During the second trimester, increased levels of hormones will stimulate the production of milk in the mammary glands as well as the growth of milk ducts in your breasts. (Milk ducts are lobes in your mammary gland at the tip of the nipple.) But it’s not until after you deliver that you’ll begin full-scale milk production.
Right after you give birth (but sometimes as early as the second trimester), your breasts produce something called colostrum, which looks like a thick, yellowish fluid. It’s full of antibodies and will help strengthen baby’s immune system. Since baby only needs a small amount of nutrients in these first few days (because of their small tummy size), your body will naturally hold off on allowing actual breast milk to flow until after baby’s three or four days old. By this point, your body will produce milk in a supply-and-demand-like manner, making as much it needs to based on your breastfeeding patterns.