Wow, is there some lunacy out there regarding menstruation and the moon! (yeah, bad pun.) There are all sorts of New-Agey websites devoted to fertility and the moon with very few facts but lots of speculation.
As far as actual science, here’s what I found:
From the Bureau of the Census, a 1987 study entitled Investigating Lunar Cycles in Monthly Fertility Rates, which looked at the huge US Census data set-
“In order to test for the presence of full moon effects on birth and conception, deterministic regression components are added to previously developed time series models for monthly U.S. general fertility rates. These time series models included a stochastic ARIMA component and other deterministic components for outliers and calendar effects. The components are estimated jointly using efficient statistical procedures and statistical
tests are carried out to determine the significance of the lunar component. No significant lunar effects are found.”
A summary of several studies found “Scientific studies, however, have failed to find any significant correlation between the full moon and number of births (Kelly and Martens 1994; Martens et al.1988 ). In 1991, Benski and Gerin reported that they had analyzed birthdays of 4,256 babies born in a clinic in France and “found them equally distributed throughout the synodic (phase) lunar cycle” (Kelly, et al. 1996: 19). In 1994, Italian researchers Periti and Biagiotti reported on their study of 7,842 spontaneous deliveries over a 5-year period at a clinic in Florence. They found “no relationship between moon phase and number of spontaneous deliveries” ” This article goes on to give a detailed refutation of the moon/menstruation connection.
The Straight Dope has a good review of the topic and states “The smart money says it’s coincidence. In 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?”
So yeah, I am betting on coincidence.