The question, whether the seasonal variation of the birth frequencies are changing with the increase of birthcontrol practice in the population is subject of the present study. The changes of the relevant demographic parameters in this context are discussed first. The study is based on the official monthly records of live- and stillbirths from 1826 to 1979 in Land Bremen (an urban district of northern Germany). For analysis, the material is standardized to equal month length. Trend effects were eliminated using the mean curve. To eliminate stochastic effects, the material is summarized using intervals formed analog to the changes in the birth rate. The means of these intervals illustrate two typical figures; the first is characteristic for the period prior to 1905, the second is characteristic for the years following (excluding the periods from 1915 to 1922 and from 1973 to 1978). The transition of these figures occurred at the change of the century. It is regarded in context with the beginning reduction of birth rate at this time. Whereas for the 19th Century, the seasonal changes, particularly from winter to spring, are regarded as a main cause of physiologic adaptation followed by stimulation of the gonadal gland, "sociologic factors" are taken into account for the seasonality of the birth rates in the 20th Century.