In this case it is a developmental problem caused by insufficient growth in length of the major milk ducts underneath the nipple. The short ducts pull the nipple inwards causing inversion.

Often, nipple inversion develops in a previously 'normal' nipple. This is most often the result of chronic inflammation underneath the nipple (periductal mastitis), which causes scarring of the ducts. The scarred milk ducts shorten and gradually pull the nipple inwards.

Breast cancer can also pull on the breast ducts and cause nipple inversion, it is therefore very important to have a breast check if you noticed that your nipple has recently come inverted.