Diarrhea does happen in the third trimester, though constipation gets far more attention as the signature bowel complaint of late pregnancy. Roughly one in three pregnant women experiences diarrhea at some point during pregnancy, and symptom-tracking data show a clear increase in episodes as delivery nears. The causes span everything from ordinary dietary triggers to early signs of labor, and a few warrant prompt medical attention.
How the Numbers Compare to Constipation
When researchers study bowel habits in the third trimester, constipation overwhelmingly dominates the conversation. A study of third-trimester women found that about two-thirds reported constipation, with nearly three-quarters saying they needed to strain during bowel movements.1PubMed Central. Prevalence and severity of bowel disorders in the third trimester of pregnancy Urgency, though, was far from rare: about 41% of women reported it. That same study found abnormal stool consistency in roughly 68% of participants, a category broad enough to include both hard stools and loose ones.
The broader picture across all trimesters puts diarrhea at up to about a third of pregnant women at some point, though the causes tend to mirror those in the general population rather than being pregnancy-specific.2Gastroenterology Clinics of North America. Constipation and diarrhea in pregnancy In other words, diarrhea in pregnancy is common, but constipation is the one your body is more mechanically primed for, thanks to the hormonal and physical changes of the third trimester. If you’re experiencing loose stools while everyone in your prenatal group complains about the opposite problem, you’re not unusual. You’re just on a different part of the same messy spectrum.
Why the Third Trimester Stirs Up Your Gut
By the third trimester, your uterus is pressing on your intestines in ways it wasn’t earlier. That mechanical pressure alone can disrupt normal bowel patterns, sometimes slowing things down and sometimes speeding them up unpredictably. The growing baby shifts organs, compresses the colon, and changes the geometry of your digestive tract in real time.
Hormones play a role too, though their reputation is mostly tied to constipation. Progesterone, which rises throughout pregnancy, relaxes smooth muscle throughout the body, including the muscles of the intestinal wall. That slowing effect is the classic constipation pathway, but it can also set up a pattern where stool sits in the colon longer than usual, then moves suddenly once the intestines finally contract, resulting in loose or urgent bowel movements that feel a lot like diarrhea. The effect is more like an irregular schedule than a steady slowing.
Diet shifts during the third trimester can also push things toward the loose end. Many women increase their fruit and fiber intake on the advice of their providers or because of constipation earlier in pregnancy. Overcompensating with fiber, especially if it’s a sudden change, can tip the balance toward diarrhea. Dairy sensitivity can also worsen in late pregnancy for some women, even if it wasn’t a problem before.
Do Iron Supplements Cause Diarrhea?
Iron supplements are one of the most commonly blamed culprits for digestive trouble in pregnancy. They have a well-earned reputation for causing nausea, stomach cramps, and black stools. Diarrhea is on the list of possible side effects, though constipation gets blamed far more often.
Interestingly, a randomized placebo-controlled trial that tracked gastrointestinal complaints in pregnant women taking ferrous sulfate found that GI complications were not significantly different between the supplement group and the placebo group at either the 24- to 28-week or the 32- to 36-week checkpoints.3PubMed Central. Gastrointestinal Complications of Ferrous Sulfate in Pregnant Women: A Randomized Double-Blind Placebo-Controlled Trial That doesn’t mean iron never upsets anyone’s stomach. Individual reactions vary. But it does suggest that iron supplements may take more blame for third-trimester digestive symptoms than they deserve, and that the pregnancy itself, or dietary changes surrounding it, may be doing more of the work.
If you suspect your iron supplement is the problem, switching to a different formulation (ferrous gluconate, for example, or a slow-release version) is worth discussing with your provider rather than stopping the supplement entirely. Iron deficiency anemia in the third trimester carries its own risks.
Diarrhea as a Sign That Labor Is Approaching
One of the most widely repeated pieces of pregnancy lore is that loose stools in the final days or weeks before delivery can signal that labor is on its way. There is real support for this. Data from a large pregnancy symptom-tracking study showed that diarrhea increases in frequency around gestation week six and then again toward the end of pregnancy, and researchers have noted its role as a recognized precursor to labor.4npj Digital Medicine. Prevalence and course of pregnancy symptoms using self-reported pregnancy app symptom tracker data
The mechanism behind this is prostaglandins. As your body prepares for labor, it ramps up production of these hormone-like substances, which help soften the cervix and stimulate uterine contractions. Prostaglandins don’t discriminate between smooth muscle in the uterus and smooth muscle in the intestines, so they can trigger cramping and loose stools at the same time. Some women experience this a few days before labor begins; others notice it only hours beforehand, and some never do.
The practical question is whether diarrhea alone means labor is imminent. It doesn’t, at least not reliably. Many women have episodes of loose stools in the third trimester that have nothing to do with labor. But if diarrhea arrives alongside other signs such as regular contractions, a bloody show, or pelvic pressure, it fits the pattern of a body gearing up for delivery. On its own, it’s a soft signal, not a definitive one.
Foodborne Illness Hits Harder During Pregnancy
Pregnancy changes your immune system in ways that make you more vulnerable to certain foodborne infections, and the third trimester is no exception. Among the infections that especially concern obstetricians is listeriosis, caused by the bacterium Listeria monocytogenes. In a pregnant woman, listeriosis often presents with flu-like symptoms: fever, headache, muscle aches, and diarrhea or other digestive complaints.5PubMed Central. An Update Review on Listeria Infection in Pregnancy It can look deceptively mild in the mother while posing serious risks to the pregnancy, including preterm labor and stillbirth.
A case report illustrated this danger in stark terms: a woman at 31 weeks of pregnancy developed abdominal pain, diarrhea, and reduced fetal movements after eating raw sushi, and was ultimately diagnosed with Listeria infection.6PubMed Central. Relevance of microbiological cultures of cord blood and placental swabs in the rapid diagnosis of preterm newborn infection due to Listeria monocytogenes The standard food-safety advice during pregnancy, to avoid soft unpasteurized cheeses, deli meats that haven’t been reheated, and raw fish, exists precisely because of cases like this one.
Other common culprits include Salmonella, E. coli, and norovirus. Any diarrhea accompanied by a fever above 100.4°F, bloody stools, or signs of dehydration warrants a call to your provider, especially in the third trimester when dehydration can sometimes trigger preterm contractions.
Rarer Complications That Can Include Diarrhea
Most diarrhea in the third trimester has a mundane explanation, but occasionally it shows up as part of a more serious pregnancy complication. HELLP syndrome, a severe form of preeclampsia involving liver and blood-clotting problems, is one such condition. In one documented case, a hospitalized pregnant patient who initially had normal blood pressure developed abdominal pain accompanied by diarrhea as the syndrome progressed.7PubMed Central. HELLP syndrome manifesting as abnormal fetal umbilical artery blood flow and rapidly worsening laboratory indexes HELLP syndrome is uncommon, but the fact that its symptoms can mimic ordinary digestive distress is part of what makes it dangerous. Severe upper abdominal pain, nausea, and diarrhea in the third trimester, especially combined with swelling, headaches, or visual changes, need evaluation.
Cholestasis of pregnancy, another liver-related condition, primarily causes intense itching on the hands and feet. But some women also develop digestive changes, including loose stools, as bile acid levels rise. Acute fatty liver of pregnancy, an extremely rare but life-threatening condition, can present with nausea, vomiting, and diarrhea alongside jaundice and abdominal pain. These are edge cases, not the likely explanation for a day of loose stools, but they illustrate why persistent or worsening diarrhea in the third trimester deserves attention rather than dismissal.
Pre-existing Digestive Conditions and the Third Trimester
Women who enter pregnancy with a chronic digestive condition like Crohn’s disease or ulcerative colitis face a particular challenge. Pregnancy can shift the course of inflammatory bowel disease in either direction. A retrospective review of Crohn’s disease during pregnancy found that the disease worsened only slightly overall, with diarrhea exacerbations being the main problem.8PubMed. Course of Crohn’s disease during pregnancy and its effect on pregnancy outcome: a retrospective review For many women with Crohn’s, the third trimester brings a flare that complicates an already complex picture of managing medications, nutrition, and prenatal care simultaneously.
Irritable bowel syndrome follows a similar unpredictable pattern. Some women with IBS find that their symptoms improve during pregnancy, possibly because the hormonal environment suppresses their usual triggers. Others find the opposite, with the third trimester’s physical crowding and stress amplifying both diarrhea and constipation episodes. Because IBS symptoms overlap with normal third-trimester digestive complaints, distinguishing a flare from pregnancy-related bowel changes can be genuinely difficult, even for experienced clinicians.
The key for women with pre-existing conditions is continuity of care between their gastroenterologist and their obstetric team. Many medications used for IBD are safe to continue during pregnancy, and stopping them abruptly often carries more risk than continuing. The third trimester is not the time to freelance your own medication adjustments.
Managing Diarrhea Safely in the Third Trimester
Fluid replacement is the cornerstone of treating diarrhea during pregnancy, as clinical guidance consistently emphasizes that management involves two patients, not one.9Gastroenterology Clinics of North America. Diagnosis and Management of Irritable Bowel Syndrome, Constipation, and Diarrhea in Pregnancy Dehydration during the third trimester is a bigger deal than it might sound. Your blood volume is at its peak, your kidneys are filtering for two, and even mild dehydration can reduce amniotic fluid levels or trigger uterine irritability. Water alone helps, but oral rehydration solutions or drinks with electrolytes are better when diarrhea is frequent.
Dietary adjustments are usually the first practical step. Temporarily pulling back on high-fiber foods, dairy, and greasy or spicy meals can calm things down. The old BRAT approach (bananas, rice, applesauce, toast) is bland enough to be tolerable, though it shouldn’t be your entire diet for more than a day or two since it lacks the nutritional density you and the baby need in the third trimester.
Over-the-counter anti-diarrheal medications are where things get trickier. Loperamide (the active ingredient in Imodium) is the most studied, but most providers recommend using it only when necessary and only after ruling out an infectious cause, since stopping diarrhea caused by a bacterial infection can trap the pathogen in the gut longer. Your provider can help make that judgment call. Bismuth subsalicylate (Pepto-Bismol) is generally avoided in pregnancy, especially in the third trimester, because of its salicylate content and potential effects on the baby’s circulatory system.
Probiotics have drawn interest as a gentler option. A review of evidence including a meta-analysis and several randomized trials found that probiotics used during the third trimester of pregnancy did not increase the risk of adverse fetal outcomes.10Canadian Family Physician. Are probiotics safe for use during pregnancy and lactation? Whether they meaningfully reduce diarrhea episodes is a separate question, and the evidence there is less clear-cut. They are unlikely to hurt, and for women who prefer a non-pharmaceutical approach, they’re a reasonable option to discuss with a provider.
When to Call Your Provider
Most episodes of third-trimester diarrhea resolve on their own within a day or two and need nothing more than fluid replacement and a bland diet. But certain features should prompt a call rather than a wait-and-see approach:
- Duration: Diarrhea lasting more than 48 hours without improvement, especially if it’s getting worse.
- Fever: A temperature above 100.4°F alongside diarrhea suggests infection, and infections during the third trimester can affect the pregnancy.
- Blood or mucus: Bloody stools point toward something beyond a simple upset stomach and need evaluation.
- Dehydration signs: Dark urine, dizziness, dry mouth, or producing very little urine despite drinking fluids.
- Reduced fetal movement: Any change in the baby’s movement pattern alongside digestive symptoms deserves assessment.
- Contractions: Regular tightening of the uterus combined with diarrhea could be early labor, especially before 37 weeks.
Providers are used to fielding calls about digestive symptoms in the third trimester and are unlikely to consider you alarmist. The threshold for calling should be lower in the third trimester than it would be outside of pregnancy, because the stakes of missing something like preterm labor, HELLP syndrome, or a serious infection are high. A brief phone call or message through your patient portal is almost always the right move if something feels off.
The Psychological Side of Bowel Trouble Late in Pregnancy
One aspect that rarely makes it into clinical papers is how disruptive and isolating third-trimester diarrhea can feel. By the last stretch of pregnancy, many women are already dealing with fatigue, pelvic pressure, and difficulty sleeping. Adding unpredictable bowel urgency to that mix can make leaving the house feel risky, and the worry about accidents, especially given that the study mentioned earlier found that roughly 41% of third-trimester women reported bowel urgency, adds a layer of anxiety that nobody talks about in prenatal class.1PubMed Central. Prevalence and severity of bowel disorders in the third trimester of pregnancy
The social taboo around bowel problems makes this worse. Pregnant women are expected to talk about nausea, cravings, and swollen ankles, but diarrhea doesn’t make the small-talk roster. That silence can make you feel like you’re the only one dealing with it. You’re not. The data consistently show that bowel irregularities, in both directions, are a near-universal feature of the third trimester. Knowing that the experience is shared, even if nobody mentions it at the baby shower, can take some of the sting out of it.