This study aims to evaluate iron prophylaxis in pregnant women from the individual aspect, i.e. according to serum ferritin levels at the beginning of pregnancy, and to assess which dose of iron would be adequate to prevent iron deficiency (ID) and iron deficiency anaemia (IDA) during pregnancy and postpartum. A randomised, double-blind study comprising 301 healthy Danish pregnant women allocated into four groups taking ferrous iron (as fumarate) in doses of 20 mg (n=74), 40 mg (n=76), 60 mg (n=77) and 80 mg (n=75) from 18 weeks gestation (inclusion) to 8 weeks postpartum. Iron status markers [serum ferritin, serum soluble transferrin receptor (sTfR), haemoglobin] were recorded at 18, 32 and 39 weeks gestation and 8 weeks postpartum. Body iron was calculated using the serum sTfR/serum ferritin ratio. ID was defined by serum ferritin <12 μg/l in pregnancy and <15 μg/l postpartum; IDA as serum ferritin <12 μg/l and haemoglobin <5th percentile in iron-replete pregnant women. Women in the iron supplement groups were stratified according to serum ferritin levels at inclusion; 50.7% had ferritin ≤30 μg/l, 37.7% ferritin 30–70 μg/l and 11.6% ferritin >70 μg/l. At 32 weeks, women with ferritin ≤30 μg/l had an ID frequency of: 20-mg group 54.1%, 40 mg 29.7%, 60 mg 24.4%, 80 mg 20.6% (p<0.001); women with ferritin >30 μg/l had an ID frequency of: 20-mg group 20.0%, 40 mg 13.9%, 60 mg 5.7%, 80 mg 5.1% (p<0.001). Women with ferritin >70 μg/l had no ID. Postpartum, ID was found in 4.7% in 20-mg group, 2.9% in group 40 mg and 0% in group 60 and 80 mg. IDA: At 32 weeks, women with ferritin ≤30 μg/l had an IDA frequency of: 20-mg group 2.7%, 40 mg 2.7%, 60 and 80 mg 0%; none of the women with ferritin >30 μg/l displayed IDA. Body iron at 18 weeks was 10.4 mg/kg, similar in the four iron groups. Later in pregnancy body iron declined significantly, being lower the 20 mg group, and similar in the 40, 60 and 80-mg groups. Postpartum body iron rose to inclusion levels being 9.3 mg/kg in the 20-mg group and 10.5 mg/kg in the 40-, 60- and 80-mg groups. This study gives an estimate of iron dosage in individual iron prophylaxis adjusted to serum ferritin levels in early pregnancy. In the prevention of ID, we suggest 80–100 mg ferrous iron/day to women having ferritin ≤30 μg/l and 40 mg ferrous iron/day to women having ferritin 31–70 μg/l. In the prevention of IDA, we suggest 40 mg ferrous iron/day to women having ferritin ≤70 μg/l. Women with ferritin >70 μg/l have no need for iron supplement.