During the first few days after delivery, the breasts produce colostrum. This is a thin yellowish fluid that is the same fluid that sometimes leaks from the breasts during pregnancy. It is rich in protein and antibodies that provide passive immunity to the baby (the baby's immune system is not fully developed at birth). Colostrum also helps the newborn's digestive system to grow and function properly. After 3 to 4 days, breasts will begin producing milk that is thin, watery, and sweet. This quenches the baby's thirst and provides the proteins, sugar, and minerals that the baby needs. Over time, the milk changes and becomes thick and creamy. This satisfies the baby's hunger.