The highest iron demands during pregnancy occur in the third trimester, although preventive iron supplementation is recommended from early stages of the pregnancy. The aim of the study was to compare whether early supplementation with moderate iron doses better prevents the decrease of iron levels during gestation than the late supplementation. One hundred and eighty-four pregnant women participated. They received iron supplements before or after week 20 of pregnancy. At each quarter, serum ferritin (SF), transferrin saturation (TS) and hemoglobin (Hb) were determined. Gestational age-standardized hemoglobin (zHb) was calculated. The early supplementation group received a mean of 40.5 ± 15.7 mg/day and the late group 43.1 ± 11.9 mg/day. Iron deficiency (impaired SF and TS) iron deficiency anemia increased as pregnancy progressed without significant differences between the early and late supplementation groups. More than half of the women ended up their pregnancy with iron deficiency and more than 20% with iron deficiency anemia. In conclusion, although early intake of iron supplements is recommended in pregnancy, no better preventive effect is observed on the decrease of iron levels with early supplementation as compared to late supplementation when moderate iron doses are used.