Pregnancy causes heartburn primarily because rising hormone levels relax the muscle that normally keeps stomach acid out of your esophagus. About 26% of pregnant people experience heartburn in the first trimester, climbing to 33% in the second and peaking at nearly 56% in the third. It’s one of the most common discomforts of pregnancy, and understanding the mechanics behind it can help you manage it effectively.
How Progesterone Relaxes Your Acid Barrier
At the bottom of your esophagus sits a ring of muscle that acts like a one-way valve. It opens to let food into your stomach and then closes tightly so acid can’t splash back up. During pregnancy, progesterone levels rise dramatically to support the growing pregnancy, and one side effect is that this muscle relaxes and doesn’t close as firmly as it normally would.
The result is straightforward: with a weaker seal at the top of the stomach, acid escapes upward into the esophagus more easily. The esophagus has no protective lining against acid the way the stomach does, so even small amounts of reflux produce that familiar burning sensation behind the breastbone. This hormonal effect begins early in pregnancy, which is why some people notice heartburn well before their belly has grown.
Why It Gets Worse Later in Pregnancy
Hormones start the problem, but physical changes escalate it. As the uterus expands through the second and third trimesters, it pushes upward against the stomach. This increased pressure from below forces stomach contents toward the already-weakened valve. It’s essentially a two-hit problem: the seal is looser and the pressure behind it is higher.
The trimester-by-trimester statistics reflect this clearly. Heartburn affects roughly one in four people in the first trimester, one in three by the second, and more than half by the third. That steep jump in the final months corresponds to the period when the uterus is largest and exerting the most upward pressure on the digestive organs.
Changes to Digestion Speed
There’s a third factor that surprises many people. During the first trimester, the stomach empties more slowly than usual, likely because of the same hormonal shifts that relax the esophageal valve. When food sits in the stomach longer, there’s more opportunity for acid to reflux upward. Interestingly, gastric emptying returns to a more normal pace during the second and third trimesters, meaning the sluggish digestion is mainly an early-pregnancy contributor. By late pregnancy, the physical crowding from the growing uterus has taken over as the dominant cause.
Foods and Habits That Make It Worse
Certain foods lower the pressure of that esophageal valve even further or stimulate extra acid production. During pregnancy, when the valve is already compromised, these triggers hit harder than they normally would. Common culprits include citrus, tomato-based foods, chocolate, caffeine, carbonated drinks, spicy dishes, and high-fat meals. Peppermint, often reached for to settle nausea, can also relax the valve and worsen reflux.
Eating habits matter as much as food choices. Large meals stretch the stomach and increase pressure against the valve. Eating within two to three hours of lying down gives acid easy access to the esophagus once gravity is no longer helping keep it in place. Tight clothing around the waist can add external pressure that mimics the effect of the growing uterus.
Practical adjustments that tend to help include eating smaller, more frequent meals throughout the day, staying upright for a while after eating, and elevating the head of your bed a few inches so gravity works in your favor overnight. These changes won’t eliminate heartburn entirely when hormones and anatomy are working against you, but they can meaningfully reduce how often and how severely it flares.
Medications Generally Considered Safe
When lifestyle changes aren’t enough, over-the-counter antacids containing calcium carbonate (like Tums) or aluminum and magnesium combinations (like Maalox) are generally regarded as safe during pregnancy. These work by directly neutralizing stomach acid and tend to provide quick, short-term relief.
If antacids alone don’t control symptoms, acid-reducing medications that block histamine receptors in the stomach are the typical next step. These have been studied in pregnant populations without significant adverse effects, though most experts suggest avoiding them during the first trimester when possible, simply because fewer studies have evaluated their use that early. The general approach is to start with the simplest option (smaller meals, avoiding triggers) and step up to medication only as needed.
When the Pain Isn’t Heartburn
One important distinction worth knowing: pain just below the ribs during pregnancy can occasionally signal something more serious called HELLP syndrome, a dangerous complication related to preeclampsia. The key differences are specific. HELLP pain is not a burning sensation, does not spread upward toward the throat, and does not improve with antacids. It tends to be severe and is often accompanied by tenderness over the liver area, headaches, or vomiting.
Standard pregnancy heartburn, by contrast, produces a burning feeling that rises from the stomach toward the throat and typically responds at least partially to antacids or positioning changes. If you experience intense upper abdominal pain that doesn’t behave like your usual heartburn, especially in the second half of pregnancy, that warrants prompt medical attention.