Why placental progesterone suppressed or drop before onset of labour?
Pregnenolone -the precursor of progesterone is principally converted to estradiol instead of progesterone.Thus the oestrogen:progesterone ratio increases prior to parturition.



  Fetal cortisol stimulates activity of placental 17 α hydroxylase/17, 20 lyase (CYP 17) enzyme, which catalyzes the conversion of pregnenolone to estradiol.





Confinement of the foetus in the uterus stimulates the stress responses.



  Stress leads to a rise in foetalACTH. The rise in ACTH causes production of cortisol from the foetal adrenal glands. The rise in foetal cortisol leads to an increased oestrogen:progesterone ratio by upregulating aromatase enzymes, which convert androgens to oestrogens. Cortisol also reduces progesterone production.