High levels of estrogen during pregnancy have been hypothesized to increase the risk of breast cancer in offspring. Some studies have reported a positive association of estrogen level during pregnancy with fetal size, which has been linked to the subsequent risk of breast cancer in offspring. We examined whether maternal diet, including fat and alcohol intake, was associated with hormone levels during pregnancy, as well as with birth weight. The concentrations of estradiol, estriol, and testosterone were measured in the maternal serum and umbilical cord blood of 189 women during pregnancy and at delivery. Intakes of fat, alcohol, and other nutrients were assessed by 5-day diet records at approximately the 29th week of pregnancy before blood sampling. Intake of polyunsaturated fatty acids was moderately but significantly positively correlated with the umbilical cord estriol level (r = 0.17, P = 0.03) after controlling for covariates. The positive association between intake of polyunsaturated fatty acids and birth weight was of borderline significance (r = 0.14, P = 0.06). Intake of long-chain n-3 fatty acids was significantly inversely correlated with the umbilical cord estradiol and testosterone levels (r = -0.18, P = 0.02 and r = -0.24, P = 0.002, respectively). Alcohol intake was significantly positively correlated with the maternal estradiol level in the 29th week of pregnancy (r = 0.19, P = 0.01), but was unrelated to birth weight. Estrogen level during pregnancy may be regulated by dietary polyunsaturated fatty acids and mediate their effects on fetal growth.