Thirty women were studied for the impact of pregnancy iron status on hemoglobin (Hb) and serum ferritin (Ft) during the course of pregnancy. Blood samples were taken 4 weeks before their last menstrual period, at 8, 12, 16, 20, 24, 28, 32 and 36 weeks of pregnancy, during labor and 1 month after delivery. Two groups were formed based on prepregnancy Ft concentration: with high Ft (Ft greater than or equal to 20 micrograms/l) and with low Ft (Ft less than 20 micrograms/l). In women with high Ft, Hb decreased significantly during the first weeks of pregnancy while Ft increased, followed by a gradual fall as pregnancy progressed. In women with low Ft, Hb increased during the first weeks of pregnancy while Ft remained practically unchanged but very close to 0 microgram/l throughout pregnancy. Our findings suggest that besides the hemodilution characteristic of pregnancy, iron deficiency does develop during this period. The fall in Ft in well-nourished women during the first 30 weeks of pregnancy suggests an increased utilization of iron stores, while for those with already low levels of prepregnancy serum Ft, iron stores are not available for this physiological response. Therefore, iron supplementation should be given to women with reduced iron stores before they start a pregnancy so as to insure sufficient iron deposits to cope with the increased iron requirements during this period.