Concerns have been raised about the benefits of Fe-containing supplements on infant birth weight among women with normal/high Hb levels at baseline. Thus far, no clinical trials have examined whether the effects of prenatal Fe-containing supplements on birth weight vary by maternal Hb levels. We compared the effects of Fe–folic acid (IFA) or multiple micronutrients (MMN) with folic acid (FA) supplements on birth weight among pregnant women with mild/no anaemia or high Hb levels. A double-blind randomised controlled trial was conducted in 2006–2009. In total, 18 775 pregnant women with mild/no anaemia (<100 g/l) were enrolled from five counties in north China. During the period from before 20 weeks of gestation to delivery, the women randomly received a daily supplement containing the following: (1) FA (400 μg); (2) IFA (FA, 400 μg; Fe, 30 mg); or (3) MMN (FA, Fe and thirteen additional vitamins and minerals). Birth weight was measured within the 1st hour of birth. Maternal Hb concentration was determined at enrolment. Among women with normal (≤132 g/l) or high (133–145 g/l) baseline Hb levels, IFA or MMN supplementation had no effect on birth weight. Among women with very high (>145 g/l) baseline Hb levels, IFA and MMN supplements increased birth weight by 91·44 (95 % CI 3·37, 179·51) g and 107·63 (95 % CI 21·98, 193·28) g (P<0·05), respectively, compared with the FA group. No differences were found between the IFA and the MMN group, regardless of maternal Hb concentration. In conclusion, the effects of Fe-containing supplements on birth weight depended on baseline Hb concentrations. The Fe-containing supplements improved birth weight in women with very high Hb levels before 20 weeks of gestation.