Objective To estimate the shape and magnitude of associations between maternal Hb levels in the first and third trimesters of pregnancy, and pregnancy outcomes in a high-income setting. Design Prospective cohort studies Setting Two population based pregnancy cohorts from the UK Population The Avon Longitudinal Study of Parents and Children(ALSPAC) and Pregnancy Outcome Prediction Study(POPS). Methods 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 Preterm labour, low birth weight, small for gestational age(SGA), pre-eclampsia(PET), and gestational diabetes mellitus(GDM). Results There was no strong evidence of associations between a higher Hb (1g/dL) in the first trimester and preterm birth (1.07: 95% CI 0.96,1.21), low birth weight(1.09: 0.96, 1.24) and SGA (1.05; 0.96, 1.14). Higher Hb in the third trimester was associated with preterm birth (1.43:1.28,1.61), low birth weight(1.68: 1.48,1.90) and SGA (1.41:1.30, 1.53). Higher Hb in the first and third trimesters were associated with PET in ALSPAC(1st trimester- 1.38:1.07,1.76, 3rd trimester- 1.57: 1.28,1.94) but not in POPS(1st trimester- 1.10: 0.92, 1.30, 3rd trimester- 1.10: 0.92, 1.31). In ALSPAC(1st trimester- 1.37:0.96,1.95, 3rd trimester- 1.35:0.97,1.78) and POPS(1st trimester- 0.94:0.77, 1.17, 3rd trimester- 0.85: 0.69, 1.01), there were no associations with GDM. Conclusion Higher maternal Hb, in late pregnancy, may indicate a suboptimal increase in blood volume and therefore, women at risk of adverse pregnancy outcomes. Further research is required to investigate if this association is causal, and to identify underlying mechanisms.