Down syndrome risk calculation for a twin fetus taking account of the nuchal translucency in the co-twin

Development of prenatal screening--A historical overview

The first prenatal screening test to be introduced was based on a single maternal serum marker of neural tube defects. Since then various prenatal screening concepts have been developed, the most successful being Down syndrome risk estimation using multiple serum and ultrasound markers. Today a completely new approach to aneuploidy screening is available based on maternal plasma cell-free DNA testing. This has the potential to markedly improve screening performance but routine testing is currently too expensive in a public health setting. However, it can be cost-effective when used in combination with existing multi-maker tests. Some are beginning to broaden prenatal screening to include pregnancy complications such as pre-eclampsia that can be prevented using soluble low-dose aspirin treatment started before 16 weeks of gestation. Prenatal screening for cardiac abnormalities, fragile X syndrome and recessive genetic disorders is underutilized and public health planners should considered a more widespread application of available methods.

Screening and Invasive Testing in Twins

Prenatal screening and testing for trisomy 21 in twin pregnancies poses a number of challenges: the exact estimate of the a priori risk of trisomy 21, the choice of prenatal screening test and/or invasive techniques to employ for the diagnosis and the impact of the result on the options of treatment in case of discordant results within a twin pair or among multiples. These different aspects are discussed below while recognizing that many issues remain unresolved.

Nuchal translucency in twins according to mode of assisted conception and chorionicity

OBJECTIVES

The impact of nuchal translucency (NT) screening in twins conceived after use of assisted reproduction technology (ART) has not been addressed properly in the literature. The aim of the current study was to assess, in a large cohort from a single center, NT in twins following various modes of ART as compared with NT in spontaneously-conceived twins and to differentiate results with regard to chorionicity.

METHODS

Retrospective data from 825 twin pregnancies were available for this study. All pregnant women underwent NT screening and chorionicity determination between 11 + 0 and 13 + 6 weeks' gestation in our center. Information about mode of conception and pregnancy outcome was obtained from hospital records and by telephone interviews. Twins were classified as having been conceived after in-vitro fertilization (IVF) with the woman's own or a donated embryo, after non-IVF ART or spontaneously. NT distribution in subgroups of fetuses was compared on the basis of the multiple of the normal median (MoM), SD of log10 MoM estimated by the 90(th) -10(th) centile range divided by 2.563 and the proportion of NT values exceeding the normal 95(th) centile which was 1.641 MoM in the singletons.

RESULTS

NT values were statistically significantly higher in ART pregnancies as compared with spontaneously-conceived pregnancies (Wilcoxon rank-sum test, P < 0.05) with medians of 1.073 and 1.038 MoM, respectively, but the proportion with values above the normal 95(th) percentile did not differ (chi-square test, P = 0.89). Among the ART twins, NT values were significantly higher for those conceived using IVF methods (P < 0.005), with a median of 1.082 MoM, compared with a median of 1.022 MoM using other methods. However, although a greater proportion exceeded the 95(th) centile (7.3% compared with 4.8%) this difference did not reach statistical significance (P = 0.17). There were no differences in NT values according to chorionicity (Wilcoxon rank-sum test, P = 0.75). The standard deviation of log10 NT was similar for all subgroups of twins. The correlation coefficient between fetuses in ART pregnancies was statistically significantly lower than that in spontaneously-conceived pregnancies (P < 0.05) but no significant differences were found between types of ART or according to chorionicity.

CONCLUSIONS

There was a small but significant increase in NT levels among ART pregnancies, which appeared to be confined to those conceived using IVF, and a small reduction in the correlation between fetuses. These effects probably have little impact on the estimated risk based on algorithms in use today.

Down syndrome screening in assisted conception twins: an iatrogenic medical challenge

OBJECTIVE

The objective of this study was to provide a critical analysis of the impact of assisted conception on prenatal screening for Down syndrome (DS) in twin pregnancies and the value of various screening modalities for early detection of anomalies.

METHODS

The literature was searched using PubMed and the Cochrane Library focusing on prenatal screening and antenatal care of assisted-conception twin pregnancies.

RESULTS

Serum screening alone is of limited value in detecting aneuploid twins, because the unaffected cotwin can "mask" the abnormal serum results of an affected one. In addition, this test can designate the pregnancy as at high risk but not identify the affected fetus. Nuchal translucency (NT) screening is the best available modality and a highly effective screening method for twin pregnancies. Among twins, NT alone has a 69% DS detection rate, first-trimester combined NT and serum biochemistry has a 72% DS detection rate, and an integrated screen will have an 80% DS detection rate at a 5% FPR. The data in the literature concerning the effect of assisted conception on maternal serum screening markers in twin pregnancies are scarce.

CONCLUSIONS

Down syndrome screening in assisted-conception twins presents clinical and technical challenges. Therefore, assisted-conception twins need close monitoring from conception to delivery, by a practitioner familiar with the available screening modalities and their relative accuracy.

Prenatal Screening Using Maternal Markers

Maternal markers are widely used to screen for fetal neural tube defects (NTDs), chromosomal abnormalities and cardiac defects. Some are beginning to broaden prenatal screening to include pregnancy complications such as pre-eclampsia. The methods initially developed for NTDs using a single marker have since been built upon to develop high performance multi-maker tests for chromosomal abnormalities. Although cell-free DNA testing is still too expensive to be considered for routine application in public health settings, it can be cost-effective when used in combination with existing multi-maker marker tests. The established screening methods can be readily applied in the first trimester to identify pregnancies at high risk of pre-eclampsia and offer prevention though aspirin treatment. Prenatal screening for fragile X syndrome might be adopted more widely if the test was to be framed as a form of maternal marker screening.