[Seasonal fluctuations in the sex ratio of new borns and the influence of the declining birth rate on the sex ratio (author's transl)].

Seasonal fluctuations in the sex ratio of new borns are investigated. The largest predominance of male deliveries over female deliveries was found to be during the conception months May-June with an almost constant number of deliveries. The highest value was in June. Seasonal maximum of the male to female sex ratio is concomitant to the seasonal maximum of conceptions. The investigation of the yearly sex ratio from 1958-1978 showed changes in the secondary sex ratio concomitant to the increase or decrease of the delivery rate. When the delivery rate decreased below 10 deliveries by 1000 population a reverse in the sex ratio was noted and the lowest number of male deliveries was found. This occurred although the number of first deliveries increased further with the decrease of multiparities. The reasons for this reverse trend in the sex ratio are discussed and a possible relationship to preceeding oral contraception is mentioned.