Seasonal variation in conception does not appear to influence the rate of prenatal diagnosis of nondisjunction.

A seasonal effect on ovulation patterns and conception rates is well documented in animal studies. The etiology of reported variation in human fertility is, however, uncertain. Abnormal ovulation has been implicated in the etiology of aneuploid conceptions, many of which abort spontaneously. To investigate the possibility that the frequency of aneuploidy reflects seasonal variation in ovulation patterns, we analyzed 5,292 consecutive prenatal karyotypes obtained from 1985 to 1987. Karyotypes were logged according to month and year of conception. Results were evaluated for cyclicity using frequency spectrum and time series analysis. Variation in the number of prenatal diagnostic procedures corresponded to known peaks in conception and delivery rates. No significant monthly or quarterly trend in the total rate of trisomies or in the rate of trisomy 21 was demonstrated. Although the sample size in this study may not be adequate to detect minor effects, seasonal variation does not appear to be a major determinant of nondisjunction rates in the population referred for prenatal diagnosis.