Mastitiscan be caused by a build-up of milk within the breast (milk stasis) or damage to the nipple, which may lead to a bacterial infection.

Mastitis in breastfeeding women

Milk stasis

Many cases of mastitis in breastfeeding womenare thought to be caused by milk stasis. This occurs whenthe milk isn't properly removed from your breast during breastfeeding.

Itcan be caused by:

  • a baby not properly attaching tothe breast during feeding thismay mean that not enough milk is removed; see breastfeeding position and attachment foradvice on helping your child feed correctly
  • ababy 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 ofyour 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 aren't 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 milkfor a bacterial or viral infection and responds by inflaming the breast tissue in an effortto stop the spread of infection.


Fresh human milk doesn't 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 hasn't been conclusively proven.

The bacteria that usually live harmlessly on the skin of your breast may enter through a small crack or break in yourskin, or bacteria present in the baby's mouth and throat may be transferred during breastfeeding.

Youmay be at greater risk ofdeveloping infective mastitis if your nipple is damaged forexample, as a result ofusing a manual breast pump incorrectly or becauseyour baby has 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 haven't improved symptoms within 12 to 24 hours.

This can occur as a result ofbacteria getting into the milk ducts through a cracked or sore nipple, or a nipple piercing.

This type of mastitis is known as periductal mastitis. Itusually affects women 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 whenthe milk ducts behind the nipple get shorter and wider as the breasts age. It typically occurs in women approaching the menopause.

Duct ectasiais usually nothing to be concerned about, but in some casesathick, stickysecretion can collect in the widened ducts, and this can irritate and inflame the duct lining.

Content supplied by the NHS Website

Medically Reviewed by a doctor on 28 Nov 2016