An inverted nipple (occasionally invaginated nipple) is a condition where the nipple, instead of pointing outward, is retracted into the breast. In some cases, the nipple will be temporarily protruded if stimulated. Both women and men can have inverted nipples. Causes
The most common causes of nipple inversion include:

Born with condition
Trauma which can be caused by conditions such as fat necrosis, scars, or a result of surgery
Breast sagging, drooping or ptosis
Breast cancer
Breast carcinoma
Pagets disease
Inflammatory breast cancer
Breast infections or inflammations
Mammary duct ectasia
Breast abscess
Mastitis
Genetic variant of nipple shape, such as:
Weaver syndrome
Congenital disorder of glycosylation type 1A and 1 L
Kennerknecht-Sorgo-Oberhoffer syndrome
Gynecomastia
Recurrent infections
TuberculosisMost common nipple variations that women are born with are caused by short ducts or a wide areola muscle sphincter. Inverted nipples can also occur after sudden and major weight loss. Grading system
The three grades of inverted nipples are defined on how easily the nipple may be protracted and the degree of fibrosis existent in the breast, as well as the damage it has caused on the milk ducts. Inverted nipple grade 1 refers to nipples that can easily be pulled out, by using finger pressure around the areola. The grade-1 inverted nipple maintains its projections and rarely retracts. Also, grade-1 inverted nipples may occasionally pop up without manipulation or pressure. Milk ducts are usually not compromised and breast feeding is possible. These are "shy nipples". It is believed to have minimal or no fibrosis. There is no soft-tissue deficiency of the nipple.
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 breastfeed. 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 any problem. An infant that latches on well may be able to slush 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. Frequent stimulation such as sexual intercourse and foreplay (such as nipple sucking) also helps the nipple protract. Piercing
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 jewellery may prevent the nipple from returning to its inverted state. The success of both of these methods, from a cosmetic standpoint, is mixed. The piercing may actually correct the overly taut connective tissue to allow the nipple to become detached from underlying connective tissue and resume a more typical appearance. Other corrective strategies
Other strategies for protracting inverted nipples include regularly stimulating the nipples to a protruding state, in an attempt to gradually loosen the nipple tissue.