Regulation of fetal placental blood flow in the lamb.

Inflatable occluders were placed on the distal aorta of 11 fetal lambs. After 1 wk of control measurements, fetal placental blood flow was reduced to about two-thirds of its control value for an average period of 2 wk. No allowance was made for fetal growth. During the period of flow restriction, fetal growth was 2%/day. Femoral arterial blood pressure was reduced from a control value of 41 to 27 mmHg (P less than 0.001). There was an insignificant increase in carotid arterial blood pressure from 48 to 50 mmHg. Placental resistance to flow did not decrease more than could be accounted for by the increase in gestational age in the course of the experiment. It is concluded that fetal placental blood flow is not under feedback control, since neither of the determinants of flow (i.e., driving pressure and resistance) responded to its chronic reduction.