First trimester aneuploidy screening in the presence of a vanishing twin: implications for maternal serum markers

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Abstract
Objective: To assess the impact of a vanishing twin on the levels of the biochemical markers used in the first trimester aneuploidy screening.Methods: A retrospective analysis of free β‐hCG and PAPP‐A levels in 270 women with a normal singleton fetus with ultrasound evidence of a vanishing twin pregnancy. Marker levels (as MoM) were compared in three groups—76 women with a second empty gestational sac, 194 women with a second gestational sac containing a dead fetus with a measurable crown rump length (CRL), and 1360 matched singleton pregnancies.Results: In women with a second empty gestational sac, the median free β‐hCG and PAPP‐A MoMs (0.968 and 1.040, respectively) were not significantly different from the 1.0 MoM in singleton pregnancies. In the group with a vanished twin with a measurable—CRL—there was a significantly increased median PAPP‐A MoM (1.317) but the median free β‐hCG MoM was not changed (1.024). Modelling this bias in PAPP‐A MoM the detection rate for trisomy 21 would fall from 85 to 75%.Conclusion: First trimester screening in the presence of a vanishing twin may lead to errors in risk estimation. In such circumstances it may be advisable to restrict screening to the use of nuchal translucency (NT) alone. Copyright © 2009 John Wiley & Sons, Ltd.

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