Paternal age and twinning in the Jerusalem Perinatal Study.

Objective: To investigate whether incidence of twin deliveries is related to father's age, independently of mother's age, and whether it differs for same-sex or opposite-sex twin sets.

Study Design: In a program of research on effects of paternal age, this study used data from a prospective cohort of 92,408 offspring born in Jerusalem from 1964 to 1976. Of the 91,253 deliveries in the Jerusalem Perinatal Study, 1115 were twin deliveries. The data were analyzed with General Estimate Equations to inform unconditional logistic regression.

Results: After controlling for maternal age, odds ratios (ORs) and 95% confidence intervals (95% CI) associated with father's ages 25-34 and 35+ were 1.3 (1.1, 1.7) and 1.5 (1.2, 2.1) respectively, compared with fathers <25 years old. The effect of maternal age was partly explained by paternal age. The ORs for opposite-sex twin sets and male-male twin sets increased slightly with paternal age, while the OR for same-sex and female-female twin decreased.

Conclusion: Studies of twins are used to estimate effects of genes and environment in a variety of diseases. Our findings highlight the need to consider paternal as well as maternal age when analyzing data on twins to explore etiology of diseases.