Most lumps found under the breast are benign. Cysts are the single most common cause of breast lumps overall, and many other possibilities, from fatty growths to irritation from a bra underwire, are equally harmless. That said, any new lump deserves attention so you can rule out the small percentage of cases that need treatment.
The area under the breast sits right where skin folds meet breast tissue, ribs, and cartilage, so a lump there can originate from several different structures. Understanding what each type feels like can help you describe it to a doctor and ease some of the anxiety while you wait for an appointment.
Fluid-Filled Cysts
Breast cysts are closed, fluid-filled sacs that can form anywhere in the breast, including along the lower edge near the bra line. Near the surface, a cyst feels like a smooth, round blister. When it sits deeper in the tissue, it can feel surprisingly hard because layers of breast tissue cover it. Cysts are often tender and may change size with your menstrual cycle, growing larger in the days before your period and shrinking afterward. They are not cancerous.
Fibroadenomas
Fibroadenomas are solid, noncancerous tumors that feel rubbery, smooth, and firm. The hallmark feature is that they move freely when you press on them, almost like a marble sliding under the skin. They are typically painless. Fibroadenomas are most common in women in their 20s and 30s, though they can appear at any age. They sometimes grow slowly over months or years but rarely require removal unless they become large or bothersome.
Lipomas
A lipoma is an overgrowth of fat cells just beneath the skin. It feels soft, doughy, and movable, and it’s usually painless. Lipomas grow slowly and can appear anywhere on the body, including the underside of the breast. They are almost always benign and are often left alone unless they grow large enough to cause discomfort.
Skin and Bra-Related Causes
The fold under the breast is a friction zone. An ill-fitting bra, especially one with a rigid underwire, can dig into this crease and cause localized swelling or a small, irritated bump. Repeated friction and moisture in the skin fold can also lead to blocked glands or minor infections.
An abscess, a pocket of pus from a bacterial infection, can develop in this area too, particularly after a small skin break or injury. Abscesses usually come with obvious signs: the skin turns red and warm, the lump is painful, and you may feel feverish or unusually tired. These need medical treatment, typically drainage and antibiotics.
Fat Necrosis After Injury or Surgery
If you’ve had any trauma to the chest (a seatbelt injury, a fall, a prior biopsy, or breast surgery), damaged fat cells can die and form a firm, painless, round lump called fat necrosis. It is completely benign, but it poses a diagnostic challenge: on both physical exam and imaging, fat necrosis can look a lot like cancer. If you have a history of chest injury or surgery, mention it to your doctor so they can factor that into their assessment.
Fibrocystic Breast Changes
Many people notice that their breasts feel generally lumpier or more tender at certain points in their cycle. This is fibrocystic change, driven by fluctuating hormone levels, especially estrogen. The lumps tend to be most noticeable just before your period, then shrink back to their usual size once your period starts. Both breasts are typically affected, and the texture can feel ropy or granular rather than like a single distinct mass.
If your symptoms follow a predictable monthly pattern, that’s a strong clue that hormones are driving them. Some doctors recommend tracking your symptoms across two or three cycles to confirm the pattern. Hormonal birth control can sometimes smooth out the fluctuations and reduce discomfort.
Costochondritis: A Chest Wall Mimic
Not every lump in this area actually comes from breast tissue. Costochondritis is inflammation of the cartilage that connects a rib to the breastbone. It can produce a tender, swollen spot that feels like a lump sitting right under or behind the breast. The key difference is that the pain worsens when you take a deep breath, cough, sneeze, or twist your torso. It often affects more than one rib and tends to show up on the left side. A related condition, Tietze syndrome, adds visible swelling over the affected cartilage. Both resolve on their own over time, though anti-inflammatory medication can speed things along.
When a Lump Needs Prompt Attention
The physical characteristics of the lump itself offer useful clues. A cancerous lump is typically hard (not soft or squishy), has irregular or jagged edges rather than smooth borders, and stays fixed in place when you try to move it. Most breast cancers are painless, which is why “it doesn’t hurt” is not reassuring on its own.
Beyond the lump, watch for changes in the skin or nipple on that side:
- Dimpling or puckering of the skin, sometimes described as an orange-peel texture
- Redness, rash, or flaky skin on the breast or around the nipple
- Nipple discharge that isn’t breast milk, especially if it’s bloody or comes from only one side
- A nipple that suddenly pulls inward
- Swelling or thickening in part of the breast, even without a distinct lump
- Any change in breast size or shape that’s new and one-sided
Any of these signs warrants a prompt visit to your doctor, regardless of the lump’s other characteristics.
What Happens at the Doctor’s Office
Your doctor will start with a physical exam, pressing on the lump to assess its size, texture, and mobility. From there, the standard next step for a palpable lump is imaging. For most people, that means a mammogram and a breast ultrasound, done in either order. If you’re pregnant or breastfeeding, ultrasound comes first because it doesn’t use radiation.
If imaging suggests the lump is a simple, fluid-filled cyst, that may be all you need. If the results are unclear or look suspicious, a biopsy is the next step. The most common type is a fine-needle aspiration, where a thin needle draws out fluid or a small tissue sample. It takes about 10 to 15 minutes under local numbing and is done in an office or radiology suite. A core needle biopsy uses a slightly larger needle to collect more tissue and takes 20 to 60 minutes, also under local anesthesia. In either case, a pathologist examines the sample under a microscope to determine whether the cells are benign or cancerous. Surgical removal of the lump (excisional biopsy) is reserved for situations where less invasive methods don’t provide a clear answer.
The vast majority of breast lumps evaluated this way turn out to be benign. But getting a definitive answer, rather than monitoring a lump on your own, removes the guesswork and the worry.