Restriction of placental size in sheep enhances efficiency of placental transfer of antipyrine, 3-O-methyl-D-glucose but not of urea

When placental growth is restricted, fetal growth is reduced but the fetal to placental weight ratio increases, suggesting that the efficiency of placental transfer may have increased. Therefore, placental transfer of antipyrine, 3-O-methyl-D-glucose and urea was measured in control pregnant sheep and in sheep with restricted placental growth (pre-pregnancy excision of endometrial caruncles). Clearance of each decreased with placental weight but clearance of antipyrine and of 3-O-methyl-D-glucose per kg of placenta increased as placental weight decreased. The small placenta exhibited increased efficiency of flow-determined transfer of antipyrine and of facilitated-diffusion transfer of glucose but not of passive transfer of the hydrophilic substance, urea. These compensatory changes should help to maintain oxygen and glucose to the fetus when the growth of the placenta has been limited by reduction of the number of placental attachment sites.