Constipation during pregnancy is driven by a combination of hormonal shifts, physical changes in the gut, and common supplements like iron. It affects a large number of pregnant people: a University of Eastern Finland study found that 44% of women experience constipation in the second trimester and 36% in the third. Understanding what’s behind it can help you manage it more effectively.
How Pregnancy Hormones Slow Your Gut
The primary driver is progesterone, a hormone that rises steadily throughout pregnancy. Progesterone relaxes smooth muscle tissue across your body, including the muscles that line your intestinal walls. These muscles normally contract in rhythmic waves to push food and waste through your digestive tract. When they relax, everything moves more slowly. Stool sits in the colon longer, which gives your body more time to pull water out of it, leaving it harder and more difficult to pass.
Research on colonic function during pregnancy confirms this effect directly. The colon absorbs significantly more water and sodium during pregnancy than it does outside of pregnancy. That extra absorption is what turns normal stool into the dry, compacted kind that’s difficult to move. This isn’t a sign that something is wrong with your body. It’s a predictable consequence of the hormonal environment that sustains the pregnancy.
Why the Second Trimester Is Often the Worst
Many people assume constipation gets worse as the baby grows larger, but the data tells a slightly different story. Constipation peaks in the second trimester at 44%, then actually drops slightly to 36% in the third trimester. Progesterone levels are climbing rapidly during the second trimester, and many people start prenatal iron supplements around this time as well, both of which compound the problem. By the third trimester, some people have adjusted their diet or found management strategies that bring partial relief, even as the growing uterus adds physical pressure on the intestines.
That physical pressure matters too. As the uterus expands, it compresses the bowel and can further slow the movement of waste through the colon. But hormone-driven muscle relaxation remains the bigger factor throughout.
Iron Supplements and Prenatal Vitamins
If your constipation started or worsened around the time you began taking prenatal vitamins, iron is the likely culprit. The NHS notes that constipation is a common side effect of iron supplementation during pregnancy. The most widely prescribed form, ferrous sulfate, is particularly known for causing hard stools and bloating.
Iron is essential during pregnancy because your blood volume increases by nearly 50%, and your body needs more iron to produce the extra red blood cells. Stopping iron entirely isn’t a good option for most people, but there are alternatives. Some iron formulations are gentler on the gut, and your provider can help you find one that causes fewer digestive issues. Taking iron with a small amount of food (though not dairy or calcium-rich foods, which block absorption) can also reduce the constipation effect. Spacing your iron supplement away from your prenatal vitamin, if they’re separate, gives your gut less to process at once.
Other Contributing Factors
Several everyday factors stack on top of the hormonal and supplement-related causes:
- Lower physical activity. Fatigue, nausea, and physical discomfort lead many people to move less during pregnancy. Exercise stimulates the muscles of the digestive tract, so reduced activity compounds the slowdown progesterone already causes.
- Dietary changes. Food aversions, nausea, and shifting appetites can lead to a diet lower in fiber than what you ate before pregnancy. Fiber adds bulk and moisture to stool, making it easier to pass.
- Not drinking enough water. Your body’s fluid needs increase substantially during pregnancy. The American College of Obstetricians and Gynecologists recommends 64 to 96 ounces (8 to 12 cups) of water per day for pregnant people. If you’re falling short of that, your colon pulls even more water from stool to compensate.
- Stress and disrupted routines. Changes to your daily schedule, including sleep disruptions and anxiety, can affect the regularity of your bowel habits. Your gut responds to stress hormones, and pregnancy is often a time of heightened emotional and physical stress.
What Actually Helps
The most effective approach targets several causes at once rather than relying on a single fix. Increasing your fiber intake to around 25 to 30 grams per day through fruits, vegetables, whole grains, and legumes gives your stool more bulk and helps it retain moisture. If you’re dealing with food aversions that make high-fiber foods unappealing, a fiber supplement like psyllium husk mixed into water or a smoothie can bridge the gap.
Hydration is equally important. Aim for the recommended 8 to 12 cups of water daily. Fiber without adequate water can actually make constipation worse, because dry fiber creates more bulk without the moisture needed to move it through. Drinking a glass of warm water first thing in the morning can help stimulate a bowel movement for some people.
Regular movement, even a 20 to 30 minute walk, promotes gut motility. You don’t need intense exercise. Gentle, consistent activity is enough to make a measurable difference. Swimming and prenatal yoga are also effective options that tend to feel comfortable across all trimesters.
If dietary changes and activity aren’t enough, certain over-the-counter options are generally considered safe during pregnancy. Stool softeners work by drawing water into the stool, and osmotic laxatives pull water into the intestine to help things move. Your provider can confirm which specific products are appropriate for your situation, especially if you’re managing other pregnancy complications.
Signs That Need Medical Attention
Most pregnancy constipation is uncomfortable but not dangerous. However, UT Southwestern Medical Center identifies several symptoms that warrant a call to your provider: rectal bleeding, bloody stools, persistent or worsening digestive symptoms, severe abdominal pain or cramping, and dizziness or lightheadedness. Dark or unusually small amounts of urine suggest dehydration, which both worsens constipation and poses its own risks during pregnancy.
Chronic straining from constipation can also lead to hemorrhoids, which are swollen veins around the rectum. These are common during pregnancy even without constipation, but straining makes them significantly more likely and more painful. Managing constipation early is one of the most effective ways to prevent them.