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Which one of the following hormones stimulates the "let-down" (release) of milk from the mother's breasts when the baby is sucking?
A) Progesterone
B) Oxytocin
C) Prolactin
D) Relaxin

Last updated date: 25th Jun 2024
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Hint: It stimulates uterine contractions at the time of childbirth that is needed to move the child out through the birth canal. This hormone is also called Pitocin. It is often called the birth hormone. Its function in males and non-pregnant females is not clear.

Complete step-by-step answer:
Prolactin is a peptide hormone synthesised and secreted by the adenohypophysis or anterior pituitary. Prolactin has many effects, the most important is to stimulate the mammary glands to produce milk (lactation). Increased serum concentrations of prolactin during pregnancy cause enlargement of mammary glands of the breasts and increases the production of milk. However high levels of progesterone during pregnancy act directly on the breasts to stop its lactogenic effect. Milk production normally starts when the levels of progesterone fall by the end of pregnancy.
After birth, stimulation of the breast by the infant feeding stimulates the posterior pituitary to release oxytocin. Oxytocin promotes the release of milk from the mammary glands acting on the surrounding cells to contract. Oxytocin is released from the posterior pituitary or neurohypophysis. However, the posterior pituitary does not synthesise hormones. This hormone is synthesised in the hypothalamus and is transported axonally to the neurohypophysis.
So the correct answer is option B .

Note: Prolactin also gives negative feedback to the hypothalamus to stop the secretion of GnRH which in turn decreases the secretion of FSH and LH from the anterior pituitary. It stops ovulation during pregnancy.