Fetal and infant death in mono- and dizygotic twins in England and Wales 1982-91

AIM: To quantify the level of risk for stillbirth and infant death in singleton compared with twin pregnancies, using national data; to determine the independent effects of zygosity, sex, and birthweight on these risks in twin pregnancies. METHODS: A retrospective national study was carried out of all singleton and twin birth and death registrations in England and Wales 1982-91, according to sex and birthweight group. Weinberg's rule was applied to the twin pairs to differentiate mono- from dizygotic twins. Relative risks for mono- compared with dizygous twins for both twins being stillbirths and for one of the pair being a stillbirth were determined. For twins where one was stillborn and the other live born, the relative risk of neonatal and infant mortality in the surviving co-twin was determined. RESULTS: There were 6 563 834 registered singletons and 70772 registered twin pairs for the period under study. Monozygotic twins had a relative risk of: 18.91 (95% CI 12.48-28.64) for both twins being stillborn; 1.63 (95% CI 1.48-1.79) for one twin being a stillbirth; and 2.26 (95% CI 1.45-3.52) for the live born co-twin dying as a neonate. When both twins were live born and among singletons, the odds ratio for neonatal mortality of being male was 1.41 (95% CI 1.37-1.45) and there was a highly significant negative association with birthweight. After adjusting for birthweight group and sex, twins had a reduced neonatal mortality compared with singletons: odds ratio 0.91 (95% CI 0.85-0.96). CONCLUSIONS: Fetal death in one of monozygotic twins has serious implications for survival of the co-twin. Monochorionicity is probably the essential feature of the increased risk to the co-twin. It is imperative that all fetal deaths in multiple pregnancies are recorded and chorionicity determined if parents are to be adequately counselled.