Maternal and environmental factors affecting twin births in Canadian cities.

Details of 4972 pairs of twins born to mothers resident in 14 large Canadian cities in the years 1952, 1957, 1962, and 1967 were compared to those of a random sample of 4969 single births. Both dizygotic and monozygotic twinning rates, derived by Weinberg's formula, showed increases with both maternal age and with the number of previous livebirths. Dizygotic twinning rates also showed a seasonal variation with an October maximum. Twinning rates were lower in Oriental women, but did not vary between other ethnic groups, or by city of residence, legitimacy, paternal age or maternal history of stillbirths.