One of the scrub techs that I worked with this past week, asked me "Dr Bates, what causes a nipple to be inverted?"
So I begin to launch into the different grades of inverted nipples and how they can be treated, but I reined myself in. "Usually the problem is the ducts are too short. Why?"
"I have a friend who has an inverted nipple. She called and asked me. Why do they do that -- I'm just a scrub tech."
"How long has hers been inverted? And how old is she?" I ask.
"Only a little over a month. She's 35 yo."
Not an inverted nipple, but a retracted one. One of the changes you look for during a breast exam. "She needs to go see her doctor and get a mammogram. She will most likely need a biopsy of the nipple. She very likely will have breast cancer."
"I think that's what she was afraid of. Me, I'd want to know, but she's afraid to find out."
"Please, tell your friend not to put it off. I know she is afraid, but the sooner it's diagnosed the better."
There are benign and malignant reasons for a retracted nipple, but all require a history & physical and a diagnostic mammogram. There is a very nice set of algorithms for evaluation of breasts here, including a larger version of this one (photo credit).
Even though this article is over one hundred years old, it is still relevant. It helps keep it all in perspective.
On the Diagnostic Value of the Retracted Nipple as a Symptom of Disease of the Breast; Br Med J. 1866 December 8; 2(310): 635–637; Thomas Bryant (PDF file)