In general a high activity level combined with a diet not based on refined
carbs yeailds lower body fat levels than what we see in modern humans.
During a normal menstral cycle estrogen stimulates the growth of epithelium
cells.  Mid way through the cycle progesterone concentration increases (as
does est.) which primes the endometrium for either pregnancy or
menstruation.  If estrogen levels are abnormaly elevated due to excessive
body fat the cycle will not only extend in duration but will also repeat
more quickly effectively increasing the total time of fertility.  This
estrogen effect is seen in men as well when excessive body fat can
contribute to andropause and the development of secondary female sexual
characteristics.  If you have a physiology/endocrinology book handy you can
find a more indepth description of the normal cycle.
Robert