Is it coincidental that the human menstrual cycle is about the same length as the Moon cycle?

The human menstrual cycle is only the same length as the lunar month – it isn’t synchronised with a particular phase.

22nd June 2015
Is it coincidental that the human menstrual cycle is about the same length as the Moon cycle?

Asked by: Alex Sutherland, Banffshire

Charles Darwin thought that the 28-day human menstrual cycle was evidence that our ancestors lived on the seashore and needed to synchronise with the tides. The Moon’s phase certainly has an effect on the behaviour of many animals. Fiddler crabs are more active at full and new Moons because the tides are higher, so their burrows are uncovered for longer. Gerbils avoid foraging at night during a full Moon, because the extra light makes them more likely to be eaten by owls. But the human menstrual cycle is only the same length as the lunar month – it isn’t synchronised with a particular phase.

One frequently cited study, published in the American Journal Of Obstetrics And Gynecology in 1980, found some evidence of synchronisation, but the effect was very weak. Of the sample of 312 women, 244 had cycles that were longer than 29 days or shorter than 27, and only 70 per cent of the rest actually started their period within two weeks of the full Moon.

If locking our reproductive cycle to the lunar month was advantageous, you might expect other animals to do the same. Orangutans and possums have 28-day cycles, but our closest relatives, the chimpanzees, have 35-day cycles. Non-primate mammals have an oestrous cycle, which works differently to menstruation, but none of them synchronise their reproduction with the phases of the Moon.

 


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