The endocrine regulation of human labour

Placental corticotrophin releasing hormone (CRH) determines the length of pregnancy. CRH drives fetal adrenal production of dehydroepiandrosterone sulfate (DHEAS). DHEAS is converted into estriol in the placenta. Estriol and estradiol are competitive antagonists at equimolar concentrations but both are estrogen agonists at high concentrations. Labor is associated with an excess of estriol over estradiol. High levels of estriol lead to myometrial expression of connexin 43 and cyclo-oxygenase. Myometrial expression of connexin 43 and prostaglandins leads to synchronised myometrial contractions and the onset of labor