General Question

ipso's avatar

Why is a woman’s fertility cycle essentially the same as that of the moon?

Asked by ipso (4481points) November 21st, 2010

Can this really just be a huge coincidence?

Does lunar cycles effect humans gravitationally, like the ocean’s tides? If so, then why are women’s cycles not generally in sync? (Yet women who spend time together can sometimes “sync up” their cycles independent of their history, or the moon cycle.)

Did countless pre-history generations somehow select women with this periodicity, thus altering the genetic proclivity to this?

How else might this be explained?

Inspired by @john65pennington’s question here:

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13 Answers

Seaofclouds's avatar

What do you mean by it being essentially the same as the moon? Are you talking about it lining up with the phases of the moon (as in woman starting their cycle with the new moon)? If that’s the case, mine doesn’t and never has as far as I can remember. I also don’t have a normal 28 day cycle and know a lot of women that don’t have a 28 day cycle. Have you seen anything that says a woman’s cycle is essentially the same as the moon cycle?

kess's avatar

Because the moon in this age is the image of the feminine from which the woman recieve her qualities.

zenvelo's avatar

this actually happened, I can’t remember if it was Pakistan or India, but an early effort at population control involved giving the pill to a mostly illiterate population. To provide a handy reference, the women were told to coordinate taking the pill with phases of the moon. Pretty soon, large portions of the women in the country were on the same menstrual cycle.

iamthemob's avatar

As @Seaofclouds says, women are on different cycles from each other, many have irregular periods, and most periods differ in lengths from 4–8 days (e.g., 28 days for one, and then 24 days until the next, etc.). Women living together often end up having a similar cycle. So…there really isn’t anything to suggest that the moon is the reason for the length of the cycle – if it were, women’s cycles would match the phases of the moon globally, more than likely – they’d all pretty much be the same or close to it.

More importantly, among mammals who are our close relatives, the average is significantly different than the human 28-day cycle. This wouldn’t happen if the moon were linked to cycles.

It’s pretty certain that it’s a coincidence.

absalom's avatar

“Every month there is a moon, gigantic, round, heavy, an omen. It transits, pauses, continues on and passes out of sight, and I see despair coming towards me like famine. To feel that empty, again, again. I listen to my heart, wave upon wave, salty and red, continuing on and on, marking time.”
—Margaret Atwood, The Handmaid’s Tale

It’s a passage about menstruation, thought it might be relevant.

But the connection of monthliness is a neat coincidence, possibly a reason behind the moon’s alignment with the feminine in some cultures, and nothing more than that.

ipso's avatar

The question was about approximate same amount of days.

Found this

“The word “menstruation” is etymologically related to “moon”. The terms “menstruation” and “menses” are derived from the Latin mensis (month), which in turn relates to the Greek mene (moon) and to the roots of the English words month and moon— reflecting the fact that the moon’s period of revolution around the earth (27.32 days) is similar to that of the human menstrual cycle. The synodical lunar month, the period between two new moons (or full moons), is 29.53 days long.”

It goes on:

“Other animals’ menstrual cycles may be greatly different from lunar cycles: while the average cycle length in orangutans is the same as in humans—28 days[60]—the average for chimpanzees is 35 days.[61] Some take this as evidence that the average length of humans’ cycle is most likely a coincidence.[62][63][64][65][66]”

So I guess it’s a coincidence. That just seems almost impossible to me. (I guess I would have made a good witch then.)

crisw's avatar

“Can this really just be a huge coincidence?”


Rarebear's avatar

@iamthemob and @crisw Are absolutely correct. It’s pure coincidence.

MissAnthrope's avatar

Hold on – it may not be coincidence. I have read that during the times of human history in which we did not rely so much on artificial lighting, women’s menstrual cycles did sync up with the moon. This would have been a time when people rose and slept with the sun cycle and artificial light at night was not as extensively used. There are some studies that appear to support this idea.

Some authors believe women in traditional societies without nightlighting ovulated with the full moon and menstruated with the new moon. A few studies in both humans and animals have found that artificial light at night does influence the menstrual cycle in humans and the estrus cycle in mice (cycles are more regular in the absence of artificial light at night), though none have demonstrated the synchronization of women’s menstrual cycles with the lunar cycle. It has also been suggested that bright light exposure in the morning promotes more regular cycles. One author has suggested that sensitivity of women’s cycles to nightlighting is caused by nutritional deficiencies of certain vitamins and minerals.

Other animals’ menstrual cycles may be greatly different from lunar cycles: while the average cycle length in orangutans is the same as in humans—28 days—the average for chimpanzees is 35 days. Some take this as evidence that the average length of humans’ cycle is most likely a coincidence. (Wikipedia)

crisw's avatar


I’ll have to look a bit more into that, but in most diurnal animals that are seasonal ovulators (like goats and deer) it’s day length that triggers ovulation, not moon cycles. The fact that artificial light affects a cycle would support this theory just as well.

crisw's avatar

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.

JLeslie's avatar

Mine was either 26 or 28 days almost my entire menstruating life, more years it was 26. Now, as I am into my 40’s some months it is 24. The lunar month is not every 28 days I don’t think, even though we hear that all of the time. It is more like 29 days from what I remember. A woman, when she has a perfect 28 cycle, gets her period the same day always. Always on a Monday. Or, always on a Tuesday. Just look at a calendar that shows the new moon; it is not every four weeks.

Simone_De_Beauvoir's avatar

Why is the length of an ice cream bar the same as some people’s penises? Just because two things are the same something or other doesn’t mean they’re connected. Linking ‘feminine cycles’ to the moon is dark-ages bs.

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