Thirteen Holstein-Friesian cows were used to investigate relationship among concentrations of estrone sulfate (E1S) in maternal circulation during pregnancy and weight of calf and placenta at birth. Plasma samples were obtained from the tail vein of cows once every month from days 90 to 180 of gestation, every 2 weeks from days 181 to 270 of gestation, and every day from days 270 of gestation to parturition. Maternal plasma E1S concentrations were measured by a radioimmunoassay, and newborns and placentas were weighed at birth. E1S concentrations from days 225 of gestation to one day before parturition were correlated positively with weight of calf, cotyledons, intercotyledonary membranes and total placenta at birth (P<0.05 or P<0.01). In addition, calf birth weight was correlated positively (P<0.01) with the weight of the cotyledons (r=0.81), intercotyledonary membranes (r=0.93) and total placenta (r=0.93). These results suggest a close relationship among the fetal placental function, weight of calf and placenta at birth and concentrations of E1S in maternal circulation during late gestation, which provides useful and practical information for the development and function of fetoplacental unit.