Regional blood flow distribution in fetal sheep with intrauterine growth retardation produced by decreased umbilical placental perfusion

To determine the capacity of the fetus to adapt to chronic O₂ deficiency produced by decreased placental perfusion in the early development of growth retardation, we embolized the umbilicial placental vascular bed of fetal sheep for a period of 9 days. Fetal umbilical placental embolization decreased arterial O₂ content by 39%, decreased total placental blood flow by 33%, and produced a 20% reduction in mean fetal body weight. Neither the combined ventricular output nor the regional blood flow distribution was significantly different between the 8 growth-retarded and 7 normally grown fetuses despite the 39% decrease in fetal arterial O₂ content. Thus a 33% reduction in total placental blood flow restricts normal fetal growth, but does not exceed the placental circulatory reserve capacity necessary to maintain normal basal metabolic oxygenation. Because the proportion of combined ventricular output to the placenta at rest is decreased in late IUGR fetuses but not in early IUGR fetuses, despite chronic oxygen deficiency, we conclude that the growth retarded fetus maintains a normal regional blood flow distribution until the placental circulatory reserve capacity is depleted.