Transitional milk is high-protein breast milk that a woman produces about 3 to 6 days after her baby is born. A woman's breasts are stimulated to produce transitional milk by breastfeeding her baby regularly.

The breasts make transitional milk after a period of producing colostrum, which is a thick, sticky, yellowish liquid that contains important nutrients and antibodies that a baby needs right after birth. After a mother's transitional milk comes in, she typically notices a big change in the volume and type of milk and an increase in the weight and size of her breasts.

Mature milk, which has more fat and less protein than transitional milk, starts being produced about 10 to 15 days after the baby is born.