Trends in monozygotic and dizygotic twinning rates by maternal age and parity. Further analysis of Italian data, 1949-1985, and rediscussion of US data, 1964-1985.

The dizygotic twinning rate in Italy, adjusted for maternal age and parity, declined from 8.9/1000 in 1949-1953 to 6.0/1000 in 1978-81, but may have stabilized or recovered slightly in the early 1980s. The long decline was greatest in the oldest mothers, and the recent increase, if any, has been principally in first births and in women under 30. The monozygotic rate rose from 3.8/1000 in 1969-73 to 4.3/1000 in 1982-85, having been relatively stable prior to that period. No good explanation for these trends is apparent.