Neurotoxicity is the major health effect from exposure to lead for infants and young children, and there is current concern regarding possible toxic effects of lead on the child while in utero. There is no placental-fetal barrier to lead transport. Maternal and fetal blood lead levels are nearly identical, so lead passes through the placenta unencumbered. Lead has been measured in the fetal brain as early as the end of the first trimester (13 weeks). There is a similar rate of increase in brain size and lead content throughout pregnancy in the fetus of mothers in the general population, so concentration of lead probably does not differ greatly during gestation unless exposure of the mother changes. Cell-specific sensitivity to the toxic effects of lead, however, may be greater the younger the fetus. Lead toxicity to the nervous system is characterized by edema or swelling of the brain due to altered permeability of capillary endothelial cells. Experimental studies suggest that immature endothelial cells forming the capillaries of the developing brain are less resistant to the effects of lead, permitting fluid and cations including lead to reach newly formed components of the brain, particularly astrocytes and neurons. Also, the ability of astrocytes and neurons to sequester lead in the form of lead protein complexes occurs only in the later stages of fetal development, permitting lead in maturing brain cells to interact with vital subcellular organelles, particularly mitochondria, which are the major cellular energy source. Intracellular lead also affects binding sites for calcium which, in turn, may affect numerous cell functions including neurotransmitter release.