Down's syndrome screening in twin pregnancies by nuchal translucency measurement: Current concept

During the last 2 decades various non-invasive Down syndrome (DS) screening methods were introduced in clinical practice. However, specific problems were encountered when these methods were applied for twin pregnancies. The current review aims to explore the problematic issue of antenatal DS screening in twins. The implication and the adjusted management in the attempt to achieve the best evaluation for this type of gestation is discussed. Overall more women with twin pregnancies (mainly those who conceived via assisted reproduction) are found false positive for DS. This is because the standard screening algorithms include maternal age. In addition, mid-trimester maternal serum screening is associated with a higher false positive rate; secondary to changes in the feto-placental endocrinologic metabolism in assisted conception pregnancies. Therefore, in twins mid-trimester maternal serum screening is of limited clinical value. In those pregnancies, DS screening by means of nuchal translucency measurements at 10-14 weeks is associated with a lower false positive rate than mid-trimester serum screening. In addition, this screening method enables to specifically identify those fetuses at risk of DS and other anomalies, and thus contributes to a better outcome.