Does a “vanishing twin” affect first-trimester biochemistry in Down syndrome risk assessment?

Objective The purpose of this study was to evaluate the impact of spontaneous reduction in multifetal pregnancy on first-trimester maternal serum biochemistry. Study design We evaluated first-trimester pregnancy associated plasma protein-A (PAPP-A) and free β-human chorionic gonadotropin levels in singleton euploid pregnancies. Biochemical values in pregnancies with evidence of spontaneous reduction were compared to other singleton pregnancies. Mann-Whitney U, Student t test, Fisher exact test, and logistic regression analysis were used for statistical comparison. Results There were 41 cases (0.9%) of spontaneous reduction. Though spontaneous reduction was not associated with different levels of either analyte, reduction within 4 weeks was associated with higher levels of both PAPP-A (1.79 vs 1.18; P = .002) and free β-hCG (1.28 vs 0.96; P = .03) compared with other pregnancies. Spontaneous reduction was associated with a higher frequency of PAPP-A >95th %ile (17.1 vs 4.7%; P = .003) and free β-hCG >95th %ile (17.1% vs 5.0%; P = .004). Logistic regression identified independent associations between spontaneous reduction and both high PAPP-A and high free β-hCG. Conclusion Recent spontaneous reduction is associated with higher values of PAPP-A and free β-hCG. These differences have the potential to affect risk assessment for fetal aneuploidy.