Changes in blood flow to the ovine chorion and amnion across gestation.

Objective: The purpose of this study was to determine the developmental changes in blood flow to the network of fetal microvessels in ovine chorion and amnion.

Methods: Colored microspheres (15.10 +/- 0.02 [standard deviation] mu in diameter) were infused into the superior vena cava in nine chronically catheterized fetal sheep with gestational ages ranging from 103-141 days (term 147). After euthanasia, chorion, amnion, and cotyledons were separated and microspheres were counted to determine blood flow rates. Standard correlation and regression analyses were used to analyze the data.

Results: Chorionic blood flow rate increased linearly (r = 0.82, P = .006) from 12 mL/minute at 103 days' gestation to 70 mL/minute at 141 days, and averaged 10.3 +/- 1.2% of the total umbilical blood flow. Weight-normalized chorionic flow (18.4 +/- 2.0 [standard error] mL/minute/kg of fetus) did not change significantly across gestation. Absolute and weight-normalized blood flow to the amnion (0.82 +/- 0.31 mL/minute and 0.34 +/- 0.11 mL/minute/kg fetus) increased with advancing gestation until 130 days and declined thereafter. Absolute cotyledonary blood flow rate increased with gestational age (r = 0.81, P = .008), and weight-normalized cotyledonary blood flow decreased with advancing gestation (r = -0.89, P = .002). Absolute but not weight-normalized chorionic and cotyledonary blood flow rates correlated positively.

Conclusions: Blood flow to the microvascular network in the ovine chorion is high and increases with advancing gestation. Blood flow to the amnion is low but not insignificant. Therefore, intramembranous exchange may play an increasingly important role in determining amniotic fluid volume and composition as gestation proceeds.