context: No one knows the exact reason for irritable bowel syndrome ( IBS), a digestive disorder that affects up to 15% of Americans. It causes belly pain, cramps, and bloating, as well as diarrhea and constipation. The one thing that experts are certain about: Your gender plays a role. Women are about twice as likely to have IBS as men. A growing body of research shows that sex hormones, like estrogen and progesterone, may be the reason. They can trigger IBS symptoms, which may explain why you have more flare-ups at different points of your menstrual cycle. Estrogen and progesterone affect IBS symptoms in a few ways, from how your intestines work to how much pain you feel. Cells in your gut have things called receptors that let these hormones latch on to them. This suggests that your digestive system is designed to sense and react to them. Here are the main ways they affect IBS: Digestion: They control the smooth muscle in your intestines, which dictates how quickly food travels through your system. In one study, animals took longer to empty their intestines when they received a low dose of the hormones than when they got a higher one. This may explain why low levels of sex hormones can lead to constipation. Pain level: These hormones affect how much your cramps bother you. A dip lowers your pain threshold, in part because estrogen boosts the production of serotonin, a feel-good chemical in your brain. A jump in estrogen can reduce some of the ouch factor, so your bellyaches or cramps don't hurt as much. Inflammation: Sex hormones can raise levels of inflammation throughout your body. That makes your IBS symptoms worse. Most research has linked estrogen and progesterone with IBS. But scientists have also found that male sex hormones, like testosterone, may protect against the condition. This may be partly why men are less likely to get the disorder. Because these hormones rise and fall throughout the month, it makes sense that they can affect IBS symptoms. One study found that roughly 40% of women with IBS said it affects their menstrual cycle symptoms. This cycle, which spans roughly 28 days, has four stages: Menses (days 1-5): If you're not pregnant, you shed the lining of your uterus during menstruation. At this stage, estrogen and progesterone levels are at their lowest. Follicular (days 6-14): Estrogen rises, causing the uterine wall to thicken. Ovulation (day 14): The egg is released. Luteal (days 15-28): Progesterone rises to prepare the uterus for pregnancy. If that doesn't happen, your estrogen and progesterone levels drop quickly during the late luteal phase, around days 24 to 28. IBS worsens as hormone levels fall. During the late luteal phase, you're more likely to get bloated and maybe get constipated or have diarrhea. As hormone levels fall to the lowest point during menstruation, symptoms -- like stomach pain, discomfort, and constipation or diarrhea-- become more common and intense. To make matters worse, IBS patients with painful periods, a condition called dysmenorrhea, are twice as likely to have this increase in symptoms. Hormone levels rise when you're expecting, so your IBS symptoms may improve. Some research shows that you're able to handle more pain during pregnancy. This could mean you have fewer cramps and less discomfort. But moms-to-be also tend get constipated more often. Your sex hormone levels also drop with "the change." But it's unclear how this affects IBS. In some women IBS improves after menopause, when these hormonal changes stop. On the other hand, more than a third of menopausal women in one recent study reported IBS-type symptoms, like gas and heartburn. Experts say that more research is needed on the topic. How do birth control pills, which give you a steady dose of estrogen and progestin (the man-made form of progesterone), affect your IBS symptoms? So far, research suggests they don't. Scientists have found no difference in symptoms between women with IBS who are on the pill and those who aren't. Both groups saw a drop in the sex hormones before their periods started. Some experts think that continuous birth control -- where hormone levels don't change and you skip periods altogether -- may ease IBS symptoms. We'll need more research to know for sure.