Many women feel an unusual lump in their breast at some point during their lives; thankfully, only a tiny percentage of those lumps are cancer. A common source of breast lumps is the breast cyst, a benign yet annoying condition experienced by about 1 in 3 pre-menopausal women. What are cysts? If you feel a breast lump, can you tell if it’s a cyst? And what should you do about a cyst in your breast?

Q. I was in the shower this morning, and felt a lump in my breast. I check my breasts most days while showering, and I know this lump wasn’t there yesterday. Can breast cancer appear that quickly?

A. No, breast cancer can’t grow overnight. It’s possible, if a woman is unfamiliar with the usual shape and feel of her breasts, that she may notice a lump that seems to have “just appeared,” and that the lump turns out to be cancer. But most lumps that actually do crop up overnight are cysts.

Q. What’s a cyst? Is it dangerous?

A. A cyst is like an internal blister: a fluid-filled sac. There’s no known cause for cysts, though they’re quite common in younger women; and this leads some researchers to believe that their formation is tied to hormones, particularly estrogen.

A cyst forms when connective tissue and glands in the breast grow too quickly, for some reason; again, this may be hormonal. This overgrowth of tissue can block a milk duct in the breast, causing it to fill with fluid; this fluid-filled duct is called a cyst.

Cysts can be tiny, and disappear on their own; or they may grow as large as a golf ball, become painful, and need to be drained. But cysts aren’t considered dangerous; they don’t signal cancer, nor will a cyst turn into a cancerous tumor.

Q. Can I tell by feeling the lump if it’s a cyst?

A. You can make an educated guess, but only testing can determine for sure whether the lump you feel is a cyst.

Cysts generally feel squishy to the touch (think water balloon), and moveable, rather than firmly fixed in place. Most cysts feel like a smooth round or oval; though at times they may feel irregularly shaped.

Even if the lump you feel seems hard or firm, however, don’t panic; cysts that form deep within the breast can push the tissue around them out of shape, making normal breast tissue feel like a lump.

Another indication that the lump may be a cyst is pain. Cancerous breast lumps are seldom painful; cysts often are. If you don’t see a doctor right away, and notice during the next several weeks that the lump becomes painful at times (particularly in the days before your period), this is another indication that it’s a cyst, not cancer.

Q. So, what’s the next step?

A. It’s perfectly reasonable to wait a few weeks and see if the lump disappears. If it does, then it was most likely something connected with your menstrual cycle; breast cancer doesn’t come and go in synch with your period.

If the lump doesn’t disappear, then you’ll want to see your doctor.