[A decrease in twin births and factors involved in multiple pregnancy]

The object of this investigation were the causes of the decrease of the frequency of twin births during the last two decades. In Moscow this frequency decreased from 1,19% in 1956 to 0,7% in 1973. Among the births the proportion of first births was observed to increase, as well as the proportion of very young mothers. The effect of this process on the women characterized by a tendency to polyembryony was more conspicuous, than in the average on the population as a whole. During the last decades the process of acceleration of puberty was observed both in the mothers of twins and in all the women in general. However a somewhat later onset of first menstruations is characteristic of the women having a tendency to polyembryony-Hormonal changes taking place in the course of acceleration are apparently one of the causes of the decrease of the twin births frequency.