Breast engorgement is the overfilling of the breasts with milk. This can happen when milk isn't being removed well from the breasts by breastfeeding, pumping, or expressing by hand. Severely engorged breasts become increasingly hard, swollen, and tender. The nipples and areolae can become hard and flattened, making it difficult for a baby to latch on to the breast properly.
A mother with a regular breastfeeding routine can become engorged if she cannot nurse or pump as much as usual or suddenly stops breastfeeding. A mother who doesn't begin breastfeeding after childbirth will have several days of mild to moderate breast engorgement. This gradually goes away when the breasts aren't stimulated to produce more milk.
Severe breast engorgement can cause a slight fever and tender lymph nodes in the armpits. Without treatment, severe engorgement can lead to blocked milk ducts and breast infection (mastitis).