A statistical analysis of the annual pattern in white births of maternal age groups in the United States for the years 1973 thru 1980

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Abstract
The annual trend of white births of maternal age groups in the United States was analyzed for the years 1973 thru 1980. Monthly means for each age group were subjected to time series analysis with sine curves, using a stepwise, forward‐going regression analysis. The births in the maternal age groups also were compared with an analysis of variance in order to detect differences in trend. Adjacent age groups exhibited different annual trends. The general pattern of births for the period of 12 months was bimodal. The youngest and oldest maternal groups exhibited approximately equal peaks. In the middle age groups the bimodal peaks were unequal, with a major peak in September and a minor peak in February or March. Other smaller differences were present that characterized each of the eight maternal age groups, and minor changes in the annual trends occurred within the period of the study for three of the eight age groups. The relevance of maternal age groups to the peak shift from August to September that occurred in the 1940's was determined: because some maternal age groups exhibited peaks in August and others in September, their effect on the peak shift was estimated and found to be of an order of magnitude to account for the shift. The inadequacy of single sine curve representations of the data were demonstrated and the so‐called “cosinor”; method criticized. A brief consideration of the cause of the annual trends in births emphasized the heterogeneity of the USA population and suggested that homogeneous subgroups may be available and useful. The existence of different annual trends in adjacent age groups may indicate a new category of causative factors, the maturational stage of the response system.