Normal variation in the length of the follicular phase of the menstrual cycle: effect of chronological age.

Normal probability plots were used to analyse the distribution of follicular phase length in a population of 293 apparently ovulatory menstrual cycles from women aged between 18 and 39 years. The length of the follicular phase was defined as the interval (in days) from the onset of menstruation up to, but not including, the day of the LH peak. Follicular phase length appeared to be log-normally distributed and graphical inspection of the probability plot suggested that the geometric mean (and 95% confidence limits) of follicular phase length in this study group was 12.9 (10.3 to 16.3) days. There was a significant decrease (P less than 0.001) in follicular phase length with chronological age, from 14.2 days in women aged 18-24 years to 10.4 days in women aged 40-44 years.