The secular decline in dizygotic twinning rates in italy.

It is shown that the recent secular decline in DZ twinning rates in Italy was not solely due to secular changes in birth order or maternal age. The extent of the decline differed in different maternal age-parity categories. It seemed to be greater in younger women than older ones for constant parity. The data gave no clear support to the suggestion that the decline is associated with social class. It seems that this secular decline in Italy has not been greatly affected by the use of ovulation-inducing drugs.