First Trimester Screening in Multiple Pregnancy

There are challenges for aneuploidy screening in multifetal gestations and no method of screening for aneuploidy in twins is as accurate as in singleton pregnancies. In addition to limitations associated with screening tests, diagnostic testing in twins seems to confer a higher risk of loss. NT can provide a fetus-specific risk of aneuploidy. This is beneficial in multiple pregnancies, particularly in higher-order multiples for which there is currently no validated aneuploidy screening test. Similar to NT, the presence or absence of the fetal nasal bone (NB) is specific to each twin. The detection rate for Down syndrome in twin pregnancies can be increased by combining maternal age and NT with maternal serum analysis, although it is important that chorionicity be taken into consideration. Cell-free DNA screening can be performed in twin pregnancies. Overall, performance of screening for trisomy 21 by cell-free DNA in twin pregnancies is encouraging, but the total number of reported affected cases is small. Because maternal serum screening has not been adequately studied in higher order multiple and there are limited data regarding the utility of cell free DNA screening in multiple pregnancies, NT screening is still worthwhile for identifying a potentially abnormal fetus in a multiple gestation. Key words: NT, cell-free DNA, aneuploidy screening