As an isolated marker, NT detects approximately two thirds of fetuses with Down syndrome, with a false-positive rate of 5 percent (Malone, 2005b). However, NT is generally used as an isolated marker only in screening for multifetal gestations, in which serum screening is less accurate or may not be available. he NT distribution is similar in twins and singletons (Cleary-Goldman, 2005). In twin pregnancies, serum free 3-hCG and PAPP-A levels are approximately double the singleton values (Vink, 2012). Even with speciic curves, a normal dichorionic cotwin will tend to normalize screening results, and thus, the aneuploidy detection rate is at least 15-percent lower (Bush, 2005).
Maternal age afects the performance of first-trimester aneuploidy screening. Prospective trials have demonstrated Down syndrome detection rates of 67 to 75 percent in women younger than 35 years at delivery, which are 10 percent lower than the overall detection rates in these studies (Malone, 2005b; Wapner, 2003). Among women older than 35 at delivery, Down syndrome detection reached 90 to 95 percent, albeit at a higher false-positive rate of 15 to 22 percent.
Unexplained Abnormalities of First-Trimester AnaIytes. There is a signiicant association between serum PAPPA levels below the 5th percentile and preterm birth, growth restriction, preeclampsia, and fetal demise (Cignini, 2016; Dugof, 2004; Jellife-Pawlowski, 2015). Similarly, low levels of free 3-hCG have been associated with fetal demise (Goetzl, 2004). he sensitivity and positive-predictive values of isolated markers are generally too low to make them clinically useful as screening tests.
There has been renewed interest in low-dose aspirin for prevention of early preeclampsia in women identified as at risk based on mean arterial pressure, uterine artery Doppler values, and PAPP-A levels. However, these observations are preliminary (Park, 2015).