Objective To test the equivalence of two doses of intravenous iron (ferric carboxymaltose) in pregnancy. Design Parallel, two‐arm equivalence randomised controlled trial with an equivalence margin of 5%. Setting Single centre in Australia. Population 278 pregnant women with iron deficiency. Methods Participants received either 500 mg (n = 152) or 1000 mg (n = 126) of intravenous ferric carboxymaltose in the second or third trimester. Main outcome measures The proportion of participants requiring additional intravenous iron (500 mg) to achieve and maintain ferritin >30 microg/L (diagnostic threshold for iron deficiency) at 4 weeks post‐infusion, and at 6 weeks, and 3‐, 6‐ and 12‐months postpartum. Secondary endpoints included repeat infusion rate, iron status, birth and safety outcomes. Results The two doses were not equivalent within a 5% margin at any time point. At 4 weeks post infusion, 26/73 (36%) participants required a repeat infusion in the 500‐mg group compared with 5/67 (8%) in the 1000‐mg group: difference in proportions, 0.283 (95% confidence interval CI 0.177–0.389). Overall, participants in the 500‐mg arm received twice the repeat infusion rate (0.81 SD = 0.824 versus 0.40 SD = 0.69, rate ratio 2.05, 95% CI 1.45–2.91). Conclusions Administration of 1000 mg ferric carboxymaltose in pregnancy maintains iron stores and reduces the need for repeat infusions. A 500‐ mg dose requires ongoing monitoring to ensure adequate iron stores are reached and sustained.