Early first-trimester free beta-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 beta-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 beta-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 beta-hCG and PAPP-A. Trisomy 21 was identified in the two remaining pregnancies. The overall median weight-corrected MoM was 1.99 for free beta-hCG, and 2.14 for PAPP-A in all twins. Monochorionic twins have a significantly lower free beta-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.