Trophoblast invasion and oxygenation of the placenta: measurements versus presumptions.

Invasion of extravillous trophoblast into maternal tissues has a profound effect on the oxygenation of the placenta and hence the fetus. The main route of trophoblast invasion is interstitial invasion into the tissues of the decidua and myometrium. From this main route side branches reach the spiral arteries (endovascular trophoblast) as well as the uterine glands (endoglandular trophoblast) to open both structures toward the intervillous space. This enables histiotrophic nutrition in the first trimester and hemotrophic nutrition in the second and third trimesters of pregnancy. Failure of endovascular trophoblast invasion has profound effects on the oxygenation of the placenta. Interestingly, this does not lead to hypoxia as has long been presumed. Rather, all measurements available today point to increased oxygen levels within the placenta in patients with a failure of spiral artery transformation. This should lead to a rethink regarding pathological conditions such as intrauterine growth restriction and preeclampsia.