Breast-feeding is a learned skill, and poor technique can lead to milk being trapped in the breast, a main cause of mastitis. Other causes include:

  • A blocked milk duct. If a breast doesn't completely empty at feedings, one of your milk ducts can become clogged, causing milk to back up, which leads to breast infection.
  • Bacteria entering your breast. Bacteria from your skin's surface and baby's mouth can enter the milk ducts through a break or crack in the skin of your nipple or through a milk duct opening. Stagnant milk in a breast that isn't emptied enough provides a breeding ground for the bacteria. The antibacterial properties of your milk help protect your baby from the infection.
June 12, 2015