I have this theory, that since in the animal world as soon as the female becomes ripe for bearing offspring, she usually gets pregnant. And once it starts, it doesn't stop until the female becomes too old to bear children. Thus in the lifetime of female mammals menstruation is rather the exception than the norm. I would guess they spend more time being pregnant or nursing than menstruating in their fertile lifespan... Here is a woman who hasn't menstruated often in the last 20 years: http://health.discovery.com/convergence/duggars/duggarfamily.html Baby #18 is coming to the Duggar family!