What causes visible nipple damage (cracks, ulcers, bruising, or other wounds) when you're breastfeeding or lactating?

Your nipple skin can break if the mechanical pressure inside baby's mouth is too focussed on one small area of your nipple

When things are going well

The more breast tissue that is drawn up by the vacuum inside your baby's mouth when she suckles

• The closer the tip of your nipple sits to the place where baby's bony hard palate joins the soft palate in the back of her mouth, and

• The more your nipple and breast tissue inside her mouth expand.

This means that the mechanical pressure of the vacuum inside baby's mouth when she's sucking becomes spread out over a greater area on the surface of the nipple, areola and breast skin that has been drawn up. We hope to spread that pressure out over as much skin surface as possible.

When things go wrong

If there is nipple and breast tissue drag happening inside your baby's mouth, your nipple skin is experiencing a focussed area of intense stretching force, which can cause epithelial inflammation and pain, but also might break the skin on your nipple.

This results in visible trauma, ranging from cracks, blisters, and grazes to ulcers which ooze and slough. You can find a short animation of how breaks in the nipple skin happen here.

• Cracks can be small and hard to see but cause intense pain, often at the base of the nipple where it joins the areola, or buried in crevices on the nipple face.

• An ulcer occurs when the area of skin damage has spread.

• Blisters result when shearing forces are applied through your nipple skin layers, causing leakage of fluid or blood from broken capillaries so that the tissue swells with fluid or blood.

Sometimes it is possible to determine the direction of the conflicting forces inside your baby's mouth from the location of the cracks, though often we can't. For instance, if the crack is at 6 o’clock at the base of the nipple junction with the alveola, it may be that your baby is feeding higher than your breast wants to fall, causing upward tension on the nipple and breast tissue, which results in a crack at the base of the nipple.

If the crack bleeds enough during feeds, women may find that milk which dribbles from their baby’s mouth is red with blood, or that their baby pukes digested milk which is pink, or which contains fresh blood from their bleeding nipple. Sometimes, a bleeding nipple can result in your baby having black stool, or melaena stool. This can be frightening, but swallowing blood doesn’t harm your baby. It’s you we're worried about! If your baby is vomiting blood or has black stool, please see your GP.

You can find out why nipple wounds aren't cause by your baby's tongue here, even though you might often hear this said.

Too much moisture and hydration of your nipple makes the epithelium more likely to be damaged

Anything that makes your nipple moist over time makes the epithlium softer and more vulnerable to inflammation and damage from stretching pressures.

Milk itself is not a problem (unless the milk is building up against the nipple skin over a long period of time, which can happen with wet breastpads, wearable pumps, or nipple shields and silverettes). Milk is healing when it's allowed to dry on your skin, because of the many protective immune factors in it.

You can find out about this here.