Aspirin

Pregnancy

First-trimester evidence differs by dose and reason for use. Sun et al. (2022) retrospectively studied 2,763 patients with 2,856 fetuses; among the first-trimester subgroup, congenital anomalies occurred in 45 of 1,543 low-dose aspirin exposures (2.9%) and 30 of 1,079 controls (2.8%), with no significant difference. Kozer et al. (2002) reviewed 22 studies: the pooled overall malformation estimate was not statistically significant (OR 1.33, 95% CI 0.94-1.89), while the review found a gastroschisis association (OR 2.37, 95% CI 1.44-3.88). The older review did not isolate today's prescribed 81 mg regimen. Li et al. (2003) followed 1,055 pregnancies; it reported a prenatal aspirin-miscarriage association, but the aspirin-specific estimate was weaker and unstable because few participants used aspirin. The stronger around-conception association was for NSAIDs overall, not an aspirin-specific result. A 2025 systematic review of three RCTs in 656 people with unexplained recurrent pregnancy loss found no significant improvement in live birth with 50-80 mg aspirin versus no aspirin; this is an efficacy result in a distinct population, not proof that aspirin raises or lowers miscarriage risk in general. ACOG, SMFM, and USPSTF describe 81 mg daily for preeclampsia prevention after 12 weeks for people at elevated risk; ACOG and SMFM say to start between 12 and 28 weeks, ideally before 16 weeks, and continue until delivery. FDA's NSAID warning covers aspirin and describes fetal kidney dysfunction and low amniotic fluid from around week 20, limits necessary use from 20 to 30 weeks to the lowest effective dose for the shortest duration, suggests considering fluid monitoring if treatment lasts beyond 48 hours, and advises against prescribing NSAIDs from week 30 onward because of the added fetal ductus risk. FDA states that the warning does not apply to prescribed 81 mg aspirin for certain pregnancy conditions. MHRA's June 2023 notice says prolonged systemic NSAID use from week 20 may be associated with fetal kidney/fluid effects and some ductus constriction cases, and states systemic NSAIDs are contraindicated after week 28. The notice's named examples are ibuprofen, naproxen, and diclofenac, not aspirin specifically. Dathe et al. (2022) compared 1,092 pregnancies exposed to NSAIDs or metamizole in the second and/or third trimester with 1,154 exposed in the first trimester only; ductus constriction occurred in 5 of 1,092 versus 0 comparison pregnancies, including one late-second-trimester case after long-term exposure. This cohort supports class-level timing concerns, not an aspirin-only risk estimate. Recent dose evidence remains unsettled: Khander et al. (2025 e-publication) randomized 400 people to 162 mg or 81 mg and analyzed 365 with delivery data. The primary composite outcome was similar between groups (14.1% versus 17.1%, P=.4), while placental abruption occurred in 8 participants assigned 162 mg and 0 assigned 81 mg (P=.013). Ahmed et al. (2026) pooled six dose-comparison RCTs with 1,099 participants and described a possible reduction with higher doses that did not reach conventional statistical significance (P=.096); the authors graded certainty very low and noted substantial heterogeneity. A 2026 FAERS disproportionality analysis identified reporting signals among 1,647 aspirin-associated pregnancy and congenital-disorder reports, but spontaneous reports lack a population denominator and cannot establish causality or incidence. This signal is hypothesis-generating and does not distinguish dose or indication well enough to change the current dose-specific guidance.

Breastfeeding

Human low-dose milk-transfer data are small. Datta et al. (2017) collected milk samples from seven breastfeeding women after steady-state 81 mg daily: aspirin was below the assay's quantification limit in all samples, mean salicylic acid concentration was 24 ng/mL, and estimated relative infant dose was 0.4%. LactMed's April 2026 review says that with 75-325 mg daily, aspirin itself is not found in milk and salicylate levels are low; it advises monitoring for bruising or bleeding and prefers an alternative to continuous high-dose therapy. NHS SPS describes low-dose 75-150 mg milk levels as negligible, recommends cautious use with monitoring, and advises avoiding aspirin exposure if an infant has fever or viral symptoms because the milk-related Reye syndrome risk is unknown. LactMed and NHS SPS summarize isolated infant-effect reports after high or unknown maternal doses. The 16-day-old infant case included metabolic acidosis after long-term high-dose aspirin, but milk and maternal serum were not measured and direct infant dosing could not be ruled out. The primary PubMed record for that case has no abstract; the causal details remain uncertain. These reports do not describe the usual 81 mg milk-transfer exposure.

What was tested

Pregnancy evidence includes a retrospective low-dose cohort, an older meta-analysis of first-trimester aspirin studies, a prospective miscarriage cohort, randomized trials of low-dose prevention, an NSAID-class pregnancy cohort, and newer comparisons of 81 mg with 150-162 mg. Some late-pregnancy findings are class-level NSAID data rather than aspirin-only results. Breastfeeding evidence includes a small milk-level study at 81 mg and isolated high-dose infant case reports; large infant-outcome studies are lacking.