Serial first- and second-trimester Down's syndrome screening tests among IVF-versus naturally-conceived singletons

BACKGROUND: It has been reported that second-trimester serum markers may be affected by assisted reproduction, leading to a higher false-positive rate. METHODS: A total of 285 naturally and 71 IVF-conceived singletons which underwent a serial disclosure Down's syndrome screening programme were compared. The study protocol included first-trimester combined [nuchal translucency (NT), free beta-HCG and pregnancy-associated plasma protein-A (PAPP-A)] testing. The second-trimester triple serum screening included alpha-fetoprotein (AFP), intact HCG and unconjugated estriol (uE3). After excluding aneuploidies, miscarriages, anatomical anomalies and cases with incomplete follow-up, the serum samples of normal cases were assessed and correlated. RESULTS: NT measurement was not significantly changed in either group. However, the IVF group had lower PAPP-A [0.96 versus 1.05 multiples of normal median (MoM)] and higher AFP (1.13 versus 1.07 median MoM). Both groups had similar rates of first-trimester false-positive results (FPR; 7 and 9% respectively), but the IVF group had a significantly higher mid-gestation FPR rate (10 versus 5%; Pearson chi2, P = 0.029). This has contributed to amniocentesis uptake rates of 15 and 13% for the IVF and natural conception pregnancies respectively. CONCLUSIONS: The IVF group tended to have a significantly higher second-trimester FPR rate. To counterbalance this phenomenon, integrated first- and second-trimester screening tests or the use of NT alone might be a reasonable option that deserves further investigation.     

Sonographic identification of second-trimester fetuses with Down's syndrome

Fetuses with Down's syndrome are more likely than normal fetuses to have a thickened nuchal skin fold and relatively short femurs on ultrasound examination in the second trimester. We evaluated these measures in more than 5500 fetuses, including 28 who were later found to have Down's syndrome by means of amniocentesis with cytogenetic analysis. Normal femur length was established as a function of biparietal diameter in 192 of the normal fetuses. We found that with use of a nuchal skin-fold thickness of 6 mm or more and a ratio of actual to expected femur length of 0.91 or less, fetuses with Down's syndrome could be identified with a sensitivity of 75 percent and a specificity of 98 percent. When other anomalies, such as an atrioventricular canal and meconium peritonitis, were added to these two criteria, the sensitivity of sonographic detection of Down's syndrome in the second-trimester fetus rose to 82 percent. The potential predictive value of these sonographic signs far exceeds that of advanced maternal age and low alpha-fetoprotein levels, which currently identify only 10 to 30 percent of affected fetuses. We recommend that fetuses with a thickened nuchal skin fold or shortened femurs on ultrasound examination be evaluated for Down's syndrome by amniocentesis and cytogenetic analysis.     

Antenatal screening for Down's syndrome in assisted reproductive pregnancies

The wide use of assisted conception methods has risen dramatically. The greater proportion of singletons, twins and high order of multiplicity conceived by those methods have already focused the medical community to various obstetric complications. Recently, there have been suggestions that the levels of mid-gestation serum markers, particularly human chorionic gonadotrophin (HCG), might be affected by assisted conception, leading to higher false-positive results. Furthermore, women who conceived after assisted reproduction methods are on average older, and in many cases their current pregnancy was achieved after long-standing infertility and might even be their last one. This is why they are extremely wary of any invasive fetal karyotyping. Therefore, every effort should be made to provide them with the most accurate screening of Down's syndrome (DS) risk. In this respect, nuchal translucency (NT) measurement, which has been reported to be another effective screening method, might be a more reliable marker in these pregnancies. This review explores the problematic issue of antenatal DS screening in assisted conception pregnancies. For the singletons and twins, a sequential NT and second-trimester serum marker screening can be offered, thus producing a single risk estimation which seems to be more accurate. For the high order of multiplicity, the NT offers additional important data, which can be taken in consideration both as a screening tool for DS and if fetal reduction is planned.     

唐氏综合征孕中期产前筛查及产前诊断的临床价值

目的探讨唐氏综合征的产前筛查和产前诊断在预防出生缺陷中的价值。方法对68813例孕15-20w的孕妇采用时间分辨荧光免疫方法检测血清AFP和B-HCG浓度,通过Maiticale软件计算危险系数,对唐氏高危和18-三体高危孕妇取羊水获脐血作染色体诊断,对神经管缺陷（NTD）高危孕妇行系统超声检查。结果筛查68813例孕妇,其中2632例为唐氏综合征或18-三体综合征高分险病例,占总筛查人数的3.82%;NTD高风险542例,占0.79%。接受羊水或脐血染色体检查1772例（占总阳性数的67.32%）,检出异常核型67例,占异常发生率3.78%;NTD高风险行超声检查证实胎儿畸形58例,占筛查高危孕妇的10.7%。结论孕中期产前筛查结合产前诊断可以有效预防出生缺陷的发生。     

Maternal serum screening for Down's syndrome: a user's club experience

Screening of Down syndrome using maternal serum markers requires specific quality management. We report here a user's club experience (Down's Club Abbott) in surveying performances of AxSYM total beta hCG and AFP reagents combined to Maciel software. Regular analysis was carried out during three years (1998 to 2000). Values from five laboratories were collected to achieve calculation of medians for each parameter, multiple of the medians and initial positive rate (cut-off of 1/250 established in France). The large size of the observed population (3,1020 women during the studied period) increased the performances of statistical evaluations. In July 1999, five months after a change in reagents leading to new medians, these latter were recalculated in agreement with the manufacturer. Our experience exhibits that only the concomitant analysis of all parameters (medians, multiple of medians and initial positive rate) can allow the monitoring of a potential drift (bound or not to reagents). Moreover, such user's clubs enhance the individual quality management of the laboratories and allow a good follow-up of the performances of the testing in time.     

Prenatal screening for Down's syndrome with use of maternal serum markers.

BACKGROUND. Approximately 35 percent of all cases of Down's syndrome in fetuses can be detected by measuring maternal serum alpha-fetoprotein during the second trimester in the general population of pregnant women. Recent case-control studies indicate that this detection rate could be approximately doubled by measuring serum levels of unconjugated estriol and chorionic gonadotropin, which are abnormally low and abnormally high, respectively, in women carrying fetuses affected by Down's syndrome. METHODS. We prospectively screened 25,207 women and adolescents in the second trimester of pregnancy and assigned each a risk of fetal Down's syndrome with an algorithm that took into account measurements of all three serum markers in combination with maternal age. On this basis, 1661 subjects (6.6 percent) were initially assigned a second-trimester risk of fetal Down's syndrome of at least 1 in 190, and 962 (3.8 percent) were offered amniocentesis for chromosomal analysis after verification of gestational age. Gestational age was determined on the basis of the first day of the last menstrual period or, when available, by ultrasonography. RESULTS. Among the 760 women and adolescents who chose amniocentesis, 20 cases of fetal Down's syndrome were detected, along with 7 other chromosomal disorders. There was 1 additional case of fetal Down's syndrome among the 202 women who chose not to have amniocentesis. The rate of detection of Down's syndrome was thus 58 percent (21 of 36 expected cases), and the frequency of identifying a fetus with Down's syndrome in women undergoing amniocentesis was 1 per 38 amniocenteses (95 percent confidence interval, 1 in 25 to 1 in 62). CONCLUSIONS. Measuring serum alpha-fetoprotein, chorionic gonadotropin, and estriol is more effective in screening for fetal Down's syndrome than measuring maternal serum alpha-fetoprotein alone. Such an expanded protocol can readily be incorporated into existing prenatal screening programs.     

Atlantoaxial instability in Down's syndrome: clinical and radiological screening

One hundred and thirty children with Down's syndrome were screened for the presence of atlantoaxial instability, using both clinical examination and radiographs of the cervical spine taken in flexion and hyperextension views. Seven children were found to have radiological evidence of atlantoaxial instability, with an atlanto-dens interval greater than 5.0 mm in one or all positions. Although a full clinical history was obtained from the attending parent and each child underwent a complete neurological examination, there were no factors detected which differentiated between those with radiological evidence of atlantoaxial instability and those with a normal atlantodens distance. It is recommended that children with Down's syndrome be screened twice, at the ages of 5-10 years and at 15 years.     

孕中期Down＇s综合征产前筛查的血清和血片方法的一致性研究

目的验证血片方法与血清方法应用于孕中期Down’s综合征产前筛查的一致性。方法同时对1096例孕15—20孕周孕中期孕妇血清以及滤纸干血片中的AFP及β-hCG含量进行了检测,对血清以及滤纸干血片中两分析物的浓度、中倍数进行了相关性分析,比较了两种方法的筛查结果。结果血清与血片中AFP及β—hCG含量呈高度相关,相关系数分别为0．979、0．982。血清与血片中AFP及β—hCGMoM呈高度相关,相关系数为0．968、0．975。当筛查切值为1／270时,血片法和血清法筛查假阳性率分别为6．4％和5．6％。结论血片与血清中AFP及β—hCG含量呈高度相关,血片方法和血清方法应用于孕中期Down’s综合征产前筛查一致性好,血片法可以代替血清法应用于孕中期Down’s综合征产前筛查。     

Down's syndrome screening: a controversial test, with more controversy to come!

By 1998, most health authorities offered antenatal screening for Down's syndrome, usually by biochemical methods. To date, the development of this form of screening has not been coordinated by a national body and, consequently, there are wide variations in practice between localities. Fortunately, many of these variations have not led to any noticeable inequality of health provision, but the wide variation in risk cut offs used by different centres does. Other variations merely lead to potentially unnecessary expenditure; whereas it is believed that adding extra tests to the screening procedure is beneficial (such as double test to triple test), statistical evaluation of the confidence intervals for the detection rates quoted indicates that there is no evidence that the extra test provides an increase in detection. The cervical screening programme has progressively improved, partly through the auspices of a national framework. A similar national approach would benefit Down's screening and is only now being considered: the national screening committee (NSC) is currently drafting recommendations. To ensure optimum screening performance, the NSC should specify the risk thresholds applied, the screening protocols to be used--that is, an opt-in programme with a minimum (possibly even a maximum) of two biochemical analytes or a nuchal fold evaluation--and perhaps should even recommend national population parameters to be used for risk calculation. It might even be advisable for statistical work to be carried out to determine whether local derivation of medians is truly necessary. Furthermore, defined options for older women could be specified--for example, should all older patients have the option to proceed directly to amniocentesis if they wish or should National Health Service amniocentesis only be available for those with a "high risk" screening result. The difficulties that will face the NSC in deciding which screening policy to adopt are also considered; specifically, the lack of evidence to suggest that triple testing is superior to double testing, and the lack of evidence to prove the superiority of one analyte over another. This inadequacy of evidence is not from want of trying, but is caused by the problems of collecting enough data to provide statistical significance. Finally, there is one important difference between cervical and Down's syndrome screening that has a major impact on the advice given by any "expert"; namely, patents. Many aspects of Down's screening are subject to patents and, therefore, there is more potential for apparently uncontroversial decisions to rebound with future retrospective patent infringement claims. Thus, it would be sensible to insist that any member of a national body deciding upon Down's screening policy must fully disclose all potential conflicts of interest, both personal and family, before they are allowed to sit on the committee. Furthermore, if a national policy is decided upon, worldwide patent searches should be carried out to determine whether there are any possible unforeseen legal consequences of any recommendation.     

Lens culinaris agglutinin-reactive alpha-fetoprotein, an alternative variant to alpha-fetoprotein in prenatal screening for Down's syndrome.

BACKGROUND: Three serum tests, alpha-fetoprotein (AFP), human chorionic gonadotrophin and unconjugated oestriol, are now widely used for screening for Down's syndrome. Lens culinaris agglutinin-reactive alpha-fetoprotein (AFP-L3) is a variant of alpha-fetoprotein with alpha1-->6 fucose appended to the reducing terminal N-acetylglucosamine. It is the most prominent AFP detected in the serum of patients with hepatocellular carcinoma. METHODS: We investigated microheterogeneities of the carbohydrate chain on AFP in fetal liver tissues, amniotic fluids and maternal sera obtained from pregnancies with Down's syndrome using lectin affinity electrophoresis with four lectins. The percentages of AFP-L3 in maternal sera from 22 Down's syndrome and 227 unaffected pregnancies were determined. RESULTS: Unlike the case with AFP concentration, the percentage of AFP-L3 in maternal serum and amniotic fluid was similar, and apparently not influenced by membrane permeability. Knowing the percentage of AFP-L3 in maternal serum was effective for discriminating between Down's syndrome-affected pregnancies and unaffected pregnancies. The percentage of AFP-L3 in maternal serum identified 55% of Down's syndrome cases with a 5% false-positive rate. CONCLUSIONS: AFP-L3 should be an effective replacement for AFP in prenatal Down's syndrome screening.     

Twins discordant for Down's syndrome

An anterior twin with trisomy 21 at the time of amniocentesis was posterior at the termination of the pregnancy. Reversal of position must be considered in twin fetuses discordant for a genetic disorder.