Physiological hypoxia is a defining feature of early pregnancy, coordinating menstrual repair, implantation, decidualization, placental development, and fetoplacental adaptation. Hypoxia-inducible factors, HIF-1α and HIF-2α, act as master regulators of these processes by sensing oxygen tension and orchestrating cellular responses in metabolism, angiogenesis, immune regulation, and tissue remodeling. Although structurally related, HIF-1α and HIF-2α exhibit distinct spatial and temporal functions across reproductive stages. Embryonic HIF-1α is primarily involved in early embryonic development, whereas embryonic HIF-2α is required for later developmental stages. Furthermore, maternal HIF-1α acts early in pregnancy, coordinating metabolic adaptation, endometrial regeneration, decidualization, angiogenic expansion, placental organization, and maternal immune tolerance. In contrast, maternal HIF-2α regulates epithelial breakdown, trophoblast invasion, implantation mechanics, and vesicle-mediated trafficking. Mouse genetics demonstrate that disruption of either isoform leads to non-redundant defects in reproductive success, from failed implantation to placental insufficiency and fetal lethality. Pathological hypoxia or aberrant HIF signaling drives pregnancy disorders including preeclampsia, fetal growth restriction, recurrent pregnancy loss, and heavy menstrual bleeding. Defining the distinct roles of HIF-1α and HIF-2α supports the development of therapies targeting hypoxia-responsive pathways in infertility and obstetric disease.

Hypoxia is a condition in which tissues of the body do not receive sufficient amounts of oxygen supply. Numerous studies have elucidated the intricate roles of hypoxia and its involvement in both physiological and pathological conditions. This study aimed to clarify the impact of a forced low-oxygen environment in early pregnancy by exposing mice to low-oxygen conditions for 24–72 h after fertilization. The treatment resulted in the complete failure of blastocyst implantation, accompanied by vascular hyperpermeability in the uterus. A transcriptome analysis of the uterus revealed remarkable alterations in gene expression between control normoxic- and hypoxic-treatment groups. These alterations were characterized by the differentially expressed genes categorized into the immune responses and iron coordination. Furthermore, exposure to a low-oxygen environment caused apoptosis in the corpus luteum within the ovary and a reduction in progesterone secretion. Consequently, diminished plasma progesterone levels were considered to contribute to implantation failure in combination with the activation of the hypoxic pathway in the uterus. Additionally, previous studies have demonstrated the impact of hypoxic reactions on blastocyst development and the pre-implantation process in the endometrium. Our findings suggest that the corpus luteum exhibits elevated susceptibility to hypoxia, thereby elucidating a critical aspect of its physiological response.