One hundred pregnant women, not anemic and not receiving iron or vitamin supplements, were chosen at random among the three trimesters of pregnancy to determine the incidence of unexpected iron, plasma folate and/or cobalamin deficiency, and the significance of fluctuating levels of their respective binding proteins. Pregnant females had a nonsignificant fall in serum iron and a fourfold decline in serum ferritin in the 3rd trimester compared with 1st trimester values. There was a steady decrease in plasma cobalamin and cobalophilin levels in every trimester but no difference in transcobalamin II values. Unsaturated cobalamin (UBBC) and unsaturated transcobalamin binding capacity (UTCBC) were lower in the 1st trimester fully recovering afterwards. Plasma folate levels were not lower, although there was a steady reduction in total and unsaturated folate binding capacity throughout pregnancy.