Figure 3.

Oxygenation of the fetus is clearly dependent on the partial pressure gradients between maternal blood, placental tissue, fetal blood, and tissue. The level of intervillous oxygen concentration varies along gestation. Studies performed in human fetuses have shown that before the 12th week of gestation, the intervillous partial pressure of oxygen (PIVO₂) has a median value around 18–20 mmHg, presumably to protect the embryo, which is highly sensitive to ROS. However, PIVO₂ rises in the following weeks reaching a peak value (60 mmHg) around 14–16 weeks of gestation. Thenceforth, PIVO₂ decreases slowly, reaching values of 45–48 mmHg at 36 weeks of gestation (9).