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  • Title: Changes in seasonality of birth rates in Germany from 1951 to 1990.
    Author: Lerchl A, Simoni M, Nieschlag E.
    Journal: Naturwissenschaften; 1993 Nov; 80(11):516-8. PubMed ID: 8264806.
    Abstract:
    Annual birth rates have been found to be affected up to 10-20% by seasonal variation. Variations affecting birth patterns are attributed to biological and social factors: annual rhythms of climatic conditions, or the photoperiod and varying sexual activity. A recent study placed greater priority on biological factors as responsible for seasonal variation: annual variations in sperm quality, serum levels of luteinizing hormone and testosterone in men, and ovulation variations in women by time of day. Seasonal variations in birth rates were found in Munster, Germany, between 1890-99, 1965-74, and 1981-90. Further examination was made to ascertain whether the variations pertained to the entire country of Germany. Data were obtained on monthly birth rates from 1951 to 1990 from the states of the former West Germany. Annual means were calculated and the monthly deviations from the mean computed (centered moving average with a window size of 13 months), as well as monthly means within 5-year intervals. Yearly cosine functions with 2 harmonics were also fitted to the data. The results showed maximum births in the first half of the year between 1951-75 and later months in the following years until stabilization after 1981. The amplitudes of the rhythms declined from 1972 until stabilizing and increasing after 1978. The annual patterns were smooth and stable with peaks in February and March, and a "shoulder" peak in September, which conforms to long suspected seasonal conceptions in December. The annual rhythms appear to support biologic seasonal variation until 1970. The shift after 1970 to conceptions in the winter months for Germany has also been observed with US data since 1950, and in southern hemisphere countries. The explanation may be environmental changes. The workplace environment has replaced the photoperiod influences and temperature fluctuations, but the shift by 6 months would not support this deseasonalization hypothesis. It is suggested that social reasons have affected the shift, such as the enactment of the liberal abortion law in 1976 and the pregnant woman employment protection act of 1968.
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