Naproxen, like other NSAIDs, is associated with an increased risk of miscarriage, particularly when taken around the time of conception or for more than a week during early pregnancy. A population-based cohort study published in the BMJ found that NSAID use during pregnancy raised the risk of miscarriage by 80% overall, and the risk climbed sharply with longer use and earlier timing.
How Much the Risk Increases
The clearest numbers come from a large cohort study that tracked pregnant women who did and did not use NSAIDs. For any NSAID use during pregnancy, the absolute risk of miscarriage rose by about 10 percentage points compared to non-users (from roughly 15% to 25%). That’s already significant, but the timing and duration of use mattered far more than whether someone took a pill at all.
Women who used NSAIDs around the time of conception faced a much steeper risk. Their absolute miscarriage rate was approximately 50%, compared to 15% in non-users, an additional 35 percentage points. Those who took NSAIDs for longer than one week saw the highest risk of all: a miscarriage rate around 67%, representing a 52-percentage-point increase over baseline. The hazard ratio for use lasting more than a week was 8.1, meaning the risk was roughly eight times higher than in women who didn’t use these drugs.
These findings reflect NSAID use as a class, which includes naproxen, ibuprofen, and similar medications. The study did not break results down by individual drug, so the numbers apply to the group rather than naproxen alone. However, naproxen works through the same mechanism as other NSAIDs, and there is no evidence it carries less risk.
Why NSAIDs Affect Early Pregnancy
Naproxen works by blocking an enzyme that produces prostaglandins, which are hormone-like chemicals involved in inflammation and pain. That’s what makes it effective for headaches, cramps, and joint pain. But prostaglandins also play a critical role in the earliest stages of pregnancy. They help the uterine lining prepare for a fertilized egg and support the process of implantation, where the embryo attaches to the uterine wall.
When naproxen suppresses prostaglandin production during this window, it can interfere with implantation or disrupt the blood supply to the developing pregnancy. This explains why the risk is highest around conception and drops somewhat as pregnancy progresses past the implantation stage. The relationship also appears to be dose-dependent: one study found that women who used NSAIDs around conception for 1 to 14 days had a miscarriage rate of about 22%, while those who used them for more than 14 days had a rate of 29%.
Naproxen and Fertility
If you’re trying to conceive, naproxen may also reduce your chances of getting pregnant in the first place. Research on women actively trying to conceive found that naproxen users had a fecundability ratio of 0.71, meaning they were about 29% less likely to conceive in any given cycle compared to women not using analgesics. Interestingly, this fertility reduction was specific to naproxen in that study. Other NSAIDs like ibuprofen and aspirin did not show the same association, though the reasons for this difference aren’t fully understood.
Risks in Later Pregnancy
The miscarriage concern is concentrated in the first trimester, but naproxen carries different risks later in pregnancy. In 2020, the FDA issued a safety warning that NSAID use at 20 weeks of pregnancy or later can cause kidney problems in the developing baby. Reduced kidney function leads to lower levels of amniotic fluid, the protective liquid surrounding the baby. Low amniotic fluid can in turn cause complications including restricted limb movement and delayed lung development.
These kidney effects can appear as soon as 48 hours after starting an NSAID, though they more commonly develop after days to weeks of use. The good news is that low amniotic fluid is often reversible once the NSAID is stopped. After 30 weeks, NSAIDs carry the additional risk of causing premature closure of a blood vessel in the baby’s heart that normally stays open until birth.
Safer Alternatives for Pain
Acetaminophen (Tylenol) is the most widely recommended pain reliever during pregnancy. It has been studied in thousands of pregnant women without showing increased risks of birth defects or adverse pregnancy outcomes at standard doses. It works differently from NSAIDs, targeting pain signaling in the brain rather than blocking prostaglandins throughout the body, so it doesn’t carry the same implantation or miscarriage concerns.
If you took naproxen before you knew you were pregnant, a single dose or a couple of days of use carries far less risk than the prolonged-use scenarios described in the research. The studies consistently show that duration matters: brief, incidental exposure is not the same as taking NSAIDs for a week or more during early pregnancy. That said, once you know or suspect you’re pregnant, or if you’re actively trying to conceive, switching to acetaminophen removes the concern entirely.