The menstrual cycle of humans is complicated. It consists of a luteal and follicular phase, follows a roughly 21 day cycle, and has many effects beyond the shedding of the uterine lining. In addition to the cramps, bloating, hunger, exhaustion or nausea that some people deal with, many people also experience period related poop problems. Menstrual cycles are regulated by changing hormone levels, namely progesterone and estradiol (an estrogen), with some minor inputs from other biologically active molecules. One of these molecules is a prostaglandin that is released by the cells of the uterine lining when they die, that triggers the uterine contractions that expel the lining. The problem is that some of these prostaglandins escape the uterus and are detected by the smooth muscle cells of the large intestine, which are then triggered to contract. This results in, predictable, more frequent bowel movements during your period. It also turns out that the progesterone regulating your period can also effect your intestines, though contrarily to prostaglandins- by relaxing smooth muscle. This often manifests as fewer trips to the bathroom, or even constipation, during the luteal phase (from ovulation to just pre-menstruation) of the menstrual cycle.