In breastfeeding women, mastitis is often caused by a build-up of milk within the breast – known as milk stasis. In some cases, this build-up of milk can also become infected.

In non-breastfeeding women, mastitis most often occurs when the breast becomes infected as a result of damage to the nipple.

Mastitis in breastfeeding women

Milk stasis

Many cases of mastitis in breastfeeding women are thought to be caused by milk stasis. This occurs when the milk is not properly removed from your breast during breastfeeding. It can be caused by:

• a baby not properly attaching to the breast during feeding – this may mean that not enough milk is removed (see breastfeeding position and attachment for advice on helping your child feed correctly)
• a 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)
• infrequent or missed feeds – for example, when they start to sleep through the night
• favouring one breast for breastfeeding – for example, because one of your nipples is sore; this can lead to milk stasis developing in the other breast
• a knock or blow to the breast that damages the milk duct or the glands in your breast
• pressure on your breast – for example, from tight-fitting clothing (including bras), seat belts or sleeping on your front

Milk stasis can cause the milk ducts in your breasts to become blocked, and can cause milk to build-up within the affected breast.

Experts are not sure exactly why breast milk can cause the breast tissue to become inflamed. One theory is that the pressure building up inside the breast forces some milk into the surrounding tissue.

Your immune system may then mistake proteins in the milk for a bacterial or viral infection and responds by inflaming the breast tissue, in an effort to stop the spread of infection.

Infection

Fresh human milk does not usually provide a good environment in which bacteria can breed. However, milk stasis can cause milk to stagnate and become infected. This is known as infective mastitis.

Exactly how bacteria enter the breast tissue has not been conclusively proven. It may be that the bacteria that usually live harmlessly on the skin of your breast enter through a small crack or break in your skin, or that bacteria present in the baby's mouth and throat are transferred during breastfeeding.

You may be at greater risk of developing infective mastitis if your nipple is damaged – for example, as a result of using a manual breast pump incorrectly or your baby having a cleft lip or palate (an opening or split in their lip or roof of their mouth).

Mastitis in breastfeeding women is more likely to be caused by an infection if self-help measures to express milk from the affected breast have not improved symptoms within 12-24 hours. Read more about treating mastitis.

Mastitis in non-breastfeeding women

In women who don't breastfeed, mastitis is often caused by a bacterial infection. This can occur as a result of bacteria getting into the milk ducts through a cracked or sore nipple, or a nipple piercing.

This type of mastitis is known as periductal mastitis. It usually affects women who are in their late 20s and early 30s, and is more common among women who smoke.

Occasionally, mastitis can occur in non-breastfeeding women as a result of duct ectasia. This is when the milk ducts behind the nipple get shorter and wider as the breasts age. It typically occurs in women approaching the menopause.

Duct ectasia is usually nothing to be concerned about, but in some cases a thick, sticky secretion can collect in the widened ducts, and this can irritate and inflame the duct lining.