Why Do Nipples Get Hard?

Yes, erect nipples can happen seemingly randomly. Here's what's going on when your nips stand at attention.

You probably don't spend too much time thinking about your nipples — until they demand your attention. Maybe that's when you're working out sans shirt and all of a sudden it's like there's a pair of headlights poking out from your sports bra. Or maybe every time the temperature drops below a certain degree, you feel like they could cut glass.

But what actually causes your nipples to get hard? Just as no two people have the exact same breast shape, no one's nipples respond the same to different scenarios. But, anatomically, there is a general explanation for erect nipples.

The Science Behind Why You Get Erect Nipples

Anatomy time: "Underneath the nipple and areola (the area surrounding the nipple), there are tiny muscles that contract and pull on the skin and push the nipple out," says Michelle Lee, M.D., a plastic surgeon based in Beverly Hills. In response to any kind of stimulation, "the sympathetic nervous system — part of the body's neural wiring that makes your heart race, skin get goosebumps, and palms sweat — sends signals to the nerves in those tiny little muscles that cause them to contract," she explains.

Of course, that stimulation might be sexual. You probably already know your nipples are a sensitive erogenous zone. Arousal — whether it's from physical stimulation (e.g. touch) or psychological stimulation (e.g. fantasizing about something hot and heavy) — triggers certain parts of your brain that "cause those muscles in your nipples to become erect, similar to the effect those aroused feelings can have on your genitals," says Heather Irobunda, M.D., an ob-gyn based in Queens, New York.

Just like a clitoris or penis, nipples are comprised of erectile tissue, a type of tissue that receives a ton of blood flow, says Dr. Irobunda. "When the genital sensory cortex is activated, that blood flow can cause that tissue to get harder or stiffer," she adds. The genital sensory cortex is the same part of the brain turned on by stimulation of the clitoris, vagina, and cervix. Studies show that nipple stimulation — or just thinking about the nipples being stimulated — activates this part of the brain.

But stimulation isn't just about arousal. If your nipples get hard as rocks when the AC is on high, that's totally normal. "When the temperature drops, those tiny muscles underneath the skin contract to trap the warm air near the skin and lose less heat from radiation coming off your skin," says Dr. Lee. "This is why the areola also contracts when the nipple is exposed to the cold. The contracted skin then pushes the nipple outward," she explains.

And sometimes, it's not even about stimulation. Hormones can also make your nipples hard when there's no other obvious reason. "During your period or even ovulation, changes in your hormone levels — especially estrogen — can cause your nipples to become more sensitive or even be more prone to become harder," notes Dr. Irobunda.

Other hormone-related nipple changes start in pregnancy and continue through breastfeeding. "During pregnancy, the hormone progesterone causes the formation of more milk lobules and duct systems," explains Dr. Lee. And during the late stages of pregnancy, many other hormones cause the areola to get larger and the nipple to get erect in preparation for breastfeeding, she adds. "During breastfeeding, mechanical stimulation of the nipple from the baby causes the body to release oxytocin; this hormone acts on the milk glands to contract and push milk out to the baby," says Dr. Lee.

What About Nipple Piercings?

If you've ever seen someone with their nips pierced, their erect nipples are probably pretty obvious — and you may have wondered whether those piercings alter how your nipples get hard. The answer? Yes and no.

Besides the fact that placing a large piercing at the base of the nipple will physically push it out, "having jewelry there causes increased sensitivity due to the constant stimulus to the nerves in the nipples," explains Dr. Irobunda. A nipple piercing doesn't mean that your nipples will be hard 100 percent of the time, but it can certainly make them more noticeable and, because they're more sensitive, they may become hard more easily.

If You're Concerned About Your Erect Nipples (or Lack Thereof)

Whatever your nipples do, there's probably no reason to be concerned. "Nipples come in all shapes and sizes, and depending on the size and position of the nipple, some nipples may appear to be never hard or always hard," says Dr. Lee. "How sensitive your nipples are to stimulus is genetically predetermined," she adds. (Note: Nipples that never get hard are commonly referred to as flat or inverted nipples.) But if you're self-conscious to the point where you don't want to take your shirt off in a workout class or at the beach, you may want to talk to your doctor.

And, on a more serious note, "if the nipple suddenly becomes retracted or inverted; or if you notice nipple discharge, dimpling, pain, or lump, it's important to see a doctor and figure out the reason why," says Dr. Lee.

And "if your nipples are not only hard but painful for an extended period of time (versus during certain parts of your cycle), I would recommend going to see your doctor," says Dr. Irobunda. "This may be indicative of a larger health problem that needs to be addressed," she adds. (See also: 11 Signs of Breast Cancer Every Woman Should Know)

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