Journal Article

The effect of age on time until ovulation in female menstrual cycles. A longitudinal data analysis based on mixed models

Abstract

We study the effect of age on the number of days until ovulation in female menstrual cycles. This is of specific relevance to daily fecundability studies, and the literature has so far investigated the problem either from a cross-sectional point of view or within a time-series framework. We re-analyse here some data collected by Marshall, with several cycles for each woman. We also use a Gaussian linear mixed model, showing that the length of the pre-ovulatory phase declines with age and that at ages close to menopause there is some extra variability due to exceptionally long cycles. When one disregards such cycles, the variability within women is almost constant.