A wet nurse is a woman who breast feeds and cares for another's child. Wet nurses are employed when the mother is unable or chooses not to nurse the child herself. Wet-nursed children may be known as "milk-siblings", and in some cultures the families are linked by a special relationship of milk kinship. Mothers who nurse each other's babies are engaging in a reciprocal act known as cross-nursing or co-nursing.
A wet nurse can help when a baby's natural mother is unable or chooses not to feed the infant. Before the development of baby formulas in the 20th century, when a mother was unable to breastfeed her baby, the baby's life was put in danger if a wet nurse was not available. There are many reasons why a mother is unable to lactate or to produce sufficient breast milk. Reasons include the serious or chronic illness of the mother and her treatment which creates a temporary difficulty to nursing. Additionally, a mother's taking drugs (prescription or recreational) may necessitate a wet nurse if a drug in any way changes the content of the mother's milk. Some women choose not to breastfeed for social reasons.
Wet nurses have also been used when a mother cannot produce sufficient breast milk, i.e., the mother feels incapable of adequately nursing her child, especially following multiple births. Wet nurses tend to be more common in places where the maternal mortality is high.