The Little Pill With a Big Job: Why We Recommend Baby Aspirin During Pregnancy

You’re newly pregnant, settling into your first prenatal appointments, and your provider hands you a short list of things to start taking. Prenatal vitamins makes sense. Folic acid you’ve heard about that one. And then: 81mg aspirin. Daily.

Wait, isn’t aspirin something you’re not supposed to take during pregnancy?

It’s one of the most common questions we hear and it’s a great one. The recommendation can feel confusing at first, but once you understand what’s behind it, it makes complete sense. This tiny tablet the same one your grandparents called “baby aspirin” is actually one of the most powerful preventive tools we have for protecting both you and your baby from a very serious pregnancy complication called preeclampsia.

Let’s talk about why.

First: What Is Preeclampsia and Why Does It Matter?

Preeclampsia is a pregnancy complication characterized by high blood pressure and signs of organ damage most often affecting the kidneys and liver. It typically develops after 20 weeks of pregnancy and can progress rapidly, sometimes with little warning.

In the US, the rate of preeclampsia increased by 25% between 1987 and 2004 and women giving birth in 2003 were at an increased risk of severe preeclampsia compared to women giving birth in 1980. ACOG says on a global perspective ten million women develop preeclampsia each year and approximately 76,000 pregnant women die annually from preeclampsia and related hypertensive disorders.

Even in the US, where we have access to excellent obstetric care, 16% of maternal deaths can be attributed to hypertensive disorders. Then the financial toll is staggering. The estimated cost of preeclampsia in the US within the first 12 months of delivery was $2.18 billion in 2012 alone, the majority of which was driven by premature births.

The only definitive treatment for preeclampsia is delivery. That’s what makes prevention so critically important and it’s exactly where baby aspirin comes in.

So How Does a Tiny Aspirin Actually Help?

Here’s where it gets genuinely interesting. Aspirin works differently at different doses and that difference is everything.

ACOG says at low doses (81 mg or sometimes 162 mg), aspirin irreversibly inhibits a specific enzyme called COX-1, which decreases platelet synthesis of something called thromboxane A2, without affecting the vascular wall’s production of prostacyclin. Thromboxane A2 causes blood vessels to constrict and promotes clotting. Prostacyclin does the opposite. It keeps blood vessels open and relaxed. In women who develop preeclampsia, research has found that this balance tips in the wrong direction, with too much thromboxane and not enough prostacyclin. The result is the hallmark feature of preeclampsia: dangerously high blood pressure and impaired blood flow to the placenta.

Low-dose aspirin essentially acts as a gentle corrective, nudging that balance back toward the side of healthy circulation protecting placental blood flow and reducing the cascade of events that leads to preeclampsia. At full-strength doses, aspirin blocks both enzymes and loses this targeted effect.

It’s a precise biochemical intervention hiding inside a small orange tablet most people associate with headache relief.

What ACOG Says: The Official Recommendation

The American College of Obstetricians and Gynecologists (ACOG) the leading authority on women’s health in the US has clear, evidence-based guidance on this topic. Low-dose aspirin at 81mg per day is recommended for women at high risk of preeclampsia and should be initiated between 12 weeks and 28 weeks of gestation and continued daily until delivery. Although, sometimes 162 mg could be prescribed for very high risk pregnant women. Timing, it turns out, is everything. A review of 45 randomized trials including over 20,000 pregnant women found significant evidence of prevention of preeclampsia, severe preeclampsia, and fetal growth restriction when low-dose aspirin was initiated before 16 weeks’ gestation. There’s one more detail that surprises most patients: research shows aspirin is most effective when taken at bedtime, compared to morning, afternoon, or evening.

Who Needs It? Know Your Risk Factors

ACOG has identified two tiers of risk factors that guide whether low-dose aspirin is recommended for a given patient.

High-risk factors if you have even one of these, ACOG recommends aspirin:

Prior history of preeclampsia, especially with a poor outcome
Carrying multiples (twins, triplets, etc.)
Chronic high blood pressure
Type 1 or Type 2 diabetes
Kidney disease
Autoimmune conditions such as lupus or antiphospholipid syndrome

Moderate-risk factors, if you have more than one of these, aspirin prophylaxis should be considered: first pregnancy, maternal age of 35 or older, a BMI greater than 30, family history of preeclampsia, and certain sociodemographic and personal history factors.

Is It Safe for My Baby?

This is usually the first follow-up question and the answer is reassuring. At the 81mg dose, low-dose aspirin has been studied extensively in pregnancy and has a well-established safety profile at this dosage for both mother and baby. The U.S. Preventive Services Task Force (USPSTF) specifically reviewed potential harms as part of its recommendation process and found the benefit-to-risk profile strongly favorable for women who meet the criteria.

It is worth noting that regular-strength aspirin (325mg) is a different matter entirely and is not recommended during pregnancy.

What This Means for Your Care at North Florida Women's Care

At your first prenatal visit, we review your full medical and obstetric history specifically to identify whether low-dose aspirin is right for you. It is not a one-size-fits-all recommendation. It is a personalized clinical decision based on your individual risk profile, made in partnership with you.

If aspirin is recommended, we’ll walk you through exactly when to start, how to take it, and what to watch for. If it isn’t indicated for your pregnancy, we’ll explain why and discuss what other steps we are taking to monitor your blood pressure and watch for early warning signs throughout your pregnancy.

The goal is always the same: to get you and your baby safely to the other side of a healthy pregnancy. Sometimes that means a simple daily tablet is one of the most important things you can do.

If you have questions about your preeclampsia risk or low-dose aspirin, bring them to your next visit. We are always here to talk it through with you.

Sources:

American College of Obstetricians and Gynecologists (ACOG) & Society for Maternal-Fetal Medicine (SMFM). Low-Dose Aspirin Use for the Prevention of Preeclampsia and Related Morbidity and Mortality. Practice Advisory, December 2021. https://www.acog.org/clinical/clinical-guidance/practice-advisory/articles/2021/12/low-dose-aspirin-use-for-the-prevention-of-preeclampsia-and-related-morbidity-and-mortality

Reviewed by the clinical team at North Florida Women’s Care to ensure accuracy and alignment with current medical guidelines.