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Causes of mastitis

NHS Choices Medical Reference

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Mastitis can have different causes depending on

whether it is infectious or not.


Non-infectious mastitis

Most cases of non-infectious mastitis are thought to be caused by milk stasis. Milk stasis occurs when the milk is not properly removed from your breast during breastfeeding. It can be caused by:

  • your baby not being properly attached to your breast during feeding - which may mean that not enough milk is removed or it may cause pain in your nipple
  • your baby having problems sucking - for example, because they have a  tongue-tie (a piece of skin between the underside of their tongue and the floor of their mouth)
  • your baby having infrequent feeds or missing feeds - for example, when they start to sleep through the night
  • your baby favouring one breast for breastfeeding - this can lead to milk stasis developing in the other breast
  • pressure on your breast - for example, from tight-fitting clothing, an over-restrictive bra or sleeping on your front

Milk stasis can cause the milk ducts in your breasts to become blocked.

Experts are not sure why breast milk can cause the breast tissue to become inflamed. However, one theory is that it may be due to the presence of special proteins called cytokines in your breast milk.


Cytokines are proteins used by the immune system. They are passed on to your baby to help them resist infection.

It may be that your immune system mistakes cytokines for a bacterial or viral infection and responds by inflaming the breast tissue in an effort to stop the spread of infection.

Infectious mastitis

Fresh human milk does not usually provide a good environment in which bacteria can breed. However, if your milk ducts become blocked, milk can stagnate and an infection can develop.

Exactly how bacteria enter the breast tissue has not been conclusively proven. It may be that:

  • bacteria that usually live harmlessly on the skin of your breast enter through a small crack or break in your skin
  • bacteria present in the baby's mouth and throat are transferred during breastfeeding

You may be at greater risk of developing infectious mastitis if:

  • your nipple is damaged - for example, as a result of using a manual breast pump incorrectly; a breast pump is a device that is used to express milk from your breast
  • your baby has a  cleft lip or palate (an opening or split in their lip or roof of their mouth); this can also damage your nipple

Mastitis in non-breastfeeding women

In women who don't breastfeed, mastitis is caused by an infection.

This can be due to bacteria getting into the milk ducts through a cracked or sore nipple, or a nipple piercing.

There are two different types of mastitis that affect women who are not breastfeeding. These are:

  • periductal mastitis - which usually affects women who are in their late 20s and early 30s, and is more common among smokers
  • duct ectasia - which tends to affect women in the years before the  menopause (when a woman's periods stop), or after the menopause
Medical Review: June 11, 2012

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