Dizygotic Twinning and Fecundity – Is There an Association between a Family History of Twinning and Birth Intervals in Contemporary Sweden?

Martin Kolk, Stockholm University

Researchers have for a long time suspected an association between dizygotic twinning and female fecundity but direct evidence for this relationship is limited. We examine the relationship between parental twinning and own fecundity, using a research design inspired by Alter & Pison 2008. We apply survival analysis models to the transition to 2nd birth in Sweden, a transition characterized by almost universal transitions and birth hazards thus to a large degree depend on female fecundity. We apply Swedish register data of more than 3 million individuals and find a small but significant effect of parental twinning on hazard of 2nd birth in Sweden for women who are not twins themselves. The effect is 50% stronger for opposite sex twins (100% dizygotic) than for same sex twins (50-60% dizygotic), and the effect is non-significant for hazard of 2nd birth for men, which suggests that the association is causal.