translucency measurement and maternal serum screening was evaluated against second-trimester maternal serum triple screening in a hypothetic population. Screening sensitivities and screen-positive rates were 91% and 5% for the first-trimester approach and 70% and 7.5% for the second-trimester approach, respectively. The costs of fetal Down syndrome, live-born Down syndrome cost, and total costs (screening plus live-born costs) were calculated for each screening program.

RESULTS

First-trimester screening was associated with lower screening and live-born Down syndrome costs versus second-trimester serum screening. Total Down syndrome screening costs were 29.1% lower with first-trimester screening.

CONCLUSION

In this hypothetic model, combined first-trimester screening for fetal Down syndrome was more cost-effective than universal second-trimester triple serum screening.

Comparison of models of maternal age-specific risk for Down syndrome live births

OBJECTIVES

To display and compare the different published formulae that specify the association between maternal age and the risk of a Down syndrome live birth.

METHODS

Papers published since 1987 on the prevalence of Down syndrome live births in relation to maternal age were located using MEDLINE and the references given in other papers. The data series and the models fitted to them were plotted to obtain a visual idea of their similarities and differences.

RESULTS

The observed and modelled age-specific rates for Down syndrome births were remarkably similar in all published series of data for women up to the age of 35, were reasonably similar for women aged 35 to 45, but differed for women older than 45.

CONCLUSION

In practice, the overall small differences in age-related risk between the different studies did not materially affect the performance of antenatal screening for Down syndrome. If a choice is to be made, the analysis based on the National Down Syndrome Cytogenetic Register (NDSCR) has marginal advantages since it is based on the largest data set and the corresponding model fits the data well. More data is needed to clarify the pattern of risk with maternal age among women over 45 years of age.

Bedside estimation of Down syndrome risk during first-trimester ultrasound screening

OBJECTIVE

To construct tables for 'bedside' estimation of Down syndrome risk based on maternal age and nuchal translucency measurements.

METHODS

Likelihood ratios were calculated using the log multiple of median Gaussian model. The parameters for the model (mean and standard deviation) were derived from 5560 normal and 51 Down syndrome-affected pregnancies scanned during the first trimester in three different centers. Equations for calculating maternal background risk and median values were obtained from previous reports. The results were compared to two modalities using the log Gaussian model and software that uses the delta-value model.

RESULTS

The distribution fitted the data well, and the parameters obtained in the study group for the log multiple of median model were a mean of 0 and a standard deviation of 0.12356 among normal pregnancies and a mean of 0.305312 and a standard deviation of 0.240337 among Down syndrome-affected ones. The likelihood ratios obtained for the various combinations of fetal crown-rump lengths and nuchal translucency measurements were comparable to other modalities reported earlier.

CONCLUSIONS

The results of the current study provide useful tables for simple and accurate 'bedside' estimation of Down syndrome risk without the need for computerized software or complicated calculations.

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.

One-stop clinic for assessment of risk for trisomy 21 at 11-14 weeks: a prospective study of 15 030 pregnancies

OBJECTIVE

To evaluate the performance of a one-stop clinic for assessment of risk (OSCAR) for trisomy 21 by a combination of maternal age, fetal nuchal translucency (NT) thickness and maternal serum free beta-human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11-14 weeks of gestation.

METHOD

Screening for trisomy 21 was carried out by OSCAR in 15 030 singleton pregnancies with live fetuses at 11-14 weeks. The estimated risk for trisomy 21 was calculated, and the women were counseled regarding this risk and the option of invasive testing or expectant management. Follow-up of the outcome of all pregnancies was carried out. The detection and false-positive rates for different risk cut-offs were calculated.

RESULTS

Fetal NT and maternal serum free beta-hCG and PAPP-A were successfully measured in all cases. Pregnancy outcome, including karyotype results or the birth of a phenotypically normal baby, was obtained from 14 383 cases. The median maternal age of these cases was 34 (range 15-49) years and in 6768 (47.1%) the age was 35 years or greater. The median gestation at screening was 12 (range 11-14) weeks and the median fetal crown-rump length was 64 (range 45-84) mm. The estimated risk for trisomy 21 based on maternal age, fetal NT and maternal serum free beta-hCG and PAPP-A was 1 in 300 or greater in 6.8% (967 of 14 240) normal pregnancies, in 91.5% (75 of 82) of those with trisomy 21 and in 88.5% (54 of 61) of those with other chromosomal defects. For a fixed false-positive rate of 5% the respective detection rates of screening for trisomy 21 by maternal age alone, maternal age and serum free beta-hCG and PAPP-A, maternal age and fetal NT, and by maternal age, fetal NT and maternal serum biochemistry were 30.5%, 59.8%, 79.3% and 90.2%, respectively.

CONCLUSION

Screening for trisomy 21 by a combination of maternal age, fetal NT and maternal serum biochemistry at 11-14 weeks can be provided in an OSCAR setting and is associated with a detection rate of about 90% for a false-positive rate of 5%.

One-stop clinic for assessment of risk of chromosomal defects at 12 weeks of gestation

Prenatal diagnosis of trisomy 21 requires an invasive test in women considered to be at high risk after screening. At present, there are four screening tests. For a 5% false-positive rate, the sensitivities are approximately 30% for maternal age alone, 60-70% for maternal age and second-trimester maternal serum biochemical testing, 75% for maternal age and first-trimester fetal nuchal translucency (NT) scanning, and 90% for maternal age with fetal NT and maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11-14 weeks. This article examines the methodology of first-trimester screening and summarizes the results from all studies reporting on the implementation of this method.

Increased fetal nuchal translucency at 11-14 weeks

Nuchal translucency (NT) is the sonographic appearance of a subcutaneous collection of fluid behind the fetal neck. The measurement of fetal NT thickness at the 11-14-week scan has been combined with maternal age to provide an effective method of screening for trisomy 21; for an invasive testing rate of 5%, about 75% of trisomic pregnancies can be identified. When maternal serum free-beta human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11-14 weeks are also taken into account, the detection rate of chromosomal defects is about 90%. Increased NT can also identify a high proportion of other chromosomal abnormalities and is associated with major defects of the heart and great arteries, and a wide range of skeletal dysplasias and genetic syndromes. In monochorionic twins, discordancy for increased NT is an early marker of twin-to-twin transfusion syndrome (TTTS). As with the introduction of any new technology into routine clinical practice, it is essential that those undertaking the 11-14-week scan are adequately trained and their results are subjected to rigorous audit.

Screening for Down's syndrome: effects, safety, and cost effectiveness of first and second trimester strategies

OBJECTIVE

To compare the effects, safety, and cost effectiveness of antenatal screening strategies for Down's syndrome.

DESIGN

Analysis of incremental cost effectiveness.

SETTING

United Kingdom.

MAIN OUTCOME MEASURES

Number of liveborn babies with Down's syndrome, miscarriages due to chorionic villus sampling or amniocentesis, health care costs of screening programme, and additional costs and additional miscarriages per additional affected live birth prevented by adopting a more effective strategy.

RESULTS

Compared with no screening, the additional cost per additional liveborn baby with Down's syndrome prevented was 22 000 pound sterling for measurement of nuchal translucency. The cost of the integrated test was 51 000 pound sterling compared with measurement of nuchal translucency. All other strategies were more costly and less effective, or cost more per additional affected baby prevented. Depending on the cost of the screening test, the first trimester combined test and the quadruple test would also be cost effective options.

CONCLUSIONS

The choice of screening strategy should be between the integrated test, first trimester combined test, quadruple test, or nuchal translucency measurement depending on how much service providers are willing to pay, the total budget available, and values on safety. Screening based on maternal age, the second trimester double test, and the first trimester serum test was less effective, less safe, and more costly than these four options.

Expected, prenatally discovered, and born cases of Down syndrome in Denmark during the period 1980-1998

In order to elucidate the consistency between generally used age-dependent risk values for Down syndrome (DS) and estimates of the probability of miscarriage in Down pregnancies we have compared expected numbers with estimated numbers of births with DS in Denmark had no intervention at all been carried out. The expected numbers were calculated from the distribution of newborn children according to maternal age combined with the age-related risk of DS. The estimated numbers of children that actually would have been born without any intervention were estimated from observed numbers of cases of DS, i.e. the cases born plus - with corrections because of the high probability of miscarriage in DS pregnancies - a proportion of those cases discovered prenatally. The analysis was carried out separately for mothers aged 35 years or older and for younger mothers. We found a high degree of compatibility between expected and estimated numbers, probably with a minor underestimation of the expected values for the older mothers. The performance of DS screening in Denmark in the period under consideration (1980-1998) is discussed in relation to the figures presented. Despite the fact that 11.8% of all pregnancies were subjected to an invasive diagnostic procedure, only about 38% of all births with DS were prevented. This means that in the period 1990-1998, reluctance to accept serological screening has indirectly resulted in the birth of almost 300 cases of DS in Denmark and at the same time the miscarriage of an unreasonable high number of normal fetuses.

Trends in prenatal diagnosis of Down syndrome and other autosomal trisomies in Scotland 1990 to 1994, with associated cytogenetic and epidemiological findings

The present report summarizes findings on 670 cases of autosomal trisomy diagnosed in Scotland, with actual or expected dates of delivery in 1990 to 1994 inclusive. Cases were notified by cytogenetic service laboratories. There were 277 prenatal and 369 postnatal diagnoses and 24 spontaneous losses. Excluding the latter, numbers diagnosed with trisomy 21, trisomy 18, trisomy 13, and other trisomies were, respectively, 470 (72.8%), 108 (16.7%), 36 (5.6%), and 32 (5.0%). Estimated maternal age-specific birth rates for trisomy 21 were close to published values from other jurisdictions. However, comparisons with a clinically based national register of congenital anomalies suggested that 3-4% of Down syndrome births were never karyotyped, most being early neonatal deaths. There was a striking increase over the period in the proportion of cases detected prenatally, associated with increased maternal serum screening in mothers <35 years old. Over the 3 final years (1992-1994), prenatal screening followed by elective termination was estimated to reduce the birth rate in trisomy 21 by 24% in mothers aged <35 years, by 57% in older mothers, and by 35% in all mothers. The crude incidence per 1,000 births fell from 1.08 in 1990-1991 to 0.77 in 1992-1994, in spite of an upward shift in the overall maternal age distribution. For trisomies 18 and 13, the estimated overall reductions in the birth rate over the whole 5-year period were respectively, 26 and 17%. In free trisomy 18, there was a significant reduction in the sex ratio (male/female) to 0.65, in line with earlier studies.

Use of computer simulation to evaluate a putative cluster of genetic or teratologic outcomes: adjustment for "multiple hypotheses" and application to a reported excess of Down's syndrome

The identification of an apparent excess of a genetic outcome in a particular area and/or a particular time often provokes considerable public alarm about the presence of an environmental mutagen. It is often difficult to determine in any particular case whether the observation, whatever its nominal statistical significance, is due to chance concatenation of events or to an environmental factor. Statistical evaluation is made more difficult by the profuse number of possible hypotheses that could have triggered concern about an excess. This renders it difficult to calculate the actual probability of the observation (or one more extreme). By attempting to identify similar types of outcomes that could have provoked an apparent excess and then undertaking computer simulations assuming random deviations from a constant rate, one may attempt to adjust for the problem of multiple hypotheses. We apply this approach to a reported excess of Down's syndrome in Norway in 1985-1986 in younger mothers, and conclude that there is a high probability that it arose by chance.

The "genetic sonogram": comparison of the index scoring system with the age-adjusted US risk assessment

PURPOSE

To compare two ultrasonographic (US) methods for prenatal detection of fetal Down syndrome.

MATERIALS AND METHODS

Genetic amniocentesis was successfully performed in 3,303 consecutive women with high-risk pregnancies (mean gestational age, 17.1 weeks). All patients underwent a complete "genetic US" examination prospectively. Risk was assessed by using (a) various modifications of the index scoring system (ISS) and (b) the age-adjusted US risk assessment (AAURA).

RESULTS

The prevalence of Down syndrome in this population was 1.6% (53 of 3,303). By using a threshold of at least 2 points to detect trisomy 21, the best ISS had a sensitivity of 45.3%, false-positive rate of 4.9%, likelihood ratio of 9.3, and positive predictive value in the high-risk population in this study of 13.3%. Lowering the threshold to 1 point increased the sensitivity to 60.4% but increased the false-positive rate to 15.8%. Adding points for age increased the sensitivity to 67.9% but increased the false-positive rate to 24.3%. Results of using AAURA to detect trisomy 21 were nearly identical, with a sensitivity of 43.4% and false-positive rate of 4.9% at a 1 in 36 risk threshold and a sensitivity of 69.8% and false-positive rate of 26.1% at a 1 in 200 threshold. Trisomies 18 and 13 were detected with sensitivities of 80.0% and 100.0%, respectively, with either system.

CONCLUSION

The modified ISS and AAURA are equivalent in screening for Down syndrome, with detection of approximately half of all trisomy 21 fetuses at a 5% false-positive rate.

Genetic screening in patients of reproductive age. How do you advise prospective parents who want to know specific risks?

At times, determining the actual genetic condition occurring in a family can be very difficult. The most important steps in deciding when testing is appropriate are the patient's age and family history, with special attention to ethnic background. By identifying risk factors before pregnancy, prospective parents can be fully informed about their specific risk of having a child with a genetic condition. Furthermore, the pros and cons of invasive prenatal diagnostic procedures often can be fully discussed well in advance of an actual pregnancy. Clinical geneticists and genetic counselors can provide valuable assistance when difficult questions or problems arise.

Age-specific variation in aneuploidy incidence among biochemical screening programs

OBJECTIVE

Our purpose was to compare the observed age-related incidence of Down syndrome in two large screening programs with the commonly quoted incidences used in biochemical screening programs.

STUDY DESIGN

Data from two large prenatal screening programs were stratified in 5-year age groups. The age-related incidence of Down syndrome was compared with the commonly used incidence as reported by Cuckle.

RESULTS

No significant differences were found in age-related incidences of Down syndrome in any age group between the screening groups or among women ages 15 through 29 in any of the three groups. However, for women 30 to 34 and > or = 40 years old, a trend was noted toward a higher incidence in the screening groups. For women ages 35 to 39, the observed incidence was significantly greater in the screening groups compared with the data of Cuckle.

CONCLUSION

Our data suggest an underascertainment in Down syndrome risk built into the Cuckle model, particularly in high-risk patients.

UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness at 10-14 weeks of gestation. Fetal Medicine Foundation First Trimester Screening Group

BACKGROUND

Prenatal diagnosis of trisomy 21 currently relies on assessment of risk followed by invasive testing in the 5% of pregnancies at the highest estimated risk. Selection of the high-risk group by a combination of maternal age and second-trimester maternal serum biochemistry gives a detection rate of about 60%. We investigated assessment of risk by a combination of maternal age and fetal nuchal-translucency thickness, measured by ultrasonography at 10-14 weeks of gestation.

METHODS

The risk of trisomy 21 was estimated for 96127 women of median age 31 years (range 14-49) with singleton pregnancies. Ultrasonography was done by 306 appropriately trained sonographers in 22 centres. Risk of trisomy 21 was calculated from the maternal age and gestational-age-related prevalence, multiplied by a likelihood ratio depending on the deviation from normal in nuchal-translucency thickness for crown-rump length. The distribution of risks was investigated and the sensitivity of a cut-off risk of 1 in 300 was calculated. Phenotype was assessed by fetal karyotyping or clinical examination of liveborn infants.

FINDINGS

The estimated trisomy-21 risk, from maternal age and fetal nuchal-translucency thickness, was 1 in 300 or higher in 7907 (8.3%) of 95476 normal pregnancies, 268 (82-2%) of 326 with trisomy 21, and 253 (77.9%) of 325 with other chromosomal defects. The 5% of the study population with the highest estimated risk included 77% of trisomy-21 cases.

INTERPRETATION

Selection of the high-risk group for invasive testing by this method allows the detection of about 80% of affected pregnancies. However, even this method of risk assessment requires about 30 invasive tests for identification of one affected fetus.

Maternal age specific risk rate estimates for Down syndrome among live births in whites and other races from Ohio and metropolitan Atlanta, 1970-1989

Our primary objective was to estimate, by one year and five year intervals, maternal age specific risk rates for Down syndrome among whites and among other races from two different populations, metropolitan Atlanta and south west Ohio, using live birth and prenatally diagnosed cases ascertained during 1970-1989. The five year estimates were also calculated separately for each of the five four year periods during these 20 years. Additionally, we compared two different methods of estimating these risk rates by using a third population of whites, and compared two different statistical methods of smoothing the risk rates. The results indicate good agreement between the metropolitan Atlanta and south west Ohio estimates within races, but show a statistically significant difference between the two race categories. Because 86% of live births in the "other races" category in the combined population are to blacks, these data may be seen as the first estimates of maternal age specific risk rates for Down syndrome among blacks calculated by one year intervals. We found excellent agreement in the risk rate estimates among the five four year time periods, between the estimates obtained by using the two different methods of estimation, and between the estimates obtained using the two different methods of statistical smoothing. Our estimated risk rates for white women in their 20s strongly reinforce those from previous studies currently being used for genetic counselling purposes. While we did find somewhat higher rates for women under 20, and increasingly higher rates for those over 30 years of age, these differences are not substantial. Thus, this study in general supports the risk rates estimated from data collected mostly during the 1960s and 1970s.

Down syndrome serum marker screening: decision criteria and implicit values

Maternal serum markers assess the individual risk of giving birth to a fetus with Down syndrome. Because this information is a probability, and because of the infinite number of cut-off risks that can be adopted, a decision criterion is needed to define a population screening program. While a decision criterion for cut-off risks may refer to arbitrations between risks, another criterion must be considered. This criterion focuses on a societal perspective by comparing the costs of the program to the expected benefits. We will first discuss the questions that are raised when assessing, in terms of cost-effectiveness, the consequences of having adopted the policy maker's preferences for prenatal diagnosis referral. Subsequently, we will discuss the implicit values attributed to the outcomes of the program when the societal point of view is reduced to societal profitability. This is accomplished by means of a cost-benefit analysis using the 'avoided costs' approach. The consequences of screening with 'double' and 'triple' tests were assessed using a database made of 10,108 observations, including 63 Down syndrome cases. For a cut-off risk of 1:250 (resulting in a 7% amniocentesis referral rate, regardless of the technique), conclusions in terms of decision making differ according to the effectiveness indicator. Although a criterion based on resource allocation would promote the triple test, cost-benefit analysis points out the impact on results of factors such as the amniocentesis related fetal losses or the introduction of equity principles.

Reliability of statistics on Down's syndrome notifications

OBJECTIVES

To evaluate the completeness of notifications of Down's syndrome live births and terminations to the Office for National Statistics (ONS) using data from the National Down Syndrome Cytogenetic Register (NDSCR). To examine the agreement of observed birth prevalence of Down's syndrome with the expected birth prevalence derived from published maternal age specific rates.

METHODS

The number of live births (adjusted to allow for the estimated underascertainment) and the number of terminations due to fetal Down's syndrome from NDSCR were compared with those figures reported to the ONS. Subsequently, using the NDSCR figures, the live birth prevalence of Down's syndrome that would have occurred in the absence of antenatal diagnosis and selective termination was calculated in England and Wales in the years 1990-1993. These figures were compared with those derived by applying published age specific prevalences to the maternal age distribution in England and Wales.

RESULTS

It is estimated that only 48% and 46% respectively of Down's syndrome live births and terminations of pregnancy were notified to ONS between 1990 and 1993. The annual expected birth prevalences of Down's syndrome obtained by applying maternal age specific prevalences to the maternal age distribution were in close agreement with observed rates from NDSCR.

CONCLUSIONS

There is considerable underreporting of Down's syndrome births and terminations to ONS. The NDSCR data are more complete and therefore the effects of screening should be monitored using data from this source, or using estimates derived from the age specific rates of Down's syndrome.

Rates of Down syndrome at livebirth by one-year maternal age intervals in studies with apparent close to complete ascertainment in populations of European origin: a proposed revised rate schedule for use in genetic and prenatal screening

Precision and accuracy in determining rates of Down syndrome at livebirth are indispensible to algorithms which determine eligibility for prenatal cytogenetic diagnostic services. We derived Down syndrome rates by single year of maternal age which we propose as a revised rate schedule for background risk. Data on European-origin populations were obtained from 5 sources judged most likely to have complete ascertainment of cases. A "constant plus exponent" regression model and variants extending the analysis to higher powers of maternal age were applied to several ranges of maternal age. Confidence intervals about the rates were calculated. This analysis results in rates significantly higher than those in widespread use though the confidence intervals show a need for caution in assuming precision. Sources of variation in rates are also considered.

New estimates of Down syndrome risks at chorionic villus sampling, amniocentesis, and livebirth in women of advanced maternal age from a uniquely defined population

Current measures of livebirth prevalence of Down syndrome are derived from data obtained up to 20 years ago, before the introduction of the prenatal diagnostic tests amniocentesis and chorionic villus sampling (CVS). For women aged 36-52 years, but who were not tested prenatally, we proposed to make a direct estimate of current livebirth prevalence of Down syndrome. We could also determine prevalence at the time of CVS and amniocentesis in women of the same age undergoing prenatal testing. Differences in these prevalences allow an estimation of the relative loss of Down syndrome during pregnancy. In Victoria, Australia, we identified 3041 women having CVS, 7504 having amniocentesis, and 13,139 having no test. Smoothed regression estimates of age-specific livebirth prevalence were found to be higher than in the early studies. The estimate of spontaneous loss was 17 per cent between the time of CVS and amniocentesis, and 18 per cent after the time of amniocentesis. The latter figure is lower than previous estimates and may be explained by a greater likelihood of a Down syndrome fetus surviving to be liveborn, given the modern approach to early obstetric intervention. These current risk estimates of livebirth may be useful updates for genetic counselling, but perhaps more importantly, may be used as precise maternal age-related risk figures, necessary in the design and implementation of prenatal screening programmes for Down syndrome.