Situation: A 22-year-old primipara gave birth vaginally to a healthy term newborn at a lying-in clinic, and her episiotomy was repaired. She plans to breastfeed exclusively, and the nurse follows her through the first days after birth. The mother asks when her breasts will start making larger amounts of milk. The nurse's answer is based on the usual timing of lactogenesis II. When does this usually occur?

In the first days the breasts secrete colostrum, which is small in volume but rich in antibodies and protein. The milk "comes in" (lactogenesis II) around days 2 to 5, often with fullness or engorgement. Transitional milk then gives way to mature milk at about 2 weeks.

Timing of Lactogenesis II

Lactogenesis is divided into two phases. Lactogenesis I begins during pregnancy and results in the production of colostrum, a thick, yellowish fluid that is low in volume but concentrated with immunoglobulins, protein, and fat-soluble vitamins. After delivery of the placenta, the abrupt fall in progesterone allows prolactin to drive the transition to lactogenesis II, the onset of copious milk secretion. This is the phase mothers recognize as the milk "coming in," often accompanied by breast fullness, warmth, and sometimes engorgement.

Lactogenesis II typically occurs between 2 and 5 days postpartum. The earliest measurable changes are biochemical: milk lactose, citrate, and glucose concentrations rise sharply as tight junctions in the mammary epithelium close and copious secretion begins ^[3]. Clinically, the mother perceives fullness or "coming in" around the same window, although the subjective sensation may lag slightly behind the biochemical shift ^[3].

Milk volume data support this timeline. In a longitudinal study of exclusively breastfeeding women, milk transfer to the infant was low on days 1 and 2 and increased rapidly to 498 ± 129 g/day by day 5, followed by a slower rise to 753 ± 89 g/day during months 3–5 ^[1]. This steep early increase corresponds to the lactogenesis II window and confirms that the transition is well underway by the middle of the first week.

Phase	Timing	Secretion	Clinical features
Lactogenesis I	Pregnancy through first 1–2 days postpartum	Colostrum	Small volume; high immunoglobulin and protein content
Lactogenesis II	Days 2–5 postpartum	Copious milk secretion begins; transitional milk	Breast fullness, warmth, possible engorgement
Mature milk	About 2 weeks postpartum	Mature milk	Stable composition; volume continues to rise gradually

Key point! The onset of lactogenesis II is defined biochemically and clinically by the shift from low-volume colostrum to copious milk secretion, not by the attainment of full mature milk volume. Mature milk composition is reached later, around 2 weeks, which is why option 4 is incorrect for the "milk coming in" question.

Watch out! Delayed onset of lactogenesis II is defined as onset at ≥72 hours postpartum . Risk factors include higher body mass index, older maternal age, larger infant birth weight, and metabolic conditions such as diabetes . For a primipara, delayed onset is more common than in multiparas, so nurses should assess for signs of successful transition—audible swallowing, relaxed feeding, and the mother's report of breast fullness—during the first several days.

The nurse can explain to this mother that her breasts are already producing colostrum in the first day or two, and that the larger volume of milk usually arrives between days 2 and 5. Frequent, effective breastfeeding or milk removal during this window supports the hormonal and local control mechanisms that complete lactogenesis II.