Q.Explain the role of oxytocin and relaxin in parturition and lactation.

Step 1. As the foetus descends late in pregnancy, cervical/vaginal dilation triggers a neurohumoral reflex (the foetal ejection or Ferguson reflex) that causes the posterior pituitary to release oxytocin.

Step 2. Oxytocin then drives powerful, coordinated contractions of the uterine muscle (myometrium), which are what actually expel the baby through the birth canal during parturition; separately, during lactation, suckling similarly triggers oxytocin release, causing the 'let-down' reflex — the ejection of milk already made in the alveoli, out through the ducts — and it also helps the post-partum uterus contract back down toward its pre-pregnancy size.

Step 3. Relaxin, secreted by the placenta and also found in the corpus luteum, works alongside oxytocin during parturition by relaxing the pelvic joints/ligaments and assisting dilation of the cervix, both of which make it physically easier for the contractions to move the baby through the birth canal. …

Separate oxytocin's dual role (uterine contraction at birth; milk ejection during lactation) from relaxin's supporting role ( …

• Crediting relaxin with causing the uterine contractions themselves, when that expulsive force comes from oxytocin — relaxin's jo …