The impact of low‐dose aspirin on hemoglobin levels in pregnancy: A secondary analysis of a randomized controlled trial for prevention of hypertensive disorders of pregnancy

Objectives This study evaluates the association of low‐dose aspirin (LDA) with hemoglobin (Hb) levels during pregnancy and explores how changes in Hb levels relate to hypertensive disorders of pregnancy (HDP). Methods This secondary analysis of a randomized controlled trial comprised 249 pregnant women recruited from a regional hospital in Durban, South Africa (SA). Participants were randomized to receive either 162 mg of LDA daily (intervention) or standard care (control). Hemoglobin levels were measured at enrolment and again at birth. Data on maternal outcomes, including HDP, were analyzed in relation to changes in Hb levels to assess potential associations. Results At enrolment, the predominant Hb range was 9.0–10.9 g/dL (mild anemia) in 51.4% of participants, with comparable Hb distributions between groups. By birth, 43.4% remained mildly anemic, while 44.2% had normal Hb levels (11–13 g/dL). Among participants who experienced a decline in Hb, the reduction was significantly mitigated in the intervention compared to the control group (ΔHb = −0.902 vs. −1.422, P = 0.002, η 2 = 0.088). Declining Hb levels increase the odds of developing HDP compared to increasing Hb (odds ratio = 2.29 vs. 0.44, P = 0.032). However, the intervention group within this subgroup had 76% reduction in risk of developing HDP (RR = 0.24, 95% confidence interval [0.11–0.53], P < 0.001) compared to the control group. Conclusion Low‐dose aspirin was associated with a smaller decline in Hb levels and lower HDP risk; consistent with a potential hematological benefit beyond preeclampsia prevention, although this represents an association rather than causation. Hemoglobin reduction might also serve as an early predictor of HDP.