Extracoelomic fluid osmometry and electrolyte composition during early gestation in the baboon.

Objective: Access to extracoelomic fluid offers the opportunity to assess and potentially treat genetic disorders early in pregnancy. We have been using the pregnant baboon as a model to develop techniques and evaluate the feasibility of sampling extracoelomic fluid. The aim of this study was to determine the osmolality, oncotic pressures, and electrolyte composition of the baboon's extracoelomic fluid between days 39 and 41 of gestation and to compare them with those of maternal blood.

Study Design: The optimal time to perform the coelocentesis procedure was determined in 14 timed pregnant baboons. Six coelocenteses were then performed in aseptic conditions, under continuous transvaginal ultrasonographic guidance and avoiding the amniotic or yolk sacs. Between 3 and 5 mL extracoelomic fluid was aspirated from each baboon with a 10-mL syringe. Only 1 attempt at sampling was performed for each of the 6 animals. Pregnancies were tracked by transabdominal ultrasonographic evaluations on postprocedure day 3 and then weekly until day 140 of pregnancy. Oncotic pressures and biochemical measurements were determined with 1 mL extracoelomic fluid and 1 mL heparinized maternal venous blood.

Results: Data analysis suggests that maternal blood and extracoelomic fluid have similar osmolalities and concentrations of electrolytes but different colloid osmotic pressures (P <.001).

Conclusion: This nonhuman primate model can be used to gain some insight into the physiologic changes in the composition of the extracoelomic fluid and to evaluate the safety of the coelocentesis procedure. The data suggest that the chorion laeve behaves as a semipermeable membrane at 40 days' gestation.