What is the role of trophoblast?

Secretes HCG.

What is the role of HCG?

Maintains corpus luteum & stimulates corpus luteum to continue producing oestrogen & progesterone.

What is the role of placenta?

Secretes oestrogen & progesterone.

What is the effect of increasing level of oestrogen & progesterone?

They maintain the thickness of endometrium.

They also inhibit the secretion of oxytocin & prolactin by pituitary gland.

Thus they maintain a relatively low level of oxytocin & prolactin.

Why oxytocin levels still continue to rise?

Oestrogen stimulates the build-up of oxytocin receptors on foetus.

What is the role of oxytocin?

It causes the smooth muscle of uterus to contract so that the foetus is expelled out of the uterus.

Its level reach the peak when parturition occurs, that is, when placenta no longer there to secrete progesterone & oestrogen, thus releasing the inhibition to the contraction of myometrium.

What is the role of prolactin?

It works together with oxytocin, when breastfeeding occurs, to stimulate contraction of smooth muscles of mammary gland to expel the milk. The level spikes whenever breastfeeding occurs—it is stimulated by the sucking action by the baby.