A retrospective study was carried out on a sample consisting of 89,491 deliveries occurred from 1979 through 1998, in the city of São Paulo, SP, Brazil. These deliveries included 935 twin and 26 triplet births. The mean incidence per 1,000 deliveries was estimated at 10.45 for twins (5.34 for dizygotic and 5.11 for monozygotic twins) and 0.29 for triplets. The average maternal age increased significantly during this period for both twins and singletons. The temporal variation in years significantly influenced the increase of monozygotic twins and of triplets. Temporal variation and maternal age significantly influenced the dizygotic twinning rate increase. An increased proportion of monozygotic twins born to mothers aged 31 - 35 years was also detected.
Dizygotic twinning rate; monozygotic twinning rate; maternal age; gestational order; gestational age
