Objective — Data regarding low maternal hemoglobin concentration and severe maternal morbidity (SMM) are limited and potentially biased. This study evaluated the relation between early maternal hemoglobin concentration and SMM or maternal mortality.
Design — Population‐based cohort study.
Setting — Ontario, Canada, in a public healthcare system.
Population — 737,393 births with a routine outpatient hemoglobin measured at a calculated gestational age of 2‐16 weeks.
Methods — The relation between early‐pregnancy outpatient blood hemoglobin concentration and each study outcome was expressed as adjusted relative risks (aRR) and absolute risk differences (aRD), with 95% confidence intervals (CI), generated by modified Poisson regression.
Main Outcome Measures — The primary outcome was SMM or maternal mortality, from 23 weeks’ gestation to 42 days postpartum.
Results — The mean (SD) hemoglobin concentration was 126.9 (9.3) g/L. Overall, SMM or death occurred in 13,514 pregnancies (1.8%). Relative to a hemoglobin of 125‐129 g/L, the aRR was 1.07 (95% CI 1.02‐1.13) and aRD (0.09%, 95% CI 0.01‐0.18) at 120‐124 g/L; aRR 1.31 (95% CI 1.17‐1.46) and aRD 0.47% (95% CI 0.24‐0.69) at 105‐109 g/L; and aRR 4.53 (95% CI 3.59‐5.72) and aRD 5.94% (95% CI 4.12‐7.76) at < 90 g/L. 5961 women (0.8%) required red cell transfusion, with significantly higher risks at all hemoglobin concentrations below 125‐129 g/L, peaking at a hemoglobin < 90 g/L (aRR 11.82, 95% CI 9.30‐15.03).
Conclusion — There is a gradual increase in the risk of SMM or death, as well as red cell transfusion, starting from the lower level of the normal range of hemoglobin of non‐pregnant women.