During early pregnancy, trophoblast differentiation occurs in an environment of relative low oxygen tension which is essential for normal embryonic and placental development. By the term 10–12 weeks of gestation, when the intervillous space opens to maternal blood, there is an increase in РО2. This increase correlates with the time of maximal trophoblast invasion into the maternal decidua, which allows extravillous trophoblast cells to access and remodel the maternal spiral arteries. Hypoxia Inducible Factor 1 (HIF-1) is a transcription factor which activates gene transcription in response to varying oxygen concentration of cells. HIF-1 is a heterodimer composed of the inducible HIF-1α and the constitutively expressed HIF-1β/ARNT subunits. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor in the placenta during pregnancy.
