FACTORS INFLUENCING THE DECLINE OF TWINNING INCIDENCE IN A SOUTHEASTERN BRAZILIAN POPULATION

The twinning incidence from 1925 onwards was investigated in Campinas, State of Sao Paulo, Brazil. This study has shown that the decline of the twinning rate is a phenomenon that started in this city much befor e the introduction of oral anovulatories in the sixties. The decline i n twinning from 1925 to 1965 (42%) seems to be due mostly to a strikin g decrease (60%) in the proportion of Negroid parturients, together wi th the decline of parity (9%) of Caucasoid women. Maternal age did not influence twinning incidence from 1925 to 1965. The decline of the in cidence of twin births after 1965 in Campinas was influenced by the de crease of parity, of maternal age and of the prevalence of Negroid wom en in the population.