What is parturition? Which hormones are involved in induction of parturition?

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$ :

Assertion $A$ : Breast-feeding during initial period of infant growth is recommended by doctors for bringing a healthy baby.

Reason $R$ : Colostrum contains several antibodies absolutely essential to develop resistance for the new born baby.

In the light of the above statements, choose the most appropriate answer from the options given below:

Signals from fully developed foetus and placenta ultimately lead to parturition which requires the release of

Give scientific reasons : When birth time is near, two chemical signals associated and produces real parturition pain / labour pain.

What is meant by parturition ? Describe changes occurring in uterus during this period.