Fetal umbilical vascular response to chronic reductions in uteroplacental blood flow in late-term sheep.

Objective: The present study was designed to determine the effects of chronic reduction in uterine blood flow (UBF) on umbilical blood flow (UmbBF) and fetal cardiovascular hemodynamics and oxygenation.

Study Design: Sixteen sheep with singleton pregnancies were instrumented on gestational day (GD) 110; an externally adjustable vascular occluder was placed on the common internal iliac artery. UBF in control animals rose from 867 +/- 61 mL/min on GD 115 to 1520 +/- 158 mL/min (n = 8) on GD 138, whereas UBF in restricted animals was maintained at 750 +/- 50 mL/min (n = 8).

Results: UmbBF in control animals increased from 472 +/- 25 mL/min to 744 +/- 58 mL/min over the study period from GD 115 to GD 138. This was associated with normal gestational increases in fetal arterial pressure and significant reductions in calculated umbilical vascular resistance. Although restricted animals initially had a similar UmbBF on GD 115, UmbBF rose only to 545 +/- 43 mL/min over the study period (control vs restricted, P <.008). Although fetal arterial pressure showed normal gestational changes, umbilical vascular resistance failed to decrease over gestation in restricted animals as it did in control animals. Fetal heart rate and oxygenation showed normal changes in both groups.

Conclusion: Chronic reduction in UBF prevents umbilical vascular resistance from undergoing normal gestational decreases, leading to significantly lower UmbBF. This altered umbilical perfusion pattern would be expected to significantly affect fetal delivery of oxygen and nutrients and ultimately fetal growth.