Physiological implications of the materno-fetal oxygen gradient in human early pregnancy.

This study evaluates the role of the fetal fluid cavities on materno-fetal oxygen diffusion in early pregnancy. Oxygen tension (pO2) was recorded using a multiparameter sensor inserted inside the exocoelomic cavity (ECC) or in the amniotic cavity. There was no correlation between coelomic pO(2) and gestational age, but a negative correlation was found between amniotic pO(2) and gestational age. The mean (SEM) pO(2) was 19.5 mm Hg (1.83) in the ECC at 7-11 weeks and 15.4 mm Hg (1.36) in the amniotic cavity at 11-16 weeks. The volume of the ECC changed little between 7 and 10 weeks of gestation, indicating that coelomic pO(2) results from passive oxygen diffusion through the placenta and is an indicator of the overall pO(2) inside the gestational sac during the first trimester. By contrast, the amniotic cavity volume increases exponentially, whereas amniotic pO(2) decreases with gestational age, suggesting that the increase in uterine blood flow is not sufficient to compensate for the rapid increase in amniotic fluid volume during the first half of pregnancy.