Lactation Physiology and Breastfeeding
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Direct answer
Lactation is the production and release of milk by the breast, controlled by two pituitary hormones. Prolactin drives milk synthesis in the alveoli, whereas oxytocin contracts myoepithelial cells to eject milk during feeding. The WHO recommends exclusive breastfeeding for the first six months, meaning no other food or drink, including water.
What you must remember
- Prolactin reflex: suckling stimulates nipple receptors, and prolactin released from the anterior pituitary drives milk synthesis in the alveoli.
- Oxytocin reflex: suckling triggers posterior pituitary release of oxytocin, which contracts myoepithelial cells and produces milk ejection, also called let-down.
- Colostrum to mature milk: colostrum comes first, transitional milk follows from about day 4, and mature milk is established by about two weeks after birth.
- Feeding frequency: on-demand feeding about 8 to 12 times in 24 hours supports supply and weight gain.
- Milk removal: the amount removed controls production, and milk left in the breast contains a feedback inhibitor that slows synthesis.
- Exclusive breastfeeding: WHO recommends six months, followed by continued breastfeeding with complementary foods for up to two years or beyond.
- Attachment: a good latch shows a wide gape, more areola visible above the upper lip than below the lower lip, and no clicking sounds.
Tracing the reflexes from suckle to let-down
A new mother notices milk leaking when her baby cries in the next room, and asks how her body knows when to make and release milk. The answer is two linked reflexes, which act in sequence as set out below.
- Suckling stimulus: the infant's suck stimulates nipple nerve endings, and the signal travels to the hypothalamus.
- Prolactin release: the anterior pituitary releases prolactin, which acts on alveolar cells to synthesise milk.
- Oxytocin release: the posterior pituitary releases oxytocin in pulses with suckling, contracting myoepithelial cells around the alveoli.
- Milk ejection: the contraction pushes milk into the ducts so the infant can receive it, and stress or pain can inhibit this reflex.
- Supply regulation: removal of milk drives further production, while milk left in the breast slows synthesis through a feedback inhibitor.
Synthesis and ejection work as a pair, so good production with poor let-down can still leave a baby hungry at the breast. Calm surroundings, skin-to-skin contact and frequent emptying support both reflexes.
Where the exam tries to catch you
Stems usually ask which hormone does what, and which practice is unnecessary or unsafe.
- Trap: attributing milk ejection to prolactin. Correct: oxytocin causes let-down, while prolactin drives synthesis, and each reflex has a different trigger.
- Trap: discarding colostrum as inadequate. Correct: colostrum is rich in secretory IgA and clears meconium, so it should be given from birth.
- Trap: feeding by a strict clock. Correct: on-demand feeding about 8 to 12 times in 24 hours supports supply and weight gain.
- Trap: offering water to exclusively breastfed infants in hot weather. Correct: exclusive breastfeeding meets all the water needs of an infant, including in hot climates.
Frequently asked questions
When does mature milk replace colostrum?
Transitional milk follows colostrum from about day 4, and mature milk is established by about two weeks after birth. The composition changes gradually across this period rather than switching at a single point.
What is the WHO recommendation for exclusive breastfeeding?
The WHO recommends exclusive breastfeeding for the first six months, meaning no other food or drink, including water. Breastfeeding then continues alongside complementary foods for up to two years or beyond.
How can a mother tell that her baby is getting enough milk?
A baby getting enough milk has at least six wet nappies a day after the first week, regular stools, and regains birth weight by about two weeks. Feeding about 8 to 12 times in 24 hours is typical in this pattern.
Why does milk leak when a baby cries?
Leakage during crying reflects the oxytocin let-down reflex, which can be triggered by sensory cues such as an infant's cry. It is a normal conditioned response and does not indicate a problem with supply.
Does a breastfed baby need extra water in hot weather?
Breastfed infants do not need extra water, even in hot climates, because breast milk meets their fluid needs. Water supplements can displace milk intake and are not recommended during exclusive breastfeeding.