Oxygenation of the placental-fetal unit in humans.

This review focuses on the role of oxygen and the changes in oxygen levels in the different trimesters of pregnancy in the human placenta. In the first trimester, the physiological conversion of the spiral arteries restricts maternal blood flow into the intervillous space creating a low oxygen environment for the trophoblast and the embryo. In the second trimester, progressive conversion of the spiral arteries allows unhindered entrance of maternal blood into the intervillous space. In early pregnancy, pathology of spiral artery conversion may promote premature flow of maternal blood resulting in miscarriage. In more advanced pregnancy, incomplete conversion of spiral arteries impairs maternal blood flow to the placenta, causing chronic hypoxia and growth restriction of the fetus. Chronically reduced maternal supply of oxygen to the placental-fetal unit may be partially balanced by metabolic reprogramming of the placenta. Acute impairment of oxygenation in the perinatal period and its effect on the placental-fetal unit will also be discussed.