Background. Iron supplementation in pregnancy is a controversial issue. The aim of this review was to summarize the results of relevant papers on this subject. Methods. Placebo-controlled studies on iron treatment in pregnancy were identified from the Cochrane database. Results. Among fertile women, 20% have iron reserves of > 500 mg, which is the required minimum during pregnancy; 40% have iron stores of 100-500 mg, and 40% have virtually no iron stores. The demand for absorbed iron increases from 0.8 mg/day in early pregnancy to 7.5 mg/day in late pregnancy. Dietary iron intake in fertile women is median 9 mg/day, i.e. the majority of women have an intake below the estimated allowance of 12-18 mg/day. Iron absorption increases in pregnancy, but not enough to prevent iron deficiency anemia in 20% of women not taking supplementary iron. Iron-treated pregnant women have greater iron reserves, higher hemoglobin levels, and a lower prevalence of iron deficiency anemia than placebo-treated women both in pregnancy as well as postpartum. Furthermore, children born to iron-treated mothers have higher serum ferritin levels than those born to placebo-treated mothers. An iron supplement of 65 mg/day from 20 weeks of gestation is adequate to prevent iron deficiency anemia. Conclusions. In order to avoid iron deficiency in pregnancy, prophylactic iron supplement should be considered. Iron supplements may be administered on a general or selective basis. The selective approach implies screening with serum ferritin in early pregnancy, in order to identify women who can manage without prophylactic iron.