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Aspirin for Preeclampsia Prevention
Kelsey McLaughlin, PhD

Canadian and American guidelines recommend low-dose aspirin to prevent preeclampsia in pregnant women considered high-risk of this pregnancy disorder. Let's talk about the history of this therapy and how it could help to prevent the development of preeclampsia.

What is aspirin? 


Aspirin is a non-steroidal anti-inflammatory therapy used to treat mild-to-moderate pain, inflammation and fever. Also referred to as acetylsalicylic acid or ASA, aspirin is a synthetic derivative of salicylic acid, which is naturally found in the bark of some trees. It is one of the oldest drugs in use, dating back 4,000 years. Fast forward to the 20th century, the Bayer Company began producing and distributing the tablet form of aspirin, including as an over-the-counter medication in 1915.

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How does aspirin work?


Aspirin inhibits the activity of the enzymes cyclooxygenase (COX), which are involved in clotting and inflammation. The COX enzymes are involved in the production of prostaglandins and thromboxanes, which mediate inflammation, pain and fever.

Aspirin is also commonly utilized in the clinical setting for patients at risk of blood clots, autoimmune diseases and the prevention of heart attack and stroke in high-risk patients.

Evidence suggests that aspirin prevents preeclampsia


The first clinical study that assessed aspirin for the prevention of preeclampsia was published in The Lancet in 1979. This trial found that pregnant women taking aspirin throughout pregnancy had significantly lower rates of preeclampsia, when compared to pregnant women not taking aspirin (4% vs 16%). However, the mechanism by which aspirin prevented preeclampsia was not understood.

More recently, scientists have hypothesized that aspirin may not prevent all types of preeclampsia.

A meta-analysis published in the Lancet in 2007 compiled data from over 32,000 pregnant women who participated in 31 clinical trials to assess if aspirin can prevent preeclampsia. Aspirin therapy was found to reduce the risk of preeclampsia, delivery before 34 weeks' gestation and prevent poor pregnancy outcomes, including preterm birth and small for gestational age baby.

Another recent meta-analysis determined that aspirin prevented preeclampsia, severe preeclampsia and fetal growth restriction when aspirin therapy was started before 16 weeks of pregnancy. Aspirin therapy started prior to 16 weeks of pregnancy was a reduced the risk of preeclampsia by approximately 50%. These findings suggests that aspirin may impact placenta development, as the placenta is still developing at 16 weeks of pregnancy. If low-dose aspirin improved placental function, this could possibly explain how aspirin prevents preeclampsia.

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Recommendations for aspirin in pregnancy


The Society of Obstetricians and Gynaecologists of Canada, American College of Obstetricians and Gynecologists and Society for Maternal–Fetal Medicine recommend low-dose aspirin therapy for women at high-risk of developing preeclampsia; aspirin therapy is recommended to be started before 16 weeks of pregnancy. The U.S. Preventive Services Task Force also says with moderate certainty that daily low-dose aspirin use is effectively for reducing preeclampsia, preterm birth and fetal growth restriction in high-risk pregnant women.


Trials have not found low-dose aspirin therapy during pregnancy to be associated with bleeding complications, placental abruption, postpartum hemorrhage, mean blood loss, congenital anomalies, neonatal hemorrhagic complications or adverse fetal effects (read more here, and here). 


The American College of Obstetricians and Gynecologists summarize contraindications to aspirin use in pregnant women. 


A common side effect of aspirin is gastrointestinal irritation - the COX enzyme has a role in maintaining the integrity of the gastrointestinal lining that can be inhibited by aspirin. History of gastrointestinal issues is a contraindication for low-dose aspirin therapy.



Low-dose aspirin therapy reduces the risk of preeclampsia in pregnant women at high risk of developing preeclampsia. Although this therapy does not prevent all cases of preeclampsia, it is an affordable therapy that is not associated with increased risk of bleeding complications. Researchers are investigating the mechanism by which aspirin prevents preeclampsia, with some data suggesting that aspirin may impact placental development.



Every woman and every pregnancy is unique. Pregnant women should speak to their healthcare provider to ensure maternal and fetal safety. This article is meant to provide readers with current information and opinions. All medical and treatment decisions should be discussed with your healthcare provider.

This article was written by Dr. Kelsey McLaughlin and edited by Dr. Melanie Audette.

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