Just like breasts, nipples come in all shapes and sizes.

Just as breasts come in many different shapes and sizes, so do nipples. While most people have nipples that either poke out or lie flat, some people's nipples actually poke inward—they're known as retracted or inverted nipples. And if you've had them all your life, they're totally, completely normal.

What are inverted nipples?

Inverted nipples lie flat against the areola and, in some cases, retract inward rather than stick out, says ob-gyn Alyssa Dweck, M.D.

Okay, but what do inverted nipples look like, exactly? "Inverted nipples can be bilateral or just on one breast," explains Dr. Dweck, adding that inverted nipples can sometimes appear retracted in one moment and "pop out" at other moments, often in response to stimulation from touch or cold temperatures. (Related: Why Do Nipples Get Hard?)

Typically, there are "no obvious causes" behind inverted nipples, says ob-gyn Gil Weiss, M.D., partner at the Association for Women's Healthcare in Chicago. "If you're born with inverted nipples, it's usually just a genetic difference in how your nipples were made," notes Mary Claire Haver, M.D., an ob-gyn at the University of Texas Medical Branch.

That said, in addition to genetic differences, shortened breast ducts can represent another possible inverted nipple cause, says Dr. Weiss. "Inverted nipples usually happen because breast ducts do not grow as fast as the remainder of the breast, causing [shortened breast ducts and] a retraction of the nipple," he explains. (Reminder: the breast duck, aka milk duct, is the thin tube in the breast that carries milk from the production glands to the nipple.)

Regardless of the cause, though, if you were born with inverted nipples, they don't increase your risk for health consequences, says Dr. Weiss. "Some difficulty in breastfeeding may occur, but a majority of women with inverted nipples can breastfeed without any problems," he adds.

What if you develop inverted nipples later in life?

If your nipples have always been outies and suddenly one or both pull inward, it may be cause for concern, cautions Dr. Haver. "If you develop one, this can be a sign of something more serious—like an infection or even a malignancy—and it warrants a trip to your doctor to get evaluated," she explains. Other symptoms that indicate you should get your breasts checked out: redness, swelling, pain, or any other change in the architecture of your breast. (Related: 11 Signs of Breast Cancer Every Woman Should Know About)

If you're breastfeeding and your nipple inverts, that's usually normal, Julie Nangia, M.D., medical director of breast oncology at the Comprehensive Cancer Center of Baylor College of Medicine, previously told Shape. However, sometimes an inverted nipple caused by breastfeeding can indicate something called mastitis, an infection of the breast tissue that can be caused by a blocked milk duct or bacteria that causes pain, redness, and swelling, notes Dr. Haver. (BTW, mastitis can also be behind itchy nipples.) If symptoms are mild, warm compresses and OTC painkillers usually help treat the infection. But sometimes antibiotics are needed.

Is it safe to get an inverted nipple piercing?

Interestingly enough, piercing an inverted nipple might actually help reverse the inversion, as the extra, sustained stimulation in that area might help keep the nipple erect, says Suzanne Gilberg-Lenz, M.D., board-certified ob-gyn and partner at the Women's Care of Beverly Hills Medical Group. "But it may also be more difficult or painful to pierce [an inverted nipple]," adds Dr. Gilberg-Lenz.

Plus, while some people believe an inverted nipple piercing can reverse the inversion, "no medical evidence for that exists," notes Dr. Weiss. "Risks of nipple piercing include, most commonly, pain and infection," he adds. "There is [also] risk of nipple discharge, numbness, difficulty nursing, and scar tissue with nipple piercing," confirms Dr. Dweck.

Can you "fix" an inverted nipple?

Technically, there is such a thing as inverted nipple corrector surgery, "but [it] will likely permanently damage the milk ducts and make breastfeeding impossible," warns Dr. Gilberg-Lenz. "It is only recommended for cosmetic preference and isn't considered a medical issue—I honestly would not recommend it."

"Other nonmedical procedures do exist, such as suction devices or even the Hoffman Technique (a manual home exercise that draws out the nipple by massaging tissue around the areola), but their efficacy has not been proven," adds Dr. Weiss. (Related: How a Breast Reduction Changed One Woman's Life)

Bottom line: Unless they develop out of nowhere or appear alongside other symptoms (redness, swelling pain, other changes in breast shape), inverted nipples usually aren't a cause for concern. Whether you have innies or outies, go ahead and #freethenipple.