Changes in the transfer of nutrients across the placenta during normal gestation in the rat

Transfer of labeled glucose and alpha-amino isobutyric acid from the maternal circulation into the fetus was studied in the rat at various times during gestation and the results were correlated with placental and fetal growth. It was found that the rate of transfer of both substances per gram of placental tissue increase markedly near term in coincidence with a simultaneous fetal growth spurt. The data suggest a bimodal sequence of growth of the conceptus with a first phase of rapid placental growth and reduced placental transfer and a second phase of slow placental growth, increased transfer, and rapid fetal growth.