OBJECTIVE: This study determined the placental transfer of antidepressants and their metabolites. METHOD: A total of 38 pregnant women taking citalopram, fluoxetine, paroxetine, or sertraline participated. Maternal and umbilical cord blood samples were obtained to determine antidepressant and metabolite concentrations. RESULTS: Antidepressant and metabolite concentrations were detectable in 86.8% of umbilical cord samples. The mean ratios of umbilical cord to maternal serum concentrations ranged from 0.29 to 0.89. The lowest ratios were for sertraline and paroxetine; the highest were for citalopram and fluoxetine. Maternal doses of sertraline and fluoxetine correlated with umbilical cord concentrations of these medications. CONCLUSIONS: Umbilical cord concentrations of antidepressants and their metabolites were almost invariably lower than corresponding maternal concentrations. Maternal doses predicted umbilical concentrations of fluoxetine and sertraline. Mean umbilical cord to maternal serum ratios were significantly lower for sertraline than fluoxetine, suggesting that sertraline may produce less fetal medication exposure than fluoxetine near delivery.