God designed the lunar cycle or synodic month to equal about 29.5 days. This is the period from new moon to new moon or full moon to full moon. This is quite close to the average woman's menstrual cycle of 28 days. If a woman started her period last month with a full moon, for example and the next full moon came without a period she was quite probably pregnant.
I think that I will have to defer to a quote that I found on The Straight Dope.
The smart money says it's coincidence.Science and the Paranormal (1983), astronomer George O. Abell writes, "The moon's cycle of phases is 29.53 days, while the human female menstrual cycle averages 28 days (although it varies among women and from time to time with individual women); this is hardly even a good coincidence! The corresponding estrus cycles of some other mammals are 28 days for opossums, 11 days for guinea pigs, 16 to 17 days for sheep, 20 to 22 days for sows, 21 days for cows and mares, 24 to 26 days for macaque monkeys, 37 days for chimpanzees, and only 5 days for rats and mice. One could argue, I suppose, that the human female, being more intelligent and perhaps aware of her environment, adapted to a cycle close to that of the moon, while lower animals did not. But then the 28-day period for the opossum must be a coincidence, and if it is a coincidence for opossums, why not for humans?"
With that said I am really glad that the human menstrual cycle and the rats are not the same!