Second trimester serum markers

Prenatal screening for Down syndrome in the early second trimester with multiple maternal serum markers has been available for more than 15 years. The multiple marker combination with the highest screening performance currently available is alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A, together with maternal age (so-called quad marker test). With this combination, a detection rate of 80% at a 5% false positive rate is achieved. Inhibin A, the newest addition to second trimester serum screening, is an alpha-beta subunit hormone of placental origin, and is measured using a monoclonal two-site ELISA validated for use in prenatal screening. Quality control parameters for inhibin A measurement are acceptable and are monitored through the proficiency testing program administered by the College of American Pathologists. Research into other possible second trimester screening markers has included studies on the maternal urine and serum levels of an hCG variant, hyperglosylated hCG (h-hCG; invasive trophoblast antigen). Recent data indicate that h-hCG is similar to hCG itself, although its measurement in maternal urine may improve the performance of the established serum marker combinations. With the introduction of first trimester screening markers and their use in an integrated first and second trimester marker approach to screening, and with the fact that many women do not seek prenatal care until the early second trimester, prenatal screening for Down syndrome using second trimester serum markers remains a major resource in obstetrical care.     

First trimester screening for aneuploidy: Nuchal translucency sonography

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

An Update on Current Prenatal Testing Options: First Trimester and Noninvasive Prenatal Testing

Prenatal genetic testing is rapidly evolving and requires that prenatal care providers stay up‐to‐date with accurate, evidence‐based knowledge. Noninvasive prenatal testing (NIPT), first trimester maternal serum markers, and fetal nuchal translucency are the most recently developed screening tests added to the testing repertoire for detection of chromosomal disorders such as trisomy 21 (Down syndrome). NIPT is a new, highly accurate technique that uses maternal serum and is rapidly being introduced as a first trimester screening tool and increasingly being requested by pregnant women. The American College of Obstetricians and Gynecologists recommends that all pregnant women be offered first and second trimester screening options, regardless of risk status, but does not yet recommend NIPT. It is important for prenatal care providers to be aware of and understand these testing options in order to assist women and their families in making well‐informed decisions during pregnancy. The purpose of this article is to update midwives and other prenatal care providers on the current prenatal genetic testing options available and how to appropriately offer and discuss them with their clients. We discuss how these tests work; what to do with the results; and most importantly, how to support and communicate accurate information to women and families as they navigate through an increasingly complicated array of testing choices.     

Womens' preference in Down syndrome screening

OBJECTIVE: To determine the knowledge of pregnant women about prenatal tests, and what tests they would choose if offered. Also, the preference of pregnant women for second-trimester or first-trimester screening was assessed. PATIENTS AND METHODS: Pregnant women receiving antenatal care in a decentralized primary care system (n=80), and pregnant women that were offered a prenatal diagnosis at the Academic Medical Centre (n=195), were asked to complete a questionnaire. RESULTS: The response rate was over 80%. Most women in both groups preferred a screening test for Down syndrome to be performed in the first trimester of pregnancy. A combination of nuchal translucency measurement and first-trimester serum screening was the option of choice. The screening possibilities for Down syndrome were less well known to the women in the low-risk group compared with the women in the high-risk group. The offer of a prenatal screening test would have been declined by more than 30% of women at low risk for carrying a fetus with Down syndrome. CONCLUSIONS: Our results show that women prefer screening for Down syndrome to be performed in the first trimester of pregnancy, using both serum and ultrasound tests. In women at low risk for Down syndrome the knowledge of prenatal screening methods was less, as well as the acceptance of prenatal screening being lower.     

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

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

妊娠中期二联唐氏综合征筛查法在浙南地区32188例孕妇中的筛查效率

目的 采用二联法(母血清甲胎蛋白和β-人绒毛膜促性腺激素)对浙南地区妊娠中期孕妇进行唐氏综合征筛查,评估其筛查效率. 方法 对本地区孕妇根据知情同意原则在妊娠中期取羊水进行常规二联唐氏综合征筛查,筛查出的高风险(≥1∶270)孕妇采用羊膜腔穿刺、羊水细胞培养和染色体核型分析进行产前诊断.通过本地区的三级妇幼保健网对本地区行产前唐氏综合征筛查或未行筛查的孕母分娩的新生儿进行临床随访,对可疑唐氏综合征的新生儿行外周血染色体核型分析进行诊断.正态分布计量资料采用均数±标准差(x-±s)表示,组间差异比较采用两独立样本t检验；计数资料用率表示,组间差异比较采用x2检验.唐氏综合征的危险概率用随机筛查软件进行统计分析. 结果 2007年10月至2010年5月,本地区共32 188例单胎妊娠孕妇接受筛查,唐氏综合征高风险者为1130例,低风险31 058例.高风险者中90.79％(1026/1130)接受产前诊断,确诊7例唐氏综合征胎儿均引产终止妊娠；另外104例未接受产前诊断的孕妇分娩1例唐氏综合征患儿.31 058例低风险者中新生儿出生后确诊唐氏综合征6例,发生率0.19‰.接受产前筛查者中唐氏综合征患病率为0.43‰(14/32 188).妊娠中期二联唐氏综合征筛查检出率为57.14％(8/14),假阳性率为3.48％(1122/32 188),阳性预测值为7.08‰(8/1130).同期,由于各种原因未接受唐氏综合征产前筛查的孕妇达到23 813例,分娩唐氏综合征患儿15例,患病率0.63‰.与接受筛查者中的患病率(0.43‰)差异无统计学意义(x2=1.004,P＞0.05).本地区唐氏综合征总体患病率为0.52‰(29/56 001). 结论 产前筛查和诊断可以减少唐氏综合征患儿出生.但本研究中妊娠中期二联唐氏综合征筛查法的检出率、假阳性率和阳性预测值均较低,可能与本研究所采用的正常值范围并不适用于中国人群有关.     

Between pregnancy biological variability of first trimester markers of Down syndrome: implications for screening in subsequent pregnancies.

In a group of 149 women who had undergone routine first trimester screening using fetal nuchal translucency thickness (NT) and maternal serum free beta-hCG and pregnancy associated plasma protein-A (PAPP-A) in two consecutive pregnancies the within person between pregnancy biological variability of these markers has been assessed. For fetal NT there was no correlation between NT MoM in the first and second pregnancy (r=0.0800). For maternal serum free beta-hCG MoM a significant correlation was observed (r=0.4174) as was also found for PAPP-A MoM (r=0.3270). The implications for such between pregnancy marker association is that women who have an increased risk of Down syndrome in their first pregnancy are 1.5-2 times more likely to repeat this event in their next pregnancy. This observation may be useful in counselling women in the first trimester screening of a subsequent pregnancy.     

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

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

Maternal serum-integrated screening for trisomy 18 using both first- and second-trimester markers

OBJECTIVES: To estimate the prenatal screening performance of an integrated serum test for detecting trisomy 18, which combines measurements of first- and second-trimester markers with maternal age to assign patient-specific risks. METHODS: Published and new observations of maternal serum marker levels in trisomy 18 and unaffected pregnancies are used to derive population parameters. These parameters are then combined in a multivariate Gaussian model to assign patient-specific risks for trisomy 18. RESULTS: The best combination of serum markers includes pregnancy-associated plasma protein-A in the first trimester and alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin in the second trimester. At a second-trimester risk cutoff of 1 : 100, these 4 markers, in combination with maternal age, detect 90% of trisomy 18 pregnancies at a false-positive rate of 0.1%. The odds of a trisomy 18 pregnancy among screen-positive women are 1 : 4. Without the first-trimester marker, detection is reduced to 67% at about the same false-positive rate. CONCLUSION: The algorithm described here is highly efficient for detecting trisomy 18 and should be considered by programs that offer serum-integrated screening for Down syndrome.     

Down syndrome screening in the first and second trimesters: what do the data show?

Recent advances in Down syndrome screening are providing women with additional options in how they are assessed for this condition. First trimester screening tests are at least as accurate as traditional second trimester testing and provide results earlier in a woman's pregnancy. Combining first and second trimester results increases the sensitivity and specificity of screening and reduce the number of women requiring invasive testing. This review summarizes and critiques the available data on Down syndrome screening and compares different testing algorithms. Meta-analyses of first trimester screening techniques are presented. Nuchal lucency measurements and other first trimester ultrasound markers, first trimester serum markers, and methods for combining these screens are addressed.     

Comparison and integration of first trimester fetal nuchal translucency and second trimester maternal serum screening for fetal Down syndrome

BACKGROUND: It is uncertain whether first trimester nuchal translucency (NT) is more effective than the well-established second trimester serum screening for fetal Down syndrome or whether their combination works best. We report data from a large multicentre non-interventional trial in which all subjects underwent both first and second trimester screening. METHODS: All women who attended the obstetric clinic before 15 weeks' gestation were recruited. An ultrasound examination was performed at 10 to 14 weeks to measure the NT. The nuchal measurements were not acted upon unless the fetus showed gross features of hydrops fetalis. All women had serum alpha-fetoprotein (AFP) and human chorionic gonadotrophin (hCG) assay at 15 to 20 weeks. The Down syndrome risk assigned by serum screening was disclosed and amniocentesis was offered if this assigned risk was >or=1:250 or if the women were 35 years and older. The efficacy of different combinations of screening markers was compared. RESULTS: Between January 1997 and August 2000, 17 590 women were recruited (19% >or=35 years old). After excluding subjects who miscarried, defaulted the serum test and other reasons, 16 237 pregnancies were analysed. Of these, 35 pregnancies were affected by Down syndrome (2.2 cases per 1000 pregnancies). At a false-positive rate of 5%, the detection rate of Down syndrome by NT alone, NT and age, serum hCG, AFP and age, and NT, hCG, AFP and age were 61%, 69%, 73% and 86%, respectively. CONCLUSION: Integration of NT and second trimester serum AFP and hCG assay yielded the best screening efficacy for Down syndrome.     

Between pregnancy biological variability of first trimester markers of Down syndrome and the implications for screening in subsequent pregnancies: an issue revisited

OBJECTIVES: To assess the level of correlation of first trimester biochemical and biophysical markers of Down syndrome between different pregnancies in the same individual. To assess the impact that between pregnancy biological variability has on the likelihood that women who are at increased risk in a first pregnancy being also at increased risk in a subsequent pregnancy. METHODS: During a three period women attending the OSCAR clinic at Harold Wood Hospital have had the opportunity to have first trimester screening for Down syndrome and other aneuploidies using the maternal serum biochemical markers free beta-human chorionic gonadotrophin (hCG) and pregnancy associated plasma protein-A (PAPP-A) in conjunction with fetal nuchal translucency (NT) thickness and maternal age. Of the 111,105 women undergoing such screening, the computer records were examined for women who had more than one pregnancy. The results from 1002 women with two normal singleton pregnancies were available for analysis. Marker correlations (as MoM) were established between the pregnancies and the proportion of women likely to be at increased risk in each pregnancy estimated, as was the likelihood of women being at increased risk in both pregnancies. RESULTS: For fetal NT there was no correlation between NT MoM in the first and second pregnancy (r = 0.0959, p > 0.10). For maternal serum free beta-hCG MoM a significant correlation was found (r = 0.3976, p < 0.001), as was also found for PAPP-A MoM (r = 0.4371, p < 0.001). CONCLUSION: The implication for such between pregnancy marker association is that women who have an increased risk of Down syndrome in one pregnancy are two or three times more likely to repeat this event in their next pregnancy. This information may be useful in counselling women when undergoing first trimester screening in a subsequent pregnancy.     

Accepting or declining the offer of prenatal screening for congenital defects: test uptake and women's reasons

OBJECTIVES: Prenatal screening for Down syndrome has become standard practice in many western countries. In the Netherlands, however, prenatal screening tests for congenital defects are not offered routinely. The present study aims to assess test uptake in a large, unselected population of pregnant women, and to give more insight into the decision for or against prenatal screening through nuchal translucency measurement or maternal serum screening. PATIENTS AND METHODS: The study is part of a randomized controlled trial with two groups, each being offered a different prenatal screening test, and a control group. Pregnant women received postal questionnaires at three stages of their pregnancy. RESULTS: Of the women being offered the nuchal translucency measurement or the second trimester maternal serum test, 53 and 38% respectively accepted the test offer. The main reasons for accepting were 'gaining knowledge about the health of the foetus/curiosity' (50%), 'favourable characteristics of the screening test' (18%), and 'increased risk of having a child with DS' (15%). The main reasons for declining were 'unfavourable characteristics of the screening test' (42%), 'not applicable/not necessary' (35%), 'anxiety/uncertainty' (36%), 'adverse characteristics of the invasive tests' (32%), and 'being against abortion' (15%). DISCUSSION: The uptake of prenatal screening was relatively low, and different distributions of reasons were reported, compared to other studies. These differences may be due to the specific Dutch situation in which prenatal screening is not part of standard prenatal care. The question arises as to whether informed decision-making would be reduced if prenatal screening became routinised.     

First-trimester combined ultrasound and biochemical screening for Down syndrome in routine clinical practice

OBJECTIVES: To assess the effectiveness of combined ultrasound and biochemical (CUB) screening for chromosome abnormalities in singleton pregnancies in a routine antenatal clinic and laboratory setting. METHODS: Women whose pregnancies fell within the gestational age range of 11 to 14 weeks by ultrasound assessment were offered CUB screening on the basis of measurement of nuchal translucency (NT), maternal serum free beta-human chorionic gonadotrophin (FbetahCG) and pregnancy-associated plasma protein A (PAPP-A). NT measurements were obtained using a standardised method defined by the Fetal Medicine Foundation and FbetahCG, and PAPP-A were measured using the DELFIA immunoassay system. Each screening marker measurement was converted to a multiple of the appropriate gestational median and a risk was derived using previously published parameters for each marker in chromosomally abnormal and unaffected pregnancies. A combined risk of Down syndrome and of trisomy 18/13, incorporating the maternal age risk, was calculated for all women. Invasive diagnostic testing was offered to women whose combined risk exceeded the cut-off risk of 1 in 250 (term). RESULTS: Five thousand and eighty-four women accepted a first-trimester screening test for Down syndrome, representing 75% of the eligible booking population. Out of the population eligible for CUB screening at the time of booking, NT measurements were obtained from 93% at the first clinic visit and 7% had to return for a second attempt. After excluding women who defaulted on a return visit, satisfactory NT measurements were obtained in 99.5% of pregnancies. Fifteen cases of Down syndrome and eleven pregnancies with other chromosome abnormalities were ascertained. The detection rate for Down syndrome was 93% (14/15) at a false-positive rate of 5.9% and for all chromosome abnormalities it was 96% (25/26) at an overall false-positive rate of 6.3%. CONCLUSIONS: CUB screening offers a significant improvement in sensitivity over second-trimester biochemical screening and is deliverable within a routine prenatal clinical setting.     

双胎妊娠胎儿染色体非整倍体产前筛查的研究进展

双胎妊娠胎儿染色体非整倍体疾病的发生率较单胎妊娠高,同时其筛查受到很多因素的影响。双胎妊娠的合子性质非常重要,直接影响风险率的计算模式,临床实践中多以妊娠早期绒毛膜性间接判断。目前双胎妊娠染色体非整倍体疾病筛查方案集中在妊娠早期胎儿颈部半透明层厚度(nuchal translucency,NT)筛查、妊娠早期联合筛查(NT+血清学筛查)、妊娠中期血清学筛查以及利用无创产前检测技术(noninvasive prenatal test,NIPT)进行胎儿染色体非整倍体疾病筛查。这些方案主要涉及染色体疾病中发病率较高的21-三体综合征和18-三体综合征。考虑到检出率、假阳性率以及经济学效应,妊娠早期联合筛查为目前双胎妊娠染色体非整倍体疾病筛查的主要方案,对近年来双胎妊娠染色体非整倍体产前筛查研究进展进行综述。     

Integrated serum screening for Down syndrome in primary obstetric practice

OBJECTIVES: Integrated serum screening for Down syndrome is potentially more effective than current second-trimester screening. We report results of an intervention trial of integrated serum screening that involved 229 primary prenatal care practitioners throughout Maine. METHODS: Women provided a first-trimester serum (for PAPP-A) followed by a second-trimester serum (for AFP, uE3, hCG, and DIA). These five marker measurements were used to calculate a Down syndrome risk in the second trimester. Screen-positive women (risk > or = 1:100) were managed according to standard practice. RESULTS: During 24 months' enrollment, 11 159 women provided a first-trimester sample (61% of women receiving screening services). Nine thousand seven hundred twenty-three women also provided a second-trimester sample; 8773 women satisfied gestational age criteria for testing in both trimesters. Integrated serum screening detected 14 of 16 Down syndrome cases (87%) and 79% after adjustment for trimester-of-ascertainment bias. The initial false-positive rate was 3.2% and was 2.7% when restricted to ultrasound-dated pregnancies. Performance was better than any combination of second-trimester markers. Implementation challenges included initial samples being collected too early and sample matching. CONCLUSIONS: Integrated serum screening for Down syndrome was successfully implemented in primary care settings; screening performance was consistent with predictions. It provides an accessible and acceptable alternative to screening protocols that require nuchal translucency measurements.     

Uptake of prenatal screening for chromosomal anomalies: impact of test results in a previous pregnancy

AIM: To assess whether the uptake of prenatal screening for trisomy 21 in a subsequent pregnancy is influenced by being classified in the 'increased risk' or 'not at increased risk' group in the first pregnancy. SETTING: District General Hospital Maternity Unit. METHODS: Amongst a group of women attending for maternity care at this hospital, the maternity records were examined to find women having at least two pregnancies. Any prenatal screening record for each pregnancy was retrieved from the prenatal screening database. Prenatal screening for trisomy 21 was by a combination of maternal serum alpha-fetoprotein (AFP) and free beta-human chorionic gonadotrophin (beta-hCG) in the second trimester and by maternal serum free beta-hCG and pregnancy-associated plasma protein-A (PAPP-A) and fetal nuchal translucency (NT) thickness in the first trimester. Women were stratified according to their trisomy 21 risk into an 'increased risk' group (1: <250 in the second trimester and 1: <300 in the first trimester) or 'not at increased risk' group based on their first pregnancy. In a second pregnancy, the records were examined to see if the mother accepted prenatal screening in the second pregnancy. The rate of acceptance of screening in a subsequent pregnancy, depending on whether 'at increased risk' or 'not at increased risk' in the first pregnancy, was examined using chi square tests. RESULTS: In the second trimester study, 4601 women were identified with two pregnancies during the study period. Of these, 4559 women had prenatal screening in a subsequent pregnancy. Initially, 273 women were identified in the high-risk group, and of these 252 (92.3%) elected to have prenatal screening in a subsequent pregnancy. This compared with 4307 of 4328 (99.5%) women in the low-risk group. In the first trimester study, 1077 women were identified with two pregnancies during the study period. Of these, 1072 had prenatal screening in a subsequent pregnancy. Initially, 60 women were identified in the high-risk group, and of these 56 (93.3%) elected to have prenatal screening in a subsequent pregnancy. This compared with 1016 of 1017 (99.9%) in the low-risk group. Statistically, there was no difference between the rate of declining prenatal screening in a second pregnancy amongst those in the high-risk group in a first pregnancy or those in the low-risk group (p = 0.429 for second trimester screening and p = 0.794 for first trimester screening). Similarly, no difference could be demonstrated between rates when screening in the first or second trimester (p = 0.961) for those in the high-risk group. CONCLUSION: Despite the understandable anxiety associated with being identified in the high-risk group (as a false positive finding) in a previous pregnancy, this did not seem to deter women from accepting prenatal screening in a subsequent pregnancy.     

Three-stage contingent screening for Down syndrome

OBJECTIVE: To demonstrate the potential value of three-stage sequential screening for Down syndrome. METHODS: Protocols were considered in which maternal serum pregnancy associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (hCG) measurements were taken on all women in the first trimester. Those women with very low Down syndrome risks were screened negative at that stage and nuchal translucency (NT) was measured on the remainder and the risk reassessed. Those with very low risk were then screened negative and those with very high risk were offered early diagnostic testing. Those with intermediate risks received second-trimester maternal serum alpha-fetoprotein, free beta-hCG, unconjugated estriol and inhibin-A. Risk was then reassessed and those with high risk were offered diagnosis. Detection rates and false-positive rates were estimated by multivariate Gaussian modelling using Monte-Carlo simulation. RESULTS: The modelling suggests that, with full adherence to a three-stage policy, overall detection rates of nearly 90% and false-positive rates below 2.0% can be achieved. Approximately two-thirds of pregnancies are screened on the basis of first-trimester biochemistry alone, five out of six women complete their screening in the first trimester, and the first-trimester detection rate is over 60%. CONCLUSION: Three-stage contingent sequential screening is potentially highly effective for Down syndrome screening. The acceptability of this protocol and its performance in practice, should be tested in prospective studies.     

Nuchal Translucency Thresholds in Prenatal Screening for Down Syndrome and Trisomy 18

ObjectiveTo determine if nuchal translucency (NT) can be used as a first trimester triage marker in prenatal screening for Down syndrome and trisomy 18.MethodsData from first trimester prenatal screening in 77 443 women were stratified by maternal and gestational ages. They were then analyzed to identify NT thresholds above or below which only positive (high-risk) or negative (low-risk) results were reported by a first trimester prenatal screening test combining PAPP-A, free β-hCG and NT.ResultsCombined prenatal screening was always positive for Down syndrome when NT thickness exceeded 4.0 mm. As women aged, this upper NT threshold value changed according to gestational age. In women aged 35 to 37 years, combined prenatal screening was always positive when NT exceeded 2.8 mm, 3.0 mm, and 3.4 mm at 11, 12, and 13 weeks of gestation, respectively. In women over 42 years of age, the upper threshold value for NT was 1.8 mm, 2.4 mm, and 2.7 mm at 11, 12, and 13 weeks of gestation, respectively. In women less than 35 years of age, we identified lower threshold values below which combined prenatal screening for Down syndrome was always negative.ConclusionIn prenatal screening for Down syndrome and trisomy 18, it is possible to identify NT threshold values above which biochemical screening provides no additional benefit. In pregnancies in which NT is above the established upper cut-offs, invasive prenatal screening can be offered without delay.     

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

OBJECTIVES: To compare alternative methods of antenatal screening for Down's syndrome (nuchal translucency measurement and second trimester maternal serum screening) in a low-risk population and to evaluate the consequences of a sequential estimation of risk. METHODS: In a consecutive series of 4308 women aged less than 38 with a singleton pregnancy, we examined the detection rate of nuchal translucency (NT) measurement at 10-14 weeks and maternal serum screening (MSS) by human chorionic gonadotropin and alpha-feto-protein at 14-18 weeks. Women with a NT measurement = 3 mm and women with a MSS derived risk = 1/250 were recommended to have an amniocentesis. A second trimester detailed ultrasound scan was also performed in all women. The outcome of all pregnancies was entered in a computerized database and the detection rate and false-positive rate of different screening strategies were analysed. RESULTS: Of the 4308 pregnancies that were followed (mean maternal age 30.1 years), there were 12 cases of Down's syndrome (0.28%), all detected prenatally. Seven of twelve cases had a NT measurement above 3 mm (58%), and 6 out of ten cases with available MSS had a calculated risk = 1/250 (60%). Four of the five cases with NT measurement below 3 mm were detected by subsequent MSS. At a threshold giving 5% of positive tests, the sensitivity of NT screening and MSS were 75% and 60%, respectively. DISCUSSION AND CONCLUSION: Sequential screening for Down's syndrome by nuchal translucency and second trimester biochemistry is effective and appears to increase the detection rate compared to the use of any single test. However, this strategy is likely to raise the false-positive rate and the interpretation of MSS derived risk should be combined to the first trimester NT measurement.