The ejection of the fetus by the human female is induced by what?

Some hormones prevent the secretion of milk by the mammary glands until the birth of the young ones,even though the glands are fully prepared to do so. These hormones are:

Oxytocin mainly helps in .....

Which hormone acts on the uterus during parturition?

The signals for parturition originate from the fully developed fetus and the placenta,which induce mild uterine contractions called the fetal ejection reflex. This triggers the release of which hormone?

Assertion $A :$ Colostrum provides passive immunity to the infant.
Reason $R :$ Colostrum secreted by the mother contains antibodies that protect the infant.