The answer is prostaglandins. By Francie Diep
Another reason some women poop more during their periods is because their progesterone levels fall. Progesterone is a hormone whose levels cycle with the menstrual cycle. Normally, at the middle of her cycle, a woman's progesterone levels are high. Just before her period, however, the levels drop suddenly, which helps kick off the period. Progesterone is naturally a bit constipating, too, so its sudden decline can make you feel like you really gotta go.
If a woman uses hormonal birth control--one in five women of childbearing age in the U.S. do—then her hormone and chemical levels are a bit different. Hormonal birth control often reduces how much prostaglandin the body makes, which is why it reduces painful cramps. That also means the prostaglandins aren't at work making women poop.
Nevertheless, a woman taking birth control may still feel some gastrointestinal effects just before her "period." Many birth control pills have a series of placebo pills that don't have any hormones in them, which patients are supposed to take once a cycle. Most modern birth control pills have some kind of synthetic progesterone in them. So when patients take their placebo pills, their progesterone levels fall suddenly and… voilà.