Heartburn during pregnancy is caused by a combination of hormonal changes and physical pressure on your stomach. Roughly one in three pregnant people experience it in the first trimester, and that number climbs to nearly half by the third trimester. Understanding what’s actually happening inside your body can help you manage the discomfort and know which remedies are safe.
Progesterone Relaxes the Valve That Keeps Acid Down
The primary culprit is progesterone, a hormone that rises dramatically during pregnancy. Progesterone’s main job is to relax smooth muscle tissue, which is essential for keeping your uterus from contracting too early. But it doesn’t target just the uterus. It relaxes smooth muscle throughout your body, including the ring of muscle at the bottom of your esophagus called the lower esophageal sphincter.
That sphincter normally acts as a one-way valve: it opens to let food into your stomach, then closes tightly to keep stomach acid from washing back up. When progesterone loosens it, the seal becomes weaker. Stomach acid slips upward into the esophagus, producing that familiar burning sensation behind your breastbone. This is why heartburn can start surprisingly early in pregnancy, well before the baby is big enough to physically push on anything. Your progesterone levels begin rising within weeks of conception.
Your Growing Uterus Crowds the Stomach
As pregnancy progresses, a second factor kicks in: simple mechanical pressure. Your uterus expands to accommodate the growing fetus, and by the third trimester it’s pressing directly against your stomach. That compression pushes stomach acid upward into the esophagus, even if the sphincter were functioning normally.
This explains the clear pattern researchers see across trimesters. In one study of pregnant women, reflux prevalence was about 35% in the first trimester, 31% in the second, then jumped to nearly 47% in the third. The fetus and uterus are largest in those final months, crowding the stomach and other organs the most. Many people who sailed through early pregnancy without any reflux find it appearing for the first time in the last 10 to 12 weeks.
Foods That Make It Worse
Certain foods are more likely to trigger reflux whether you’re pregnant or not, but pregnancy lowers your threshold. The usual offenders include spicy foods, greasy or fried dishes, and high-fat meals, all of which slow stomach emptying and increase acid production. Citrus fruits, tomato-based foods, and chocolate are also common triggers. Caffeine and carbonated drinks can relax the esophageal sphincter further on top of what progesterone is already doing.
You don’t necessarily need to eliminate all of these. Pay attention to which ones bother you specifically. Some people find that citrus is fine but fried food is unbearable, or vice versa. Eating smaller, more frequent meals instead of three large ones reduces the volume of acid your stomach produces at any given time, which means less acid available to reflux upward.
Sleeping Position Makes a Real Difference
Nighttime heartburn is especially common during pregnancy because lying flat lets gravity work against you. Multiple studies have found that sleeping on your left side is the best position for reducing reflux. When you lie on your left side, your stomach sits below the esophageal sphincter, making it harder for acid to flow upward. Sleeping on your back or right side increases reflux episodes.
Elevating the head of your bed by about 6 inches also helps. The key is raising the entire upper body, not just stacking pillows under your head. Pillows alone can bend you at the waist, which actually increases abdominal pressure. Placing a foam wedge under your mattress or putting risers under the bed’s headboard legs creates a gentle slope that keeps acid in your stomach throughout the night.
Which Medications Are Safe
When lifestyle changes aren’t enough, several over-the-counter options have strong safety profiles during pregnancy.
- Calcium-based antacids are generally considered the first choice. They neutralize stomach acid on contact and provide extra calcium, which most pregnant people need anyway. Avoid taking excessive amounts, but standard doses are well established as safe.
- Antacids with aluminum or magnesium are also considered safe at normal doses. Magnesium-containing antacids are best avoided in very late pregnancy at high doses, but typical over-the-counter amounts aren’t a concern.
- H2 blockers (the active ingredients in products like Pepcid) reduce how much acid your stomach produces. They’ve been used in all trimesters with no known risk to fetal development.
- Proton pump inhibitors (the active ingredient in products like Prilosec) are the strongest option and are also considered safe during pregnancy. Most human data support their use throughout all trimesters.
Antacids work fastest but wear off within an hour or two. H2 blockers and proton pump inhibitors take longer to kick in but provide relief for hours, making them better for persistent or nighttime symptoms.
When the Pain Might Signal Something Else
Most pregnancy heartburn is harmless, if annoying. But upper abdominal pain in the third trimester can occasionally be a sign of preeclampsia, a serious blood pressure condition. The difference matters. Typical heartburn produces a burning sensation that rises from the stomach toward the throat, often gets worse after eating, and responds to antacids. Preeclampsia-related pain tends to feel like intense pressure or a squeezing sensation under the right ribs. It often comes with other symptoms: sudden swelling in the face or hands, severe headaches, vision changes, or a sharp rise in blood pressure.
If your upper abdominal pain feels different from your usual heartburn pattern, doesn’t respond to antacids, or comes with any of those additional symptoms, that’s worth bringing up with your provider promptly rather than assuming it’s just reflux.