Human Milk Production
During the establishment of lactation, there are three stages of human milk: colostrum, transitional milk and mature milk.
Colostrum production begins early in pregnancy and may last for several days after birth. In most cases colostrum is replaced by transitional milk within two to four days after birth. Colostrum is a yellowish or creamy appearing fluid that is thicker (and more concentrated) than later milk and contains more protein, fat-soluble vitamins and minerals so your baby needs less volume than what will appear with later milk production.
Transitional milk is produced from the end of colostrum production for about two weeks until the mature milk comes in.
Mature milk contains about 10% solids (proteins, fats, carbohydrates) for energy and the rest is water, which is vital for maintaining hydration.