Luteolysis,[38] removal of the ovaries,[39] or administration of antiprogestational agents[40] leads to uterine activation with increased effective signaling through oxytocin or other

receptors and parturition. The difference in serum levels before parturition in mice and rats is said to make these animal a poor model for progesterone regulation in humans. However, further understanding of local progesterone check details metabolism and responsiveness is likely to reveal mechanisms that are to some extent important in humans and may be a natural stand in for women who do not respond to exogenous progesterone in the prevention of preterm birth. Rats also express an inhibitory receptor that increases in expression before parturition.[41] In guinea pigs, in which early pregnancy can be disrupted by antiprogestins,[42] maternal serum levels, similar to what is seen in humans, rise from the time of conception

to a peak in early gestation followed by a transient decrease in late gestation and increasing levels from that point beyond the time of parturition.[25] Rabbits and sheep in contrast have very low levels of progesterone in the serum as compared to humans, and in these animals, pregnancy brings a slight increase in serum progesterone and a rapid fall before parturition.[25] Another Ferrostatin-1 in vivo important endocrine system related to pregnancy is the hypothalamic pituitary adrenal axis,[43] both of the mother and the fetus. Activation of the HPA axis by stress or other factors initiates a cascade involving release of corticotropin-releasing hormone (CRH) from the hypothalamus, secretion of corticotropin (ACTH) from the anterior pituitary, and action of ACTH on the adrenal to release cortisol and other glucocorticoids which can then exert feedback suppression on their release. This system not only interacts with the immune system,

but is also thought to be part of the mechanism underlying poor pregnancy outcomes related to emotional or physiologic stress.[44, 45] CRH, a principle mediator of the HPA axis, is produced by the placenta and fetal membranes[45] and may be a mediator of local estrogen production. In pregnant women, the possibility for multiple sources of increased systemic however CRH presents an ongoing challenge in understanding the interaction between maternal stress, fetal stress, and normal HPA development in the generation of parturition or preterm birth.[46] Animal models are likely critical in the examination of this issue in that they can be used to isolate and understand the potential importance of maternal versus fetal HPA and other factors[47, 48] in this process. In related non-human primates, the placenta also expresses CRH, and development of the fetal adrenal and activation of the fetal HPA axis generate important support signals for normal labor.