IngentaConnect Seasonal variation of livebirths, stillbirths, extramarital birth...

Abstract:

A study was made of the seasonal variation in all births, and births according to maritalstatus, multiplicity and birth status (live and still) in Switzerland recorded between1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses arebased on rates. When comparing the seasonality in data sets showing markedly differentlevels, standardised indices were used. Assuming the length of pregnancies with twins tobe about one month shorter than for pregnancies with singletons, lagged twinning rateswere calculated but, in comparison with actual twinning rates, the general seasonalvariation remained. Therefore, this study was based on actual twinning rates. A monotonicincrease in the amplitude of the seasonal variation in general births was noted for theperiod 1876-1930, with strong seasonal variation holding for 1921-1980. Afterthat, a marked decline in the amplitude can be observed. Seasonality of both all birthsand twin maternities showed very similar pattern for the periods 1876-1930 and1969-1990, with maxima in the spring (March-May) and troughs in late autumn(October-December). Twin maternities showed a strong seasonality for the period1876-1930, being about 20% higher in March than in October. The twinning rate inthe period 1876-1930 was about 2.6 per thousand units higher than in the period1969-90. For twin maternities there was also a stronger seasonal variation duringthe earlier period than during the later one. The pattern of the seasonal variation forextramarital births, showing a maximum in February (conceptions in May-June) and aminimum in August (conceptions in November-December) with a difference of no lessthan 24% was more marked than for the marital births. It seems likely that thisseasonality of extra-marital maternities was due mainly to seasonal variation ofcoital rates and multiple ovulation in the early summer months coinciding with optima oflight, temperature and food supply. A strong reduction in the rate of stillbirths(gestational age more than 29 weeks) was observed during the twentieth century. Thestillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 inthe 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonalrhythm was noticed throughout the period with high stillbirth rates among births aroundMarch and low rates during the summer and autumn.

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Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland