First trimester risk assessment for trisomy 21 in twin pregnancies combining nuchal translucency and first trimester biochemical markers

Abstract

The combined test in twins appears to be a good method for Down syndrome screening with a high detection rate and an acceptable false-positive rate.

First-trimester Ultrasound and Aneuploidy Screening in Multifetal Pregnancies

“…When differentiating by chorionicity, this series showed that the combined first-trimester test allowed for detection of all the cases of Down syndrome in the 554 fetuses evaluated, with an FPR of 6.4% in dichorionic twins and 3.8% in monochorionic twins. Similar results were obtained by Prats et al, 35 who showed reductions in the FPR to 5.7% in dichorionic and 4.4% in monochorionic twins, especially in those with advanced maternal age. The improved performance characteristics in monochorionic twins is expected, as both fetuses are affected and the abnormal analyte values trend in the same direction, whereas in dichorionic twins there may be discordance.…”

“…In addition, some centers adjust for chorionicity, as PAPP-A and free B-hCG levels are lower in monochorionic twins than dichorionic twins. 35,41,42 However, in chromosomally discordant twins, usually dichorionic, an unaffected twin can mask the presence of an affected twin with this calculation, making the tests less sensitive. Therefore, using a combination of NT measurements and first-trimester biomarkers appears to be more accurate in diagnosing aneuploidy rates in twins than using biochemical markers alone, reaching detection rates of 75% to 90%.…”

“…This approach has a sensitivity equivalent to that of singletons, estimated to be over 70%. 2,3,34,35 In addition, it can be performed in triplets and high-order multiples. Although evidence supporting this practice is limited, a retrospective study of 769 monochorionic twin gestations including 6 cases of trisomy 21 reported that using the average NT of monochorionic twins, rather than the larger or smaller NT, resulted in an estimated 100% detection with a 4.2% FPR for trisomy 21.…”

“…35 Therefore, there is disagreement as to whether serum markers should be adjusted for mode of conception. As assisted reproduction comprises a large proportion of multiple pregnancies, the use of adjustment factors based on mode of conception in this population may improve biochemical risk assessment performance characteristics.…”

“…35,39 FIRST-TRIMESTER SCREENING First-trimester aneuploidy screening combining NT with serum markers (free B-hCG and PAPP-A) is well established in singleton pregnancies, with a detection rate of 82% for Down syndrome with a 5% FPR. 40 In twins, however, maternal serum markers are approximately twice as high as in singletons 41 and therefore the calculated MoM needs to account for 2 fetuses.…”