Babies of women experiencing nipple pain have higher peak vacuum pressures in their mouths while breastfeeding compared to babies of women who aren't experiencing pain. The Human Lactation Research Group found that two-thirds of women with persistent nipple pain despite standard fit and hold interventions by International Board Certified Lactation Consultants had stronger vacuums while breastfeeding compared to babies whose mothers didn't have pain.

This finding has been interpreted as showing that some babies must naturally generate very high negative suction pressures, which are more likely to damage their mothers.

But that theory doesn’t take into account the highly dynamic nature of the physical or mechanical interaction between mother and baby during breastfeeding.

In Possums Breastfeeding & Lactation, I suggest that these babies are experiencing breast tissue drag, which causes not only pain, but also a compensatory increase in vacuum pressure as baby attempts to draw up more breast tissue into the mouth (or even just to hold on to what breast tissue is in his mouth) in response to the breast tissue drag. This increased stretching load will cause pain and damage.

The stretching force generated by the vacuum in the baby’s mouth needs to be spread out over a larger surface area of nipple and breast tissue to avoid the nipple pain.