Why placental progesterone suppressed or drop before onset of labour? As we know during pregnancy predominant hormone is progesterone which is at early pregnancy about first 3-4 months produce by corpus luteum of pregnancy together with estrogen after that placenta can make its own progesterone with estrogen till birth but for initiation of labour estrogen go up and progesterone should go down because it prevents uterine contraction. Does anyone know mechanism of suppressing of progesterone and rising of estrogen before onset of labour ? In other word mechanism of initiation 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.

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* 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.