Breast milk is produced by the female to feed her childs. The milk is produced by the mammary glands and has a different composition depending on the age of the baby in order to meet its needs at different ages. There are three types of milk: colostrum, which is produced in the third trimester of pregnancy and 2-3 days after birth, transitional milk, which is produced between the first and fourth week, and mature milk.

Breast milk is the main source of nutrition for newborns. It contains fats, proteins, carbohydrates (lactose and oligosaccharides from human milk), and variable minerals and vitamins. Breast milk also contains factors that are important in protecting the infant from infection and inflammation, while contributing to the healthy development of the immune system and gut microbiome. For example, colostrum is two-thirds white blood cells, which will enable the baby to defend itself against infections quickly by making antibodies.