Inverted nipple
An inverted nipple (occasionally invaginated nipple) is a nipple that, instead of pointing outward, is retracted into the breast. In some cases, the nipple will be temporarily protrude if stimulated, but in others, the inversion remains regardless of stimulus.  Women and men can have inverted nipples.  A given individual may have one or two inverted nipples.  Some doctors estimate that up to ten to twenty percent of women have some form of inverted nipples.
An inverted nipple can be a symptom of Breast Sheet Cancer.  Sheet Cancer is hard to detect in a mammogram and is frequently in a fatal stage before detection.

# Pregnancy and breastfeeding
Women with inverted nipples may find that their nipples protract (come out) temporarily or permanently during pregnancy, or as a result of breastfeeding.  Most women with inverted nipples who give birth are able to breastfeed without complication, but inexperienced mothers may experience higher than average pain and soreness when initially attempting to nurse.  When a mother uses proper breastfeeding technique, the infant latches onto the areola, not the nipple, so women with inverted nipples are actually able to breastfeed without issue.  An infant that latches on well may be able to draw out an inverted nipple.  The use of a breast pump or other suction device immediately before a feeding may help to draw out inverted nipples. A hospital grade electric pump may be used for this purpose.  Some women also find that using a  nipple shield can help facilitate breastfeeding.

# Methods which cause inverted nipples to protract
Some people with inverted nipples consider them to be a cosmetic deformity and seek methods to protract them.
Plastic surgery is one method of protracting inverted nipples.  If a woman elects to have this surgery performed on her inverted nipples, it can permanently destroy her capacity to breastfeed.  Another method of protracting inverted nipples is to have the nipple pierced. This method will only be effective if the nipple can be temporarily protracted. If pierced when protracted, the jewelery may prevent the nipple from returning to its inverted state.  Like surgery, this method can have adverse effects.  The success of both of these methods, from a cosmetic standpoint, is mixed.
Other strategies for protracting inverted nipples include regularly stimulating the nipples to a protruding state, in an attempt to gradually loosen the nipple tissue.  Some  sex toys designed for nipple stimulation, such as suction cups or clamps may also cause inverted nipples to protract or stay protracted longer.  There are special devices specifically designed to draw out inverted nipples or a homemade nipple protractor can be constructed out of a 10 cc disposable syringe.  These methods are often used in preparation for breast-feeding, which can sometimes cause inverted nipples to become protracted permanently.
Two methods which are now discouraged are  breast shells and the Hoffman technique.  Breast shells may be used to apply gentle constant pressure to the areola in order to try and break any adhesions under the skin that are preventing the nipple from being drawn out. The shells are worn inside the bra. The Hoffman technique is a nipple stretching exercise that may help loosen the adhesions at the base of the nipple when performed several times a day. Although both techniques are heavily promoted, a 1992 study found that not only do shells and the Hoffman technique not promote more successful breastfeeding, they may actually disrupt it.