Seasonal behavior of human menstrual cycles: a biometric investigation
Sundararaj, N.; Chern, M.; Gatewood, L.; Hickman, L.; McHugh, R.
Human Biology 50(1): 15-31
1978
ISSN/ISBN: 0018-7143 PMID: 649101 DOI: 10.2307/41463034Document Number: 452514
Menstrual histories representing 38,194 woman yr were analyzed for seasonal periodicities using a simple additive model incorporating a secular trend. A recurrent sinusoidal pattern for cycle length was found which varied by 0.08 days throughout the yr, decreasing during the warmer days of spring and summer and increasing in the fall. Separate analyses of cycle variability and numbers of cycles indicated similar results but without as clear an effect. Although these variations were statistically different from random deviations occurring during the year, they were not large enough to indicate any physiological relationship.