Background: Based on research of relatively poor quality, it is generally believed that dichorionic twins have lower perinatal mortality than monochorionic twins. We assessed the relationship between the pattern of perinatal loss in twin pregnancy and chorionicity.
Methods: A cohort study of 238 consecutive sets of twin pregnancies registered in our antenatal service over a 2-year period (1996-98) had chorionicity determined by ultrasound at 12-16 weeks' gestation. Follow up included scanning at 20 weeks' gestation for anomaly, and at 2-3-weekly intervals from 24 weeks' gestation onwards for growth and well being. Hazard ratios were computed for comparing the risk of death according to chorionicity. The perinatal loss patterns were analyzed according to gestational age at demize and that at delivery using survival analysis.
Results: Overall, 17/238 (7.1%) twin pregnancies suffered mortality: 14/190 (7.3%) amongst the dichorionic and 3/48 (6.2%) amongst the monochorionic twins. The hazard ratio for mortality was 0.89 (95% confidence interval 0.27-2.97, p = 0.85), considering gestational age at demize as the outcome. For gestational age at delivery as the outcome, the hazard ratio for mortality was 0.93 (95% confidence interval 0.27-3.15, p = 0.91). Survival analysis showed that amongst the dichorionic twins the hazard of death continued to rise throughout gestation. In contrast, the hazard of death for the monochorionic twins rose gradually to a maximum at 28 weeks' gestation and was then constant.
Conclusion: Chorionicity did not affect the overall fetal loss rate amongst the twin pregnancies in our setting. There were differences in patterns of loss according to chorionicity, which require further investigation.