Armpit lumps during breastfeeding are surprisingly common, and in most cases they’re caused by breast tissue that naturally extends into the underarm area. This tissue, sometimes called the “tail” of the breast, responds to the same hormonal signals as the rest of your breast, so it can swell, harden, and become painful when your milk comes in or when milk isn’t draining well. The good news: most of these lumps respond to simple home care and resolve on their own once your body adjusts to breastfeeding or after you wean.
Why You Have a Lump There
Roughly 2% to 6% of women have extra breast tissue (called accessory or ectopic breast tissue) that sits in the armpit rather than the main breast mound. Many never know it’s there until pregnancy or breastfeeding triggers it to swell. During lactation, this tissue engorges just like the rest of your breast, producing a firm, tender lump that can range from grape-sized to much larger. There’s likely a genetic component, so if your mother or sister experienced this, you’re more likely to as well.
A second common cause is a plugged milk duct. Milk ducts extend toward the armpit, and when one gets blocked, you’ll feel a tender, sore knot. This usually affects one side at a time. Pressure from a tight bra, a seatbelt, or even sleeping on your stomach can compress ducts and trigger a blockage.
Less often, the lump is a swollen lymph node reacting to a nearby infection, or the early stage of mastitis. Telling the difference matters because the treatment changes.
Plugged Duct vs. Mastitis
A plugged duct and mastitis can feel nearly identical at first. Both cause a sore, localized lump, and both can improve within a day or two. The key difference is what happens next. Mastitis brings flu-like symptoms: fever at or above 101°F (38.3°C), chills, body aches, nausea, or fatigue. The skin over the affected area may turn pink or red, often in a wedge-shaped pattern, and feel warm or hot to the touch. You might also notice a yellowish nipple discharge.
If your lump comes with any of those systemic signs, it’s worth getting medical attention quickly. Untreated mastitis can progress to a breast abscess, which is a pocket of pus that may need to be drained.
Home Techniques That Help
Massage Toward the Armpit
The most effective self-care is massage that moves fluid from the breast toward the underarm lymph nodes. Start near the areola and stroke outward toward the armpit, using light to moderate pressure. Repeat this three times, then knead the breast gently in alternating clockwise and counterclockwise circles. Finish by repeating those long strokes from areola to armpit. Aim to do a shorter version of this (about five minutes with light pressure) at least three times a day, ideally right before nursing or hand expressing.
Warm and Cold Therapy
Before a feeding or massage session, apply a warm compress or hot towel to the lump for a few minutes. Warmth helps loosen the tissue and encourages milk to flow. After feeding, a cool compress can reduce swelling and ease soreness. Some people alternate the two throughout the day depending on what feels best.
Frequent, Effective Milk Removal
The underlying problem is almost always milk that isn’t moving. Nurse or pump frequently, and if possible, position your baby so their chin or nose points toward the lump. This directs the strongest suction toward the blocked area. If the lump is in your armpit, a “football hold” (baby tucked under your arm on the same side) often works well because it angles suction toward the outer breast and axilla.
Therapeutic Ultrasound
If home methods aren’t enough, a physical therapist or lactation-trained clinician can apply therapeutic ultrasound to the area. This involves a small probe moved slowly over the lump using pulsed sound waves, typically for about five minutes per session. Research on lactating women has found this technique, combined with massage education and proper feeding technique, helps resolve stubborn engorgement and blocked ducts that don’t clear with home care alone.
Preventing It From Coming Back
Bra pressure is one of the most avoidable triggers. A bra that’s too tight or has an underwire pressing into the outer breast can compress ducts and block drainage. Switch to a well-fitted nursing bra without underwire, and avoid anything that digs into the tissue near your armpit. The same goes for purse straps, baby carrier straps, and sleeping positions that put sustained pressure on one breast.
Keeping feedings consistent also matters. Skipping or delaying feedings allows milk to pool, which is how plugged ducts form. If your baby sleeps through a feeding, hand express or pump just enough to relieve fullness. You don’t need to empty the breast completely, but avoiding prolonged engorgement makes a real difference.
After each feeding, a quick self-massage stroking from the areola toward the armpit helps clear residual milk from the outer ducts. This takes less than a minute and can prevent the stasis that leads to lumps.
What a Doctor Looks For
If your lump doesn’t respond to a few days of home care, or if it feels different from typical engorgement, an ultrasound can clarify what’s going on. On imaging, normal breast tissue in the armpit looks just like the glandular tissue in the rest of your breast. It’s clearly different from a cyst, which appears as a round, fluid-filled black circle with bright echoes behind it, or from something more concerning like a solid mass with irregular borders.
An abscess, which can develop as a complication of mastitis, shows up as a fluid-filled pocket where the liquid visibly moves when the technician presses the probe against the skin. This distinction is important because an abscess typically needs drainage, while engorged tissue and plugged ducts do not.
The imaging is quick, painless, and gives a definitive answer. If your lump is firm, doesn’t change with feeding, keeps growing, or is accompanied by skin changes unrelated to breastfeeding, an ultrasound is a reasonable next step.
How Long Until It Goes Away
A plugged duct that responds to massage and frequent nursing often resolves within one to two days. Engorgement of accessory armpit tissue tends to be worst in the early postpartum days when your milk first comes in, then gradually improves as your supply regulates over the first few weeks.
If the lump is caused by accessory breast tissue that actively produces milk, it may remain somewhat swollen throughout breastfeeding but should become less tender as your body adjusts. After weaning, breast tissue undergoes a process called involution where the milk-producing structures shrink back. Research shows significant regression within 12 months of stopping breastfeeding, and by 18 months post-weaning, breast tissue (including any accessory tissue in the armpit) typically returns to its pre-pregnancy state. So even in stubborn cases, the lump is temporary.