Maternal hemoglobin levels and adverse pregnancy outcomes: individual patient data analysis from 2 prospective UK pregnancy cohorts

Background

Hemoglobin (Hb) is a modifiable risk factor for adverse pregnancy outcomes. Studies have reported conflicting associations between maternal Hb levels and adverse pregnancy outcomes, including preterm birth (PTB), low birth weight (LBW), and perinatal mortality.

Objective

In this study, we aimed to estimate the shape and magnitude of associations between maternal Hb levels in early (7–12 wk gestation) and late pregnancy (27–32 wk gestation) and pregnancy outcomes in a high-income setting.

Methods

We used data from 2 UK population-based pregnancy cohorts: the Avon Longitudinal Study of Parents and Children (ALSPAC) and Pregnancy Outcome Prediction Study (POPS). We used multivariable logistic regression models to examine the relationship between Hb and pregnancy outcomes, adjusting for maternal age, ethnicity, BMI, smoking status, and parity. Main outcome measures were PTB, LBW, small for gestational age (SGA), pre-eclampsia (PET), and gestational diabetes mellitus (GDM).

Results

Mean Hb in ALSPAC were 12.5 g/dL (SD = 0.90) and 11.2 g/dL (SD = 0.92) in early and late pregnancy, respectively, and 12.7 g/dL (SD = 0.82) and 11.4 g/dL (SD = 0.82) in POPS. In the pooled analysis, there was no evidence of associations between a higher Hb in early pregnancy (7–12 wk gestation) and PTB (OR per 1 g/dL of Hb: 1.09; 95% CI: 0.97, 1.22), LBW (1.12: 0.99, 1.26), and SGA (1.06; 0.97, 1.15). Higher Hb in late pregnancy (27–32 wk gestation) was associated with PTB (1.45: 1.30, 1.62), LBW (1.77: 1.57, 2.01), and SGA (1.45: 1.33, 1.58). Higher Hb in early and late pregnancy was associated with PET in ALSPAC (1.36: 1.12, 1.64) and (1.53: 1.29, 1.82), respectively, but not in POPS (1.17:0.99, 1.37) and (1.03: 0.86, 1.23). There was an association with a higher Hb and GDM in ALSPAC in both early and late pregnancy [(1.51: 1.08, 2.11) and (1.35: 1.01, 1.79), respectively], but not in POPS [(0.98: 0.81, 1.19) and (0.83: 0.68, 1.02)].

Conclusions

Higher maternal Hb may identify the risk of adverse pregnancy outcomes. Further research is required to investigate if this association is causal and to identify the underlying mechanisms.