Secular changes in the twinning rate in Denmark 1931 to 1977.

The present study examines secular changes in the rate of twinning in Denmark during the period of 1931-77. Total twinning (i.e., mono- and dizygotic twinning combined) declined from a crude rate of 15.0 twin births per 100 deliveries in 1931 to 9.6 twin births per 100 deliveries at the end of the period. After direct adjustment for maternal age and parity (between the years 1935 and 1965), a 22% decline in total twinning rates was noted. Estimates of the rates of dizygotic twinning over the same period, again adjusted for maternal age and parity, show a decline of 20%. Thus there is evidence of a real decline in twinning, independent of the effects of changing patterns of maternal age and parity. These findings are consistent with reports of a general decline in twinning rates from a number of other countries. The interpretation of such secular changes is a matter for speculation.