Risk factors for multiple births

Study objective: To analyze risk factors for multiple births. n: A case-control study. Cases were 103 women (median age 31 years, range 20-44) who delivered multiple births not related with treatment for infertility at the 'Clinica Luigi Mangiagalli' of Milan. A total of 27 women delivered monozygotic twins and 76 dizygotic ones. Controls were 308 women (median age 30, range 17-45) admitted for normal delivery on selected days to the same clinic where cases had been identified. Results: Cases tended to be less educated than controls and the relative risk of multiple pregnancy, compared with women reporting seven years of schooling or less, was 0.4 in those reporting 7-11 and 12 or more years of education. When the analysis was done separately for dizygotic and monozygotic multiple pregnancies, this relationship was restricted to dizygotic multiple pregnancies (chi1(2), trend for dizygotic pregnancies = 3.82, p = 0.05). A family history of multiple pregnancies was reported in 36 women (48%) with dizygotic multiple pregnancy and 13 (52%) monozygotic ones and 88 (30%) controls. The corresponding relative risks (RR) were 2.2 and 2.5 respectively for dizygotic and monozygotic pregnancies. Compared with nulliparae, the estimated RR of dizygotic multiple pregnancies was 0.5 in women reporting two or more births, but the trend in risk with number of births was not statistically significant. No relationship emerged with spontaneous or induced abortions, body mass index, oral contraceptive or IUD use, age at menarche and risk of multiple pregnancies. Conclusions: This study confirms the role of familiarity in the risk of multiple pregnancies and suggests some different epidemiological characteristics in dizygotic and monozygotic multiple pregnancies