This study shows a relationship between the changing phases of the moon and the propensity for menstrual onset in women. This phenomenon was demonstrated in the subset of 229 women who cycle as often as the moon cycles. Cutler's earlier report (1980a) had shown that data of non-29.5+1 day cyclers distributed randomly about the lunar month.

This was expected from mathematical considerations when one realizes the nature of the cycles. For example, if the lunar cycle were drawn as a clock that had 30 (rounded up from 29.5) "pie sectors", and a particular woman who did not have a 29.5 day cycle was charting her data, certain phenomena would emerge. As an example, consider a women who menstruates on a regular 35 day cycle. If she plotted 6 cycles of her own on the 30 day clock, and the first cycle appeared at the full moon, then the next cycle would appear 5 days after the full moon and the third cycle would be 5 days later than, and so on.

Thus, a woman who has a regular 35 day cycle would eventually distribute her onsets completely around the clock and fail to show any lunar relationship. Similar considerations occur when we chart cross-sectional data of women with non-29.5 day cycles. One would expect a rather random distribution around the clock.

It is noteworthy that a number of studies have shown that the 29.5+ 1 day lunar cycle is coincident with the most fertile menstrual cycle (Vollman 1968, 1970, 1977; Treloar et al. 1967)

In the 4 separate studies of women living naturally, visual inspection of the figures (1 through 4) suggests a common phenomenon: the highest density in every case appears to be at about the full moon: that is 15 or so days after the new moon. The small Spring sample does not appear to differ from these Autumn samples, suggesting no seasonal effect. Although only Figures 2 and 3 contain sufficiently large samples for statistically significant difference from a non-uniform distribution, the similarity in graphs is noteworthy.

The menstrual life of a woman is known to pass through three phases: 1) the pubescent occupying the first 7 years after menarche; 2) the reproductive years; and 3) the premenopausal occupying the last 7 years before menstruation ceases (Cutler and Garcia 1984). It is during the reproductive years of women that the 29.5+1 day cycle most commonly occurs and in larger scale studies, an incidence of approximately 32% is obtained (Vollman 1977). Thus, it is during the reproductive years (approximately age 20 through 42) when this lunar influence would be testable.

It is not surprising to learn of a coordinated phase relationship between the reproductive cycles of women and the repeating cycle of lunar periods because even a cursory review of the literature shows that many different animals show a reproductive system reaction to external stimuli. Exogenous influences on the fertility of organisms have been demonstrated in several species with respect to geophysically ordered time. A seasonal variation in human birth rate has been documented with troughs in Spring and peaks in Autumn (Rosenberg 1966; Pasamanick et al. 1959). Coordinated phase relationships between reproductive rhythms and lunar rhythms are documented in monkeys, genus Cercopithecus (Reiter 1972) as well as in the fiddler crabs (Brown et al. 1953). Persistent activity rhythms that are coordinated with lunar rhythms have been well documented in a variety of organisms including the frog, Rana pipiens (Robertson 1978), the crab Carcinus maenas (Naylor 1958, 1960), the marine worm Platynereis (Havenschild 1960), the hamster (Brown 1967), and planarians (Brown et al. 1975).

The demonstration that women who cycle as often as the moon tend to be the most fertile and that among these women there is an increased propensity for menstruation at or about the full moon is particularly noteworthy. Historical indication that fertility rites were scheduled with consideration for the phase of the moon may have been reflecting accurate perceptions which we have yet to discover.

Received June 1986; revision received 17 February 1987.