Which of these is not an important component of initiation of parturition in humans ?

Prolactin does not play a role in the initiation of parturition in humans.

(2) The change in the ratio of oestrogen to progesterone is one of the key signals that triggers parturition. Throughout pregnancy, progesterone levels are high, helping to maintain the pregnancy and prevent contractions of the uterus. However, towards the end of gestation, there is a relative increase in oestrogen levels. This increase in oestrogen removes the inhibitory effect or progesterone and leads to the stimulation of the contraction of the uterus. High levels of estrogens increase the number of receptors for oxytocin on uterine muscle fibres.

(3) Oestrogens stimulate the placenta to release prostaglandins. Prostaglandins help to soften the uterine cervix by inducing the production of enzymes that digest collagen. This allows the foetus to descend into the birth canal.

(4) Oxytocin released by the posterior pituitary stimulates uterine contractions. These contractions help to dilate the cervix and propel the foetus down the birth canal. Oxytocin levels continue to rise during labour via a positive feedback loop, leading to stronger and more frequent contractions and eventually to the delivery of the baby. After delivery, oxytocin stimulates the contraction of the uterus, helping it to return to its pre-pregnancy size and shape. It also stimulates the release of milk from the mammary glands, allowing the mother to nurse the baby.