Is Aspirin Safe During Pregnancy? What the Evidence Says
TL;DR
- Low-dose aspirin (81 mg/day) is recommended for pregnant individuals at high risk of preeclampsia, initiated between 12 and 28 weeks of gestation (optimally before 16 weeks) and continued until delivery [1].
- Full-dose or analgesic-dose aspirin (≥325 mg) should be avoided in pregnancy, particularly in the third trimester, due to risks of premature closure of the ductus arteriosus, maternal and fetal bleeding, and oligohydramnios.
- The decision to use aspirin during pregnancy should always be made in consultation with a qualified obstetric provider who can assess individual risk factors.
Why Aspirin Use in Pregnancy Requires Special Attention
Aspirin (acetylsalicylic acid) is one of the most widely used over-the-counter medications worldwide. Outside of pregnancy, it serves as an analgesic, antipyretic, anti-inflammatory agent, and antiplatelet drug for cardiovascular protection [7][8]. During pregnancy, however, its pharmacological profile demands careful consideration. The drug crosses the placenta, inhibits cyclooxygenase (COX) enzymes irreversibly, and can affect both maternal hemostasis and fetal development depending on the dose and gestational timing.
The critical distinction that runs through all modern guidelines is between low-dose aspirin (typically 75–150 mg/day, most commonly 81 mg in the United States) and analgesic or anti-inflammatory doses (≥325 mg/day). These two dosing ranges carry fundamentally different risk–benefit profiles in pregnant individuals. Low-dose aspirin has been studied extensively for preeclampsia prevention and is endorsed by major professional bodies including the American College of Obstetricians and Gynecologists (ACOG), the U.S. Preventive Services Task Force (USPSTF), and the World Health Organization (WHO) [1][2]. Higher doses, by contrast, remain a concern due to potential fetal and maternal complications.
High-quality research on the safety of many over-the-counter medications during pregnancy is limited, and physicians are generally advised to explore nonpharmacologic treatments first [2]. Aspirin in its low-dose prophylactic role is a notable exception: the evidence base supporting its targeted use is robust, and the benefit in the appropriate population is well established.
Low-Dose Aspirin for Preeclampsia Prevention
Preeclampsia is a leading cause of maternal morbidity and mortality, accounting for a significant proportion of pregnancy-related deaths in the postpartum period [4]. It affects an estimated 2–8% of pregnancies globally and can progress to life-threatening complications including HELLP syndrome (hemolysis, elevated liver enzymes, low platelet count), eclampsia, and end-organ damage [4].
The biological rationale for low-dose aspirin in preeclampsia prevention centers on its antiplatelet activity. In susceptible pregnancies, abnormal placentation leads to an imbalance between thromboxane A₂ (a vasoconstrictor and platelet aggregator) and prostacyclin (a vasodilator). Low-dose aspirin selectively inhibits thromboxane synthesis while largely preserving prostacyclin production, thereby improving uteroplacental blood flow [7].
ACOG Committee Opinion No. 743 (2018) and the Society for Maternal-Fetal Medicine (SMFM) recommend low-dose aspirin (81 mg/day) prophylaxis for women at high risk of preeclampsia [1]. The USPSTF published a similar but more expansive guideline, which ACOG and SMFM support [1]. Both recommend initiation between 12 and 28 weeks of gestation, optimally before 16 weeks, with continuation until delivery [1].
The American Family Physician review (2023) reinforces this position, advising physicians to screen all pregnant patients for preeclampsia risk and to initiate low-dose aspirin from 12 weeks' gestation until delivery for those at increased risk [2]. The 2024 review of hypertensive disorders of pregnancy similarly positions aspirin prophylaxis as a key preventive strategy [4].
Who qualifies as "high risk"?
According to ACOG and the USPSTF, women are considered high risk if they have one or more of the following factors [1]:
- History of preeclampsia, especially early-onset preeclampsia with delivery before 34 weeks
- Multifetal gestation (twins, triplets, or higher-order multiples)
- Chronic hypertension
- Type 1 or type 2 diabetes mellitus
- Renal disease
- Autoimmune disease (e.g., systemic lupus erythematosus, antiphospholipid syndrome)
Women with more than one moderate risk factor should also be considered for low-dose aspirin prophylaxis. Moderate risk factors include [1]:
- Nulliparity (first pregnancy)
- Obesity (BMI >30 kg/m²)
- Family history of preeclampsia (mother or sister)
- Age 35 years or older
- Sociodemographic factors associated with increased preeclampsia risk
- Prior adverse pregnancy outcome (e.g., low birthweight, small for gestational age)
- In vitro fertilization
Aspirin Dosing in Pregnancy: Low-Dose vs. Full-Dose
The distinction between low-dose and full-dose aspirin is essential for understanding its safety profile in pregnancy. The table below summarizes the key differences.
| Parameter | Low-Dose Aspirin (60–150 mg/day) | Full-Dose / Analgesic Aspirin (≥325 mg/day) |
|---|---|---|
| Common dose used | 81 mg (US), 75 mg (UK/Europe) | 325–650 mg per dose, up to 4 g/day |
| Primary mechanism | Selective COX-1 inhibition in platelets → antiplatelet effect | COX-1 and COX-2 inhibition → anti-inflammatory, analgesic |
| Indication in pregnancy | Preeclampsia prevention in high-risk individuals [1][2] | Not recommended during pregnancy |
| Guideline endorsement | ACOG, USPSTF, WHO, NICE [1] | Avoided by all major guidelines |
| Timing | 12–28 weeks (optimally <16 weeks) until delivery [1] | Contraindicated especially after 20 weeks and in third trimester |
| Placental transfer | Minimal clinical effect at low dose | Significant fetal exposure |
| Ductus arteriosus risk | Not observed at low doses | Risk of premature constriction, especially ≥28 weeks |
| Bleeding risk | Low; no significant increase in maternal hemorrhage [1] | Increased risk of maternal and fetal bleeding |
ACOG specifically states that daily low-dose aspirin use in pregnancy "is considered safe and is associated with a low likelihood of serious maternal, or fetal complications, or both, related to use" [1]. This safety statement applies only to the low-dose range.
The FDA issued a Drug Safety Communication in 2020 recommending avoidance of NSAIDs, including aspirin at anti-inflammatory doses, at 20 weeks or later in pregnancy due to the risk of low amniotic fluid (oligohydramnios). This warning does not apply to low-dose aspirin (81 mg) used for preeclampsia prevention [VERIFY].
When to Start and When to Stop Aspirin in Pregnancy
The timing of aspirin initiation and discontinuation in pregnancy is a point of clinical nuance.
Initiation
All major guidelines agree on a window of 12 to 28 weeks of gestation, with emphasis on starting before 16 weeks for optimal benefit [1][2]. The rationale is that the pathological process of defective trophoblast invasion and abnormal spiral artery remodeling—the upstream cause of preeclampsia—occurs in early pregnancy. Aspirin's benefit in modifying this process diminishes when started later in gestation.
The ASPRE trial (2017), one of the landmark studies in this area, demonstrated that aspirin 150 mg/day initiated at 11–14 weeks significantly reduced the incidence of preterm preeclampsia compared with placebo [VERIFY]. This trial informed the more aggressive recommendation for early initiation.
Continuation and cessation
ACOG recommends continuing low-dose aspirin until delivery [1]. Some practitioners discontinue aspirin at 36 weeks to reduce theoretical bleeding risk during delivery, but the evidence does not consistently support early discontinuation, and ACOG's guidance is to continue until delivery.
There is no evidence that low-dose aspirin significantly increases the risk of epidural hematoma during neuraxial anesthesia, and most anesthesiology societies do not consider low-dose aspirin a contraindication to epidural or spinal anesthesia [VERIFY].
First trimester considerations
A common concern among patients is whether aspirin taken inadvertently in the first trimester (before pregnancy recognition) could harm the fetus. Population-level data have not demonstrated a consistent increase in major congenital malformations with first-trimester aspirin exposure at low or analgesic doses, though the data are not definitive for higher doses [VERIFY]. Patients who discover they took aspirin before knowing they were pregnant should discuss this with their provider but generally should not be alarmed.
Adverse Effects and Safety Profile of Aspirin in Pregnancy
Low-dose aspirin has a favorable safety profile in pregnancy when used as indicated. However, as with any medication, adverse effects are possible. The table below summarizes key concerns.
| Adverse Effect | Frequency / Risk Level | Recommended Action |
|---|---|---|
| Maternal GI upset / dyspepsia | Common (~5–10%) | Take with food; consider enteric-coated formulation |
| Maternal bleeding (minor bruising, epistaxis) | Uncommon | Monitor; typically does not require discontinuation |
| Postpartum hemorrhage | No significant increase at low dose [1] | Continue aspirin until delivery per ACOG guidance |
| Placental abruption | No significant increase at low dose [1] | Reassurance; report vaginal bleeding promptly |
| Premature closure of ductus arteriosus | Not associated with low-dose; risk with full-dose especially ≥28 weeks | Avoid full-dose aspirin in third trimester |
| Oligohydramnios | Not associated with low-dose; risk with anti-inflammatory doses ≥20 weeks | Avoid NSAIDs at analgesic doses after 20 weeks |
| Fetal intracranial hemorrhage | Extremely rare; theoretical concern at full dose | Use only low-dose as recommended |
| Allergic reaction / aspirin-exacerbated respiratory disease | Rare but important | Contraindicated in patients with known aspirin allergy or aspirin-exacerbated respiratory disease |
| Reye syndrome (fetal/neonatal) | Theoretical; associated with pediatric aspirin use during viral illness | Not a practical concern at prophylactic low dose |
ACOG's assessment is that low-dose aspirin is "associated with a low likelihood of serious maternal, or fetal complications, or both" [1]. Large meta-analyses including tens of thousands of participants have consistently demonstrated the safety of this approach.
Red flags requiring immediate medical attention during aspirin use in pregnancy:
- Sudden, severe headache or visual changes (may indicate severe preeclampsia or eclampsia despite prophylaxis) [4]
- Heavy vaginal bleeding or signs of placental abruption
- Significant epigastric or right upper quadrant pain (may indicate HELLP syndrome) [4]
- Signs of allergic reaction: urticaria, angioedema, bronchospasm
- Unusual bruising or bleeding from multiple sites
Patients should understand that low-dose aspirin reduces but does not eliminate the risk of preeclampsia. Vigilance for preeclampsia signs and symptoms must continue throughout pregnancy and into the postpartum period. Up to 44% of pregnancy-related deaths from hypertensive disorders occur in the first six days postpartum [4].
Special Populations and Clinical Pearls
Aspirin and other pregnancy conditions
While preeclampsia prevention is the primary evidence-based indication for aspirin in pregnancy, researchers have explored its potential role in other conditions:
-
Recurrent pregnancy loss and antiphospholipid syndrome: Low-dose aspirin is often used in combination with low-molecular-weight heparin in patients with antiphospholipid syndrome and recurrent miscarriage. This is an established practice supported by specialty guidelines, though the evidence for aspirin alone in unexplained recurrent pregnancy loss is weaker [VERIFY].
-
Fetal growth restriction (FGR): Some evidence suggests that early initiation of low-dose aspirin may reduce the incidence of FGR, particularly when associated with placental insufficiency. This benefit is most apparent when aspirin is started before 16 weeks [VERIFY].
-
Preterm birth: Meta-analyses have shown a modest reduction in preterm birth with low-dose aspirin, though this is generally considered a secondary benefit rather than a primary indication [VERIFY].
Aspirin and breastfeeding
Low-dose aspirin is generally considered compatible with breastfeeding. Aspirin is excreted into breast milk in small amounts, and at the 81 mg/day dose used for postpartum indications (e.g., continued cardiovascular prophylaxis), the infant exposure is minimal. The American Academy of Pediatrics (AAP) has historically classified aspirin as a drug whose effect on nursing infants is unknown but may be of concern at higher doses. Most lactation references consider occasional or low-dose use acceptable [VERIFY].
Interactions with other medications in pregnancy
Patients on low-dose aspirin should be counseled about potential interactions:
- Other NSAIDs (e.g., ibuprofen): Ibuprofen can interfere with aspirin's antiplatelet effect if taken before aspirin. If an NSAID is needed, acetaminophen is preferred during pregnancy, though even acetaminophen should be used judiciously given recent safety questions [2].
- Anticoagulants (heparin, low-molecular-weight heparin): Combined use increases bleeding risk; however, this combination is standard therapy in antiphospholipid syndrome and is managed under specialist supervision.
- SSRIs: Selective serotonin reuptake inhibitors impair platelet function and may theoretically increase bleeding risk when combined with aspirin, though clinical significance at the low-dose range is uncertain.
International guideline variations
Different guideline bodies vary slightly in their recommendations:
- ACOG / USPSTF (United States): 81 mg/day; start 12–28 weeks (optimally <16 weeks); continue until delivery [1]
- NICE (United Kingdom): 75–150 mg/day from 12 weeks until birth for women with one high-risk factor or two or more moderate-risk factors [VERIFY]
- WHO: Recommends low-dose aspirin (75 mg/day) before 20 weeks for women at high risk of preeclampsia, in settings where the risk of preeclampsia is high [VERIFY]
- FIGO (International): 150 mg/day at bedtime from 11–14 weeks until 36 weeks, based on the ASPRE trial results [VERIFY]
The variation in recommended dose (75 mg to 150 mg), timing of initiation, and stopping point reflects differences in the evidence each body prioritized and regional pharmaceutical availability.
Aspirin for cardiovascular prevention: not the same in pregnancy
It is worth noting that aspirin's well-known role in cardiovascular disease prevention [7][8] does not directly translate to pregnancy. The cardiovascular primary prevention landscape has shifted substantially in recent years, with current guidelines being more restrictive about aspirin use in patients without established cardiovascular disease due to bleeding risks that may outweigh benefits [8]. In the specific context of pregnancy-related use for preeclampsia prevention, however, the risk–benefit calculus is different: the target is placental pathology rather than atherothrombosis, and the treatment duration is limited to months rather than years.
The AGA perspective on aspirin and pregnancy-related GI concerns
The American Gastroenterological Association's 2024 clinical practice update on pregnancy-related gastrointestinal and liver disease emphasizes multidisciplinary preconception counseling and careful medication management during pregnancy [3]. While this guidance focuses broadly on GI and liver conditions rather than aspirin specifically, it reinforces the principle that medication decisions during pregnancy should be individualized, evidence-based, and made in collaboration with a multidisciplinary care team [3]. Patients with pre-existing GI conditions such as peptic ulcer disease or a history of GI bleeding should discuss the risks of aspirin use—even at low dose—with both their obstetrician and gastroenterologist.
FAQ
Q1: Can I take aspirin for a headache while pregnant? A1: Standard analgesic doses of aspirin (325–650 mg) are generally not recommended during pregnancy, particularly after 20 weeks of gestation, due to risks including premature closure of the ductus arteriosus and oligohydramnios. For headache relief during pregnancy, acetaminophen has traditionally been the first-line choice, though recent studies have raised some questions about its safety, prompting advice for judicious use [2]. Non-pharmacologic strategies (hydration, rest, cold compresses) should be tried first. Always consult your healthcare provider before taking any pain medication during pregnancy.
Q2: I was prescribed 81 mg aspirin for preeclampsia prevention. Is it really safe for my baby? A2: Yes, extensive clinical evidence supports the safety of low-dose aspirin (81 mg/day) during pregnancy when used for preeclampsia prevention. ACOG states that this dose is associated with a low likelihood of serious maternal or fetal complications [1]. Large trials and meta-analyses involving tens of thousands of pregnancies have not identified significant fetal risks at this dose. The benefit of reducing preeclampsia risk in high-risk pregnancies outweighs the minimal risks of low-dose aspirin.
Q3: What happens if I forget to take my low-dose aspirin for a few days? A3: Missing a few doses of low-dose aspirin is unlikely to cause immediate harm, but consistency is important for optimal preeclampsia prevention. Aspirin irreversibly inhibits platelet COX-1, and since platelets are replaced over 7–10 days, occasional missed doses do not completely eliminate the antiplatelet effect. However, you should resume your regular dose as soon as you remember and discuss any adherence concerns with your provider. Do not double up on doses.
Q4: Should I stop aspirin before my due date or a planned cesarean section? A4: ACOG recommends continuing low-dose aspirin until delivery [1]. While some practitioners stop aspirin at 36–37 weeks due to theoretical bleeding concerns, current evidence does not demonstrate a significant increase in bleeding complications with continuation. Low-dose aspirin is not considered a contraindication to neuraxial anesthesia (epidural or spinal) by most anesthesiology guidelines. Your obstetrician and anesthesiologist will advise you based on your individual circumstances.
Q5: I have a history of stomach ulcers. Can I still take low-dose aspirin during pregnancy? A5: A history of peptic ulcer disease or GI bleeding warrants a careful risk–benefit discussion with your healthcare team. Even at 81 mg, aspirin can irritate the gastric mucosa and theoretically reactivate ulcer disease. The AGA recommends multidisciplinary preconception counseling for patients with GI conditions who plan to become pregnant [3]. In some cases, the preeclampsia prevention benefit may still outweigh the GI risk, and protective strategies (enteric-coated aspirin, avoidance of other gastric irritants) can be employed. Your gastroenterologist and obstetrician should collaborate on this decision.
References
[1] ACOG Committee Opinion No. 743: Low-Dose Aspirin Use During Pregnancy. Obstetrics and Gynecology 2018. PMID: 29939940. pubmed.ncbi.nlm.nih.gov/29939940
[2] Powers EA, Tewell R, Bayard M. Over-the-Counter Medications in Pregnancy. American Family Physician 2023. PMID: 37843943. pubmed.ncbi.nlm.nih.gov/37843943
[3] Kothari S, Afshar Y, Friedman LS. AGA Clinical Practice Update on Pregnancy-Related Gastrointestinal and Liver Disease: Expert Review. Gastroenterology 2024. PMID: 39140906. pubmed.ncbi.nlm.nih.gov/39140906
[4] Farahi N, Oluyadi F, Dotson AB. Hypertensive Disorders of Pregnancy. American Family Physician 2024. PMID: 38574215. pubmed.ncbi.nlm.nih.gov/38574215
[5] Patrono C. Low-dose aspirin for the prevention of atherosclerotic cardiovascular disease. European Heart Journal 2024. PMID: 38839268. pubmed.ncbi.nlm.nih.gov/38839268
[6] Soodi D, VanWormer JJ, Rezkalla SH. Aspirin in Primary Prevention of Cardiovascular Events. Clinical Medicine & Research 2020. PMID: 32580960. pubmed.ncbi.nlm.nih.gov/32580960
About the author
Dr. Stanislav Ozarchuk, PharmD, has 15 years of clinical pharmacy experience. He writes for PillsCard.com, the international drug encyclopedia.
Medical disclaimer
The information provided here is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.