What is parturition?

Hint: Parturition occurs when gestation period is over. Contractions of the uterus allow the baby to be expelled out.

Complete answer
Parturition means childbirth and it is a neuroendocrine mechanism and it occurs when a fully developed foetus and placenta produces the foetal ejection reflex. This process is divided into 3 stages. The first phase is dilation phase in which onset of labor and dilation of cervix takes place and this phase is again divided into two phases namely, latent and active phase that lead to dilation of cervix upto 4 to 10 cm and this phase lasts upto 8 to 12 hours. The second phase is the expulsion phase in which severe uterine contraction takes place. In this, the baby's head moves down through vagina and this phase takes 30 minute to 1 hour. The third phase is the placental phase which starts after the birth and ends with the delivery of the afterbirth. An endocrine pituitary hormone oxytocin plays a significant role in this act by inducing severe smooth muscle contraction of the uterus.

Parturition begins with the dilation in which the posterior lobe of the pituitary releases the oxytocin hormone (birth releasing hormone). This hormone is unique in itself because it is positively regulated (known as positive feedback regulation) that means the more the concentration of oxytocin, the more it will be secreted by the posterior lobe of the pituitary. Oxytocin is responsible for the generation of labor pain that is further responsible for the severe contraction of smooth muscles of the uterus.

Note: During pregnancy, placenta acts as endocrine gland that secretes Hcg, hpL, and Progesterone. After giving birth, the female mammary gland starts to secrete milk and the process is called lactation.