During pregnancy, the levels of prolactin increase, though lactation is inhibited until after delivery by high levels of placental steriods. Inhibition of lactation is removed after the placenta is delivered, and progesterone levels also drop after delivery. Lactation is possible anytime after 16 weeks gestation.
In a pregnant woman, both estrogen and progesterone maintain a gravid uterus, which in turn inhibit prolactin and oxytocin. Establishing a milk supply requires enough breast stimulation to cause frequent prolactin peaks, which is usually accomplished with 8-12 feeds every 24 hours. After 4-6 weeks, milk production becomes dependant on the autocrine system instead of any endocrine process. Thereafter, continued lactation is based on a supply-demand relationship driven by the infant.
Lactation begins with an outflow of colostrum, and then regular milk. Most mothers produce 750-1000mL of milk every 24 hours by postpartum day 14-21, though the mothers of twins may produce up to 2000mL in 24 hours. The components of breast milk change with the time of feeding and age of the baby.