Swollen nipples in cats most commonly signal pregnancy, but they can also point to infection, hormonal shifts, or in some cases, tumors. The cause depends heavily on whether your cat is spayed or intact, male or female, and whether the swelling affects one nipple or several. Here’s how to narrow it down.
Pregnancy Is the Most Common Cause
If your cat is an unspayed female who has been outdoors or around intact males, pregnancy is the most likely explanation. One of the earliest visible signs is called “pinking up,” where the nipples become enlarged, darker, and noticeably pink or red. This typically happens around the two-week mark after conception, well before any belly growth is visible. If multiple nipples are uniformly swollen and your cat is otherwise acting normal, pregnancy should be high on your list.
As the pregnancy progresses, the nipples continue to enlarge in preparation for nursing. Your cat may also gain weight, eat more, and seek out quiet nesting spots. A vet can confirm pregnancy with an ultrasound or physical exam.
False Pregnancy
Cats that ovulate but don’t conceive can develop a false pregnancy, also called pseudopregnancy. This is uncommon in cats compared to dogs, but when it happens, it causes mammary development that can include milk production and swollen nipples. Cats may also behave as though they’ve given birth, nesting and acting maternal toward toys or other objects.
False pregnancy typically resolves on its own within one to three weeks. If it happens, avoid squeezing or expressing the mammary glands, since that stimulates more milk production and prolongs the problem. Spaying prevents it from recurring.
Mastitis: Infection of the Mammary Gland
Mastitis is a bacterial infection of one or more mammary glands, most common in nursing cats but possible in any cat. In early or mild cases, the swelling may be subtle and your cat may seem fine. The first clue in a nursing cat is often that her kittens aren’t gaining weight normally.
As it progresses, the signs become harder to miss. The affected gland becomes increasingly swollen, firm, warm to the touch, and painful. The skin over the gland may turn red or purple. Any milk expressed from it may look cloudy, thickened, or contain visible blood or pus. In severe cases, the gland tissue can die, turning dark purple or black, and the skin may rupture into an open wound. At that stage, cats often become lethargic, stop eating, develop a fever, or start vomiting as the infection spreads into the bloodstream.
Mastitis requires veterinary treatment. If you notice a single gland that’s hot, hard, or discolored, or if your cat seems unwell alongside the swelling, that’s a situation that needs prompt attention.
Mammary Hyperplasia
Mammary fibroadenomatous hyperplasia is a benign condition where the mammary tissue swells rapidly and dramatically due to an exaggerated response to progesterone. It most commonly affects young, intact female cats, particularly those in their first heat cycle, during pregnancy, or during pseudopregnancy. It can also occur in cats (including males) that have been given progesterone-based medications, which are sometimes used as contraceptives or behavioral treatments.
The swelling from hyperplasia can affect most or all of the mammary glands and may look alarming because of how quickly the tissue enlarges. Unlike mastitis, it doesn’t typically involve milk production or signs of infection. One unusual aspect of this condition: cats can be exposed to progesterone through environmental sources, since the hormone is a component in some human creams and lotions. If your cat has contact with skin products you use, that’s worth mentioning to your vet.
Hyperplasia is not cancerous and typically resolves completely once the progesterone source is removed. Spaying is often curative, and medications that block progesterone can shrink the tissue within a few weeks.
Mammary Tumors
In older cats, a swollen or lumpy nipple area may be a mammary tumor. These often start small, feeling like a pebble or dried pea under the skin near one or more nipples. The critical fact with cats is that roughly 90% of feline mammary tumors are malignant, and they tend to spread quickly to adjacent glands and nearby lymph nodes. Left untreated, they grow larger and harder and can eventually break through the skin.
Mammary tumors are far more common in intact females, but they can occur in spayed cats and, rarely, in males. Any firm, distinct lump near a nipple that wasn’t there before warrants a vet visit, especially in a cat over eight or nine years old. Early detection makes a significant difference in outcomes.
Spaying dramatically reduces this risk. Cats spayed before six months of age have a 91% lower chance of developing mammary cancer compared to intact females. Spaying before one year still provides an 86% reduction. After two years of age, the protective benefit drops substantially.
Can Male Cats Get Swollen Nipples?
Yes. Male cats have nipples, and while swelling is less common, it does happen. The most likely causes in males are mammary hyperplasia (especially if the cat has been exposed to progesterone-containing medications or products), infection, or, less commonly, mammary tumors. If your male cat has a noticeably swollen nipple, the same red flags apply: warmth, firmness, discoloration, or a palpable lump all deserve veterinary evaluation.
Signs That Need Prompt Attention
Mild, symmetrical nipple swelling in a young, intact female cat is often hormonal and not an emergency. But certain signs suggest something more serious is going on:
- Heat or firmness in one or more glands, especially if your cat seems painful when touched
- Skin discoloration over the nipple area, particularly red, purple, or blackened skin
- Discharge that looks cloudy, bloody, or pus-like
- A hard lump near a nipple that feels distinct from the surrounding tissue
- Lethargy, fever, vomiting, or loss of appetite alongside the swelling
- Open wounds or ulcers on or near the mammary glands
Any of these, alone or in combination, point to a condition that needs veterinary diagnosis rather than a wait-and-see approach.