Menstrual cycle shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days).
Progesterone ovaries, the shutting down (whether by natural or chemical means), or removal, of those inevitably causes a considerable reduction in progesterone levels. Previous concentration upon the role of progestogens in female reproduction, when progesterone was simply considered a "female hormone", obscured the significance of progesterone elsewhere in both sexes. The tendency for progesterone to have a regulatory effect, the presence of progesterone receptors in many types of body tissue, and the pattern of deterioration (or tumor formation) in many of those increasing in later years when progesterone levels have dropped, is prompting widespread research into the potential value of maintaining