Objectives: We studied the effects of twins and triplets on perinatal health indicators in the overall population in the 1980s and 1990s in Canada, England and Wales, France, and the United States.
Methods: Data were derived mostly from live birth registration. We used rates, relative risks, and population attributable risks for twins and triplets separately.
Results: In each country, the increase in multiple births, and the increase in preterm delivery among multiple births, contributed almost equally to the rise in or stabilization of the overall rates of preterm delivery. Twins contributed a much larger proportion of the preterm deliveries and low-birthweight newborns than did triplets.
Conclusions: Twins have a major population-based impact on the trends of perinatal health indicators.