Early first-trimester free β-hCG and PAPP-A serum distributions in monochorionic and dichorionic twins

Background In the Netherlands, prenatal screening for trisomy 21 in the first trimester of pregnancy for singletons is conducted through a combined test based on maternal age, nuchal translucency measurement and maternal serum free β‐hCG and PAPP‐A. In our clinic risk calculations in twins are currently based on the NT of both fetuses instead of the combined test. In this study we looked at differences in early first‐trimester free β‐hCG and PAPP‐A between mono‐ and dichorionic twins. Methods A total of 202 twin pregnant women participated in the study and agreed to donate first‐trimester serum for research. Results The data of 200 twins with normal outcome were used for setting up reference values for free β‐hCG and PAPP‐A. Trisomy 21 was identified in the two remaining pregnancies. The overall median weight‐corrected MoM was 1.99 for free β‐hCG, and 2.14 for PAPP‐A in all twins. Monochorionic twins have a significantly lower free β‐hCG weight‐corrected MoM (1.53 vs 2.11; Mann‐Whitney U, p = 0.002) and a significantly lower PAPP‐A weight‐corrected MoM (1.59 vs 2.40; Mann‐Whitney U, p = 0.003) compared to dichorionic. Conclusion This study strengthened the need to make a distinction between mono‐ and dichorionic twins for the risk calculation in Down syndrome screening as biochemical markers are significantly lower in monochorionic than in dichorionic twins. Copyright © 2009 John Wiley & Sons, Ltd.