Role of Progesterone in Pregnancy and Parturition

Pregnancy

The action of progesterone on the uterine receptors is essential to ensure pregnancy is maintained.

• Progesterone inhibits uterine contractions and promotes uterine metabolism, which is necessary for the nutrition of the embryo and subsequently the foetus.
• Progesterone prepares, sustains and stimulates the glandular secretions of the endometrium required for embryo development, for the endometrium to receive the foetus, allows the cervix uteri to close, and regulates immune reactions against the foetus.
• Progesterone is produced in low concentrations during oestrus by luteinisation of the follicle prior to ovulation.

It also decreases GnRH release, thus inhibiting new ovulations. Progesterone profiles are similar during pregnancy and metoestrus.

Parturition

Progesterone in parturition:

• In the bitch, the placenta does not secrete progesterone, gestation is therefore maintained by the progesterone secreted by the corpus luteum until parturition
• Progesterone plateaus during the last trimester and declines (along with oestrogen) rapidly at parturition.
• Parturition is induced by a rapid increase in the oestrogen/progesterone ratio.
• At parturition, in the bitch the plasma oestrogen concentration remains constant. The fall in plasma progesterone concentrations observed 24–36 hours prior to delivery of the first pup may be considered as essential for inducing parturition.