[Decrease in twin births in the german federal republic (author's transl)].

The decrease in the frequency of twin births depends solely on the decrease of dizygotic twins (ZZ). Examination of birth registration forms in Baden-Würtemberg from 1955 to 1972 showed that the decrease of ZZ births is only partially due to changes in maternal age and parity. On the other hand one can assume that the proportion of women with genetically determined increased fertility, to whom ZZ mothers largely belong, is reduced among mothers due to widespread contraceptive practice so that ZZ births are also rarer. In addition, the possibility that ovulation inhibitors may have a negative influence on the numbers of double ovulations must also be considered.