Well, it's not. Not really. Although the Lunar 'synodic month' from Full Moon to Full Moon is 29.53 days, the female menstrual cycle for humans can be anything from 26 days to 30 days, and some women are quite irregular. Just for comparison, here are the cycle lengths for some other mammals:
Chimpanzees......... 37 days Humans.............. 26 - 30 days Opossums............ 28 days Macaque monkeys..... 24 - 26 days Cows and mares...... 21 days Sows................ 20 - 22 days Sheep............... 16 - 17 days Guinea Pigs......... 11 days Rats and mice....... 5 daysThere is no real correlation with the size of the mammal, the number of legs, the intelligence or brain size, or with the size of the eye. This means if there is a predictor, it is far more complex than some obvious external agent in the environment.