The most likely reason your poop is black during pregnancy is your prenatal vitamin or iron supplement. Iron is the single most common cause of black stool in pregnant women, and since most prenatal regimens include iron, this color change is something the majority of pregnant people notice at some point. That said, not all black stool is harmless, and the difference between a supplement side effect and something more serious is easy to spot once you know what to look for.
Iron Supplements Are the Most Common Cause
Your body can only absorb so much iron at once. When the dose in your supplement exceeds what your intestines can take in, the leftover iron passes through your digestive tract and darkens your stool. This is especially common with higher-dose iron supplements prescribed for pregnancy-related anemia, but it happens with standard prenatal vitamins too. The darker your stool, the more unabsorbed iron is passing through.
This type of black stool is typically firm or normal in consistency, and it doesn’t have an unusual smell beyond what you’d normally expect. It often appears within a day or two of starting iron and continues as long as you keep taking it. If you recently switched to a new prenatal vitamin or started a separate iron supplement, that’s almost certainly your answer.
Some iron formulas are better absorbed than others. If the color change bothers you or comes with constipation, your provider may suggest a different form of iron with higher bioavailability, meaning more of it gets absorbed and less ends up coloring your stool.
Foods and Medications That Darken Stool
Iron isn’t the only thing that can turn stool black. Several common foods do the same thing: black licorice, blueberries, blood sausage, and dark leafy greens in large quantities. If you’ve been eating more of these (blueberry smoothies are a popular pregnancy craving), they can be the culprit on their own or make iron-related darkening even more noticeable.
Activated charcoal, sometimes taken for nausea or bloating, also turns stool jet black. Bismuth-containing medications like Pepto-Bismol are another well-known cause, though it’s worth noting that the CDC recommends against using bismuth subsalicylate during pregnancy. If you’ve been taking it for stomach upset, let your provider know so they can suggest a safer alternative.
Food-related color changes typically resolve within one to three days after you stop eating the food in question. That short timeline can help you figure out whether your diet is playing a role.
How to Tell if It Could Be Blood
Black stool can also result from bleeding in the upper digestive tract, a condition called melena. When blood is digested as it moves through the stomach and intestines, it turns black. This is rare during pregnancy but important to recognize.
The key differences are texture and smell. Melena produces stool that is jet black, tarry, and sticky, almost like roofing tar. It also has a distinctly strong, foul odor that’s noticeably different from normal stool. This smell comes from blood being broken down by digestive enzymes, and people who experience it typically describe it as unmistakable. Stool that’s simply stained black by iron or food looks dark but has a normal consistency and no unusual odor.
If your black stool is tarry and sticky with an unusually offensive smell, that’s a sign of possible internal bleeding and needs prompt medical attention.
Warning Signs That Need Immediate Attention
Black, tarry stool on its own warrants a call to your provider, but certain accompanying symptoms mean you should seek care right away. The CDC identifies several urgent maternal warning signs that can overlap with internal bleeding:
- Dizziness or fainting: ongoing lightheadedness, passing out, or gaps in memory
- Fast or irregular heartbeat: pounding in your chest, skipped beats, or feeling disoriented
- Severe belly pain: sharp, stabbing, or cramp-like pain that doesn’t go away or gets worse over time
These symptoms alongside black stool could indicate significant blood loss in the digestive tract. Any combination of them during pregnancy is a reason to get evaluated immediately, not to wait for your next scheduled appointment.
A Simple Way to Check
If you’re unsure whether your iron supplement is the cause, think about when the color change started. Did it begin around the same time you started or changed your prenatal vitamin? If so, iron is overwhelmingly the most likely explanation. You can also try skipping your iron supplement for two to three days (with your provider’s okay) to see if the color returns to normal. If it does, you have your answer.
For ongoing peace of mind, pay attention to consistency and smell rather than color alone. Dark but normally formed stool with no unusual odor is almost always a harmless side effect of supplementation. Tarry, sticky, foul-smelling stool is the version that signals something different is happening and deserves a closer look.