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I've heard that the female menstrual cycle follows a pattern of the lunar cycle, mostly as an evolutionary adaptation so that menstruation (and hence detectability by predators because of the smell of fresh blood) happens on the nights where humans have the best visibility and are thus best able to see predators and defend themselves.

I'd like to know if there is any merit to this claim.

Thanks for reading.

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There was a study in 1979 that investigated this:

A double-blind, prospective study during the fall of 1979 investigated the association between the menstrual cycles of 305 Brooklyn College undergraduates and their associates and the lunar cycles. All subjects were 19-35 years old and using neither OCs (oral contraceptives) nor the IUD. Approximately 1/3 of the subjects had lunar period cycles, i.e., a mean cycle length of 29.5 +/- l day. Almost 2/3 of the subjects started their October cycle in the light 1/2 of the lunar cycle, significantly more than would be expected by random distribution. The author concludes that there is a lunar influence on ovulation.

Another study published in 1986 found:

Among the 826 female volunteers with a normal menstrual cycle, aged between 16 and 25 years, a large proportion of menstruations occurred around the new moon (28.3%), while at other times during the lunar month the proportion of menstruations occurring ranged between 8.5–12.6%; the difference was significant (p<0.01).

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    That means, 7/9 of the women of the first paragraph did not synchronize. Only 2/9 did.
    – user unknown
    Jun 25, 2011 at 1:29
  • I'm not a statistician or researcher, so I can't speak to the specifics of whether the finding is significant. But "only" 2/9 having a synchronized cycle may well be significant. If you picked out 27 people at random and discovered that 6 had the same birthday in a month you'd find that significant, wouldn't you? Quite a bit better than pure chance, no?
    – ghoppe
    Aug 13, 2012 at 6:41
  • The question isn't asking about a correlation, but for a general rule: "... the female menstrual cycle follows a pattern of the lunar cycle," The female cycle follows - not follows often or sometimes.
    – user unknown
    Aug 20, 2012 at 1:17
  • another paper just published provides an analysis of 22 women who kept long term records - worth reading through the Fig 3 legend to understand the visualizations. Helfrich-Forster et al 2021 Science Advances doi.org/10.1126/sciadv.abe1358 "While our findings do not prove causality, several of their features support the plausibility of a causal relation between the Moon’s cycle and women’s menstrual cycles. The Moon controlled not only the frequency of the menstrual cycle but also its phase. This was the case both across individuals and within individuals over time. "
    – David LeBauer
    Jan 29, 2021 at 19:44
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It's unlikely, given that many of our close relatives have a menstrual cycle of a length other than 30 days. The University of Wisconsin says

The duration of the menstrual cycle varies with species; about 29 days in orang-utans, about 30 days in gorillas and about 37 days in chimpanzees. Incidentally, the duration of estrus also varies in these species; about 4-6 days in female orang-utans, about 2-3 days in gorillas and about 10-14 days in chimpanzees. Both the menstrual cycle and estrus vary in duration somewhat among females of the same species. The figures given here are approximations or averages.

The cynomolgus monkey, meanwhile, has a menstrual cycle averaging at 29.4 days. Other animals are much further from a lunar cycle:

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.

George O. Abell, quoted by Cecil Adams.

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