Stillbirths in singletons, dichorionic and monochorionic twins: a comparison of risks and causes.

Objectives: To estimate the risk of stillbirth in dichorionic and monochorionic twins compared with singletons, and to evaluate the relevant causes of stillbirth in each group.
Study Design: A retrospective cohort analysis of all pregnancies ≥22 weeks of gestation was performed at a tertiary care center from January 1995 to June 2011. The overall fetal survival and the prospective risk of stillbirth were compared in monochorionic diamniotic (MCDA) twins, dichorionic diamniotic (DCDA) twins, and singletons. Causes of stillbirth were classified using the ReCoDe classification and were compared among the three study groups.
Results: A total of 46,200 singletons, 462 MCDA twins and 1108 DCDA twins were included in the study. Both Kaplan-Meier analysis and prospective risk calculation showed that MCDA twins had the highest risk of stillbirth (OR ranging between 13.5 95% CI 8.7-20.7 at 22.0-24.6 weeks and 4.0 95% CI 1.1-13.1 at 31.0-33.6 weeks, compared to singletons), while singletons had the lowest. Main causes of stillbirth were major congenital malformations in singletons (25.1%) and in DCDA twins (75%), and twin-twin transfusion syndrome in MCDA twins (81.5%). When excluding fetuses affected by major congenital anomalies, MCDA twins (p<0.001) but not DCDA twins (p=0.2) remained at increased risk for stillbirth compared with singletons.
Conclusion: The risk of stillbirth is significantly higher both in MCDA and DCDA twins compared with singletons. Stillbirths are mainly due to twin-twin transfusion syndrome in MCDA twins and major congenital anomalies in DCDA twins. When major congenital anomalies are excluded, DCDA twins have a similar in utero mortality to singletons.
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