Methods of screening and prenatal diagnosid in twins

Prenatal screening and testing for trisomy 21 in twin pregnancies poses a number of challenges:  the exact estimate of the prior 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.
The evaluation of the prior risk of trisomy 21 depends on the number of foetuses per pregnancy, on the gestational age and on the zigosity-chorionicity. A challenge in screening and diagnosis can include the underestimation of an ongoing twin pregnancy (“the appearing twin”) or the misdiagnosis of an ongoing singleton pregnancy as one that started as a twin pregnancy or more (“the vanishing twin” phenomenon). These two circumstances could affect the outcome of screening test so they are important to detect. The assessment of chorionicity is equally important in order to prepare the following tests and diagnosis and is fundamental for determining zigosity. The evaluation of chorionicity could be performed invasively, by direct collection of foetal cells, and by non invasive methods that include ultrasound evaluation (fetal sex), and, as recent studies suggest, maternal plasma DNA sequencing.
In twin monozygotic pregnancies, the risk of both foetuses being affected is similar to the maternal-age risk, while the risk of only one foetus being affected is virtually null. Therefore, in monozygotic pregnancies, the risk could be calculated per pregnancy. In dizygotic pregnancies, the risk could be expressed per foetus and/or per pregnancy and special algorithms for calculation have been formulated. However, many issues regarding the estimate of the a priori risk of trisomy 21 in a twin or multiple pregnancy remain unresolved. Ultrasound and biochemical markers for screening in twin pregnancies are different from those in singleton ones. Literature published sofar suggests that monochorionic twins tend to have a higher percentage of increased nuchal translucency  compared to dichorionic twins so the most effective screening  method for trisomy 21 is using the average NT measure of the two foetuses, although others use also the average of the risk calculation in the two foetuses. The use of combined test, with biochemical markers, is not excluded in twins pregnancies although some screening test practice guidelines generally emphasise its low efficiency and that is not as accurate  as desired to enable patients to make appropriate informed decisions about the pregnancy. Non invasive prenatal testing is possible applying the NIPT in twin pregnancy although problem issues may as well arise with twin dizygotic gestation. Invasive prenatal diagnosis in twins has certain peculiarities that are specific to this type of pregnancy and depending on chorionicity.

Anahtar Kelimeler