The Effect of Aspirin on Miscarriage Risk

Woman taking aspirin
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There's a lot of conflicting information about the safety and efficacy of taking aspirin during pregnancy. Some studies say it can increase your risk of miscarriage, whereas others say the opposite. Which is correct?

Confusingly, the answer could be both—or neither. However, the research is still unclear. More authoritative research on aspirin and miscarriage risk is needed to fully understand the risks or any potential benefits. Below, we explain what is known about aspirin and miscarriage risk to help you make sense of the various findings and recommendations.

What Is Aspirin?

Aspirin is a non-steroidal anti-inflammatory drug (NSAID) medication. Prescription-strength aspirin is given to relieve the pain of rheumatoid arthritis, osteoarthritis, and other rheumatologic conditions.

Nonprescription aspirin, or aspirin that can be purchased over-the-counter (OTC), is used to treat daily aches and pains, such as headaches. Nonprescription aspirin can also be used to treat fever. Furthermore, OTC aspirin is also prescribed to people with heart disease to prevent future heart attacks and stroke. In pregnancy, daily low-dose aspirin is routinely used to prevent preeclampsia.

However, according to the National Institutes of Health, research shows that millions of people unnecessarily take aspirin preventively with no discernible benefit. This could also be the case with aspirin and miscarriage.

Aspirin and Miscarriage Risk

A handful of studies have linked NSAIDs, which include aspirin as well as pretty much every other over-the-counter painkiller except Tylenol (acetaminophen), with miscarriage. However, keep in mind that the evidence is far from conclusive.

Other studies have concluded that the value and validity of some of this research linking aspirin use to miscarriage risk may be limited by indication bias. Indication bias or confounding by indication is when increased risk is related to the the need for the medication (or indication) rather than to the medication itself.

Multiple studies in the early 2000s released striking findings, including one that found that NSAIDs during pregnancy increased the risk of miscarriage by as much as 80%. A separate 2003 study replicated the findings, noting that NSAIDs were related to miscarriage, whereas Tylenol was not, leading the authors to speculate that the NSAIDs themselves could potentially be causing the miscarriages.

Many other studies, however, have had mixed results or failed to establish an association between aspirin during pregnancy and miscarriage. So, the truth isn't yet clear.

Causes of Uncertainty

It could be that some other factor is responsible for the association found in the first studies, or there may have been anomalies, errors, or biases in their data. (For example, it could be that whatever conditions led women to use NSAIDs would actually be the factor increasing risk of miscarriage.)

The verdict is still out on whether aspirin plays any role in miscarriage risk.

Right now, however, in an abundance of caution, most doctors lean toward acetaminophen (Tylenol) as the safest painkiller choice for pregnancy. Although even with acetaminophen, it is recommended to try medication-free pain relief approaches first, and then, if necessary, to take the lowest dose possible.

Could It Prevent Miscarriage?

It sounds like a contradiction to say that aspirin might also reduce the risk of miscarriage right after saying it's best to avoid using it during pregnancy. However, some limited research has shown a link with reduced risk of miscarriage in specific cases. 

For example, some studies have found that taking low dose aspirin may reduce the risk of pregnancy loss after prior miscarriage. Still, the evidence is far from clear at this point and other studies have shown no benefit at all.

The American College of Obstetricians and Gynecologists (ACOG) is not convinced of the efficacy of taking aspirin to reduce miscarriage risk. The ACOG, along with the Society for Maternal–Fetal Medicine state that "low-dose aspirin prophylaxis is not recommended for the prevention of early pregnancy loss."

While there's limited evidence that aspirin has any protective value for the average pregnant woman, some research has suggested that low-dose aspirin (also known as "baby aspirin") can be useful for women who have had recurrent miscarriages associated with antiphospholipid syndrome or other blood clotting disorders. It's speculated that aspirin may help because it slows the ability of blood to form clots.

Mini Doses for Specific Cases

Doctors often prescribe "baby aspirin" in combination with heparin or enoxaparin to prevent miscarriage in women with blood clotting conditions. Additionally, some doctors recommend aspirin to some women who have had unexplained recurrent miscarriages.

In this protocol, the dose of aspirin is typically about a fourth of what is in a standard painkiller tablet, so the effects on the body may be quite different than those of a larger dose. (Note that any aspirin use during pregnancy should be under the guidance of a physician.) However, research has been mixed on this treatment's efficacy.

For example, a 2014 study found that aspirin does not prevent pregnancy loss and should not be used for this purpose. Additionally, a comprehensive 2014 review found previous studies to be of varying (often questionable) quality and concluded: "At present, there is no evidence of a beneficial effect" of taking aspirin to prevent miscarriage.

Avoid Aspirin (and All NSAIDs) After 20 weeks

In addition to concerns about the possible increase in miscarriage risk, the Federal Food and Drug Administration (FDA) warns against pregnant women taking NSAIDs (including aspirin) after 20 weeks gestation due to the potential risk of rare, but dangerous, adverse effects for the baby and mother, including fetal kidney and heart problems, low amniotic fluid levels, hemorrhage (in childbirth), preterm birth, and stillbirth.

A Word From Verywell

Since the research is still mixed, most experts recommend avoiding aspirin while pregnant, unless your specific case warrants its use. Be sure to follow your doctor's guidance on which medications are safe for you to take while pregnant.

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