# Lactation Physiology > Lactation physiology for MBBS Physiology — prolactin and oxytocin reflexes, colostrum and mature milk composition, LAM criteria and suppression. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/lactation-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Lactation Physiology", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/lactation-physiology ## Direct answer Prolactin makes milk and oxytocin delivers it. Suckling at the nipple sends afferent impulses through the fourth to sixth intercostal nerves to the hypothalamus, which simultaneously drops its dopamine (prolactin-inhibiting hormone) inhibition so the anterior pituitary releases prolactin for the next feed, and fires the posterior pituitary to release oxytocin, which contracts myoepithelial cells around the alveoli for the let-down of this feed. During pregnancy high oestrogen primes the ducts and alveoli but blocks secretion; delivery removes the block, colostrum flows for 2–3 days, and mature milk "comes in" around day 3–4. ## What you must remember - Breast development in pregnancy: oestrogen drives duct growth, progesterone alveolar development, with prolactin, hPL and insulin permissive; the fall of oestrogen after delivery removes the secretion block (lactogenesis II). - Colostrum (first 2–3 days) is protein- and immunoglobulin-A rich with leukocytes and vitamin A but lower fat and lactose than mature milk — small in volume, exactly matched to the neonate's kidneys and gut. - Mature milk is about 88% water and 70 kcal per 100 mL; protein near 1 g/dL with a whey-to-casein ratio of about 60:40 (cow's milk 20:80), lactose near 7 g/dL, and iron that is low in content but roughly half absorbed. - Vitamin D is scarce in human milk — a 400 IU daily supplement is advised for exclusively breastfed infants; vitamin K is given by intramuscular injection at birth. - The let-down (ejection) reflex is conditioned — a baby's cry can trigger it, and stress with sympathetic activation can block it. - Dopamine agonists (cabergoline, bromocriptine) abolish prolactin secretion and suppress lactation; metoclopramide does the reverse and is used as a galactagogue. - Suckling suppresses hypothalamic GnRH, so lactation sustains amenorrhoea and anovulation. - Lactational amenorrhoea method criteria — exclusive breastfeeding day and night, infant under 6 months, and amenorrhoea — give about 98% protection, a recognised spacing method in India's family planning programme. - Feeding guidance: initiate within one hour of birth (the breast crawl), exclusive breastfeeding for 6 months, continue for 2 years or beyond. ## The let-down reflex walked through, nerve by nerve Follow the baby's first latch. Mechanoreceptors in the nipple fire into the fourth to sixth intercostal nerves, up the spinal cord to the hypothalamus. Two circuits leave. In the first, paraventricular and supraoptic neurons send impulses down the pituitary stalk to release oxytocin from the posterior pituitary into blood; within seconds it contracts the myoepithelial basket around each alveolus, squeezing milk into ducts and sinuses — the milk the baby actually drinks this feed. In the second, tuberoinfundibular dopamine neurons are inhibited; prolactin secretion rises from the anterior pituitary and drives alveolar synthesis over the next hours — the milk of the next feed. Oxytocin also contracts the uterus, which is why afterpains accompany feeding in the first days and breastfeeding helps involution. Now the clinical corollaries fall into place. A frightened mother with high sympathetic tone does not let down despite full breasts — engorgement, not empty glands; calm privacy is therapeutic. A bottle supplement replaces suckling, so prolactin drive falls and supply dwindles: demand feeding 8–10 times a day is not indulgence but the feedback loop itself. Prolactin's suppression of GnRH explains lactational amenorrhoea, and the loop's fragility explains why the protection fails once supplements, pacifiers or night gaps creep in. ## Where students slip The prolactin-oxytocin reversal is the commonest single error — write it as two columns on rough paper during the exam. Second, milk timing: colostrum is present from the first hour (that is why initiation within one hour matters), while the feeling of fullness on day 3–4 is mature milk coming in, not milk suddenly appearing — anxious mothers discard colostrum thinking it is "not milk", a counselling point every Indian intern should own. Third, LAM needs all three criteria; amenorrhoea alone protects nothing. And bromocriptine suppresses while metoclopramide stimulates — a two-mark question that reverses carelessly under stress. ## Frequently asked questions ### Which hormone produces milk and which ejects it? Prolactin from the anterior pituitary drives alveolar milk synthesis; oxytocin from the posterior pituitary contracts myoepithelial cells for ejection. ### Why is colostrum called the first vaccine? It is rich in secretory immunoglobulin A, lactoferrin and living leukocytes, coating the infant gut against infection during the first days of life. ### What are the three criteria for the lactational amenorrhoea method? Exclusive breastfeeding day and night, an infant under 6 months, and complete amenorrhoea — together conferring about 98% contraceptive protection. ### How do dopamine agonists stop lactation? Bromocriptine and cabergoline mimic prolactin-inhibiting hormone (dopamine), shutting off prolactin secretion and drying up milk production. ### Why is a vitamin D supplement advised for exclusively breastfed infants? Human milk contains little vitamin D, so a daily 400 IU supplement prevents nutritional rickets regardless of maternal diet.