Why Is My Female Dog’S Breast Swollen And Hard?

Mammary gland swelling in female dogs is a common occurrence that can be caused by a variety of conditions. The mammary glands, which are located on the underside of the dog’s belly, produce milk to nourish puppies during lactation. It is normal for mammary glands to become enlarged and swell during pregnancy in preparation for nursing. However, mammary gland swelling can also occur at other times and may indicate more serious health issues like cancer. This article provides an overview of the causes of mammary gland enlargement in female dogs and when mammary swelling should be a cause for concern.

Normal Mammary Gland Development

Mammary glands in dogs typically go through distinct developmental stages over the course of their life.

In puppies, mammary tissue begins to develop while still in the womb, starting around day 35 of gestation. The glands remain small and undeveloped until puberty.

When a female dog goes through her first heat cycle, usually around 6-12 months of age, her mammary glands will enlarge and develop further under the influence of reproductive hormones like estrogen. The mammary tissue will swell and ducts and alveoli will begin forming. However, they regress again when the heat cycle ends.

With each subsequent estrus cycle, the mammary glands go through phases of growth, development, and regression. They reach full maturity and capability for milk production after a pregnancy and whelping.

In adult non-pregnant dogs, the mammary glands remain small and undeveloped. They will enlarge again during proestrus and estrus of each heat cycle. In some dogs, the mammary glands will continue to get bigger with each successive estrus cycle throughout life.

Mammary gland development in dogs is a continuous process spanning from before birth into adulthood. The glands go through hormone-influenced cycles of growth and regression, only reaching full maturity after pregnancy and nursing puppies. Understanding the stages of normal mammary development provides context for evaluating abnormal changes.

Common Causes of Swelling

There are a few common causes for mammary gland swelling in female dogs:


Pseudopregnancy is a false pregnancy that occurs after a female dog goes through estrus or heat. Her body thinks she is pregnant even though she is not. This causes the mammary glands to swell as they prepare for nursing pups. The swelling may last 2-4 weeks before going away on its own [1].


If a female dog is actually pregnant, the mammary glands will naturally enlarge and swell in preparation for producing milk. The swelling increases progressively during the pregnancy [2].


Mastitis is an infection of the mammary glands, often caused by bacteria entering through cracks or trauma to the nipples. It causes red, swollen, hard, and painful mammary glands. There may also be pus or blood-tinged discharge from nipples. Mastitis requires veterinary treatment with antibiotics [3].


Pseudopregnancy, also known as false pregnancy, is a common condition that occurs in unspayed female dogs. It is caused by hormonal changes after estrus or heat cycles that mimic the signs of pregnancy in dogs without there being an actual pregnancy. According to the UC Davis Veterinary Medicine, pseudopregnancy affects 50-75% of intact female dogs and usually occurs 6 to 12 weeks after the last heat cycle [1].

The most noticeable symptom of pseudopregnancy is swelling or enlargement of the breasts as the dog’s body prepares for nursing pups. The breasts become enlarged and firm, and she may even produce milk. Other symptoms include lethargy, nesting behaviors, mothering toys or objects, loss of appetite, and mild vomiting according to PetMD [2].

Treatment may not be necessary if symptoms are mild. Reducing stimulation of the dog’s nipples can help breasts return to normal. Limiting contact with objects that trigger mothering behaviors can also help. In more severe cases, vets may prescribe medication to suppress lactation and regress the breasts back to normal size according to the VCA Hospitals [3]. The best prevention is spaying to prevent hormonal fluctuations that lead to pseudopregnancy.


If your dog is pregnant, mammary gland swelling and enlargement is normal as the mammary glands prepare to produce milk. Here’s what to expect:

– Your vet can confirm pregnancy through physical examination, ultrasound, x-rays, or blood tests. Look for signs like weight gain, enlarged nipples, and abdominal swelling.

– The mammary glands will enlarge during pregnancy as they develop mammary tissue and prepare for milk production. Swelling may start around the fourth week of pregnancy.

– Closer to delivery, the nipples may leak small amounts of milk. The breasts will become enlarged and distended right before whelping.

– Make sure to provide plenty of nutritional support during pregnancy. Talk to your vet about supplementing with additional calories or vitamins.

– Prepare a whelping area with nesting materials, heating pads, and anything else your dog may need for giving birth and nursing puppies.

– Contact your vet if you notice abnormal discharge from the nipples or excessive swelling/pain in the mammary glands, as these can indicate potentially serious conditions like mastitis.


Mastitis is the inflammation of one or more mammary glands (breasts) in female dogs. It often occurs when bacteria enter the teat and multiply in the mammary gland, causing an infection. Some common causes and symptoms of mastitis include:

Bacterial infection – Mastitis is primarily caused by bacterial invasion of the teat and mammary gland. Bacteria such as Staphylococcus, Streptococcus, and E. coli are often the culprits. These bacteria may be introduced during nursing, trauma to the teat, or improper hygiene.

Symptoms – Signs of mastitis include swelling, redness, heat, hardness, and pain in the mammary gland. The dog may be reluctant to nurse her puppies. There may also be discharge from the nipple and fever if the infection is widespread. [1]

Treatment – Mastitis is treated with antibiotics, anti-inflammatories, and warm compresses applied to the affected mammary gland. Puppies may need to be hand-fed with formula if nursing is too painful for the mother. In severe cases, the affected mammary gland may need to be surgically removed. With prompt treatment, the prognosis for mastitis is generally good.

Preventing mastitis involves proper nursing hygiene, allowing puppies to nurse frequently, and weaning gradually. Seeking prompt veterinary care for any signs of mammary gland swelling or pain can also help prevent progression to mastitis.

Mammary Tumors

Mammary tumors are abnormal growths of the mammary glands. They can be benign (non-cancerous) or malignant (cancerous). Approximately 50% of mammary tumors in dogs are malignant, making them the most common type of cancer in female dogs that are not spayed [1].

Benign mammary tumors are usually encapsulated nodules that can be surgically removed. They don’t tend to spread to other areas of the body. Malignant mammary tumors, on the other hand, are not encapsulated and have tentacle-like projections. They can metastasize to the lymph nodes and lungs [2].

To diagnose mammary tumors, vets will perform a fine needle aspirate to examine cells under a microscope. They may also recommend a biopsy to assess if the tumor is benign or malignant. Staging tests like chest x-rays and ultrasound will check for metastasis.

Treatment depends on the type, size, and stage of the tumor. Benign tumors can often be surgically removed. Malignant ones may require surgery plus chemotherapy and anti-estrogen drug therapy. Radiation therapy is sometimes used too. Early detection and treatment is key for the best prognosis.

When to See the Vet

It is important to have your female dog examined by a veterinarian anytime there is noticeable mammary gland swelling or firmness (Source). While some swelling can be normal, it may also indicate a more serious health issue that requires prompt veterinary care. Warning signs to watch for that warrant an urgent vet exam include:

  • Sudden, rapid swelling of one or more mammary glands
  • Mammary glands that are hot, painful, or red/purple in color
  • Nipple discharge that is bloody, purulent, or abnormal
  • Loss of appetite, lethargy, vomiting, or other signs of illness
  • Enlarged lymph nodes near the affected mammary glands
  • Fever above 102.5°F (39.2°C)

If your dog shows any of these concerning symptoms along with mammary gland swelling or hardness, contact your veterinarian right away. Timely diagnosis and treatment is essential for conditions like mastitis, cancer, or pseudopregnancy. Don’t delay – call your vet promptly whenever there are mammary changes in your female dog.


When diagnosing the cause of swollen mammary glands in dogs, the veterinarian will first perform a physical exam. They will look for signs of warmth, pain, swelling, or discharge from the nipples that may indicate infection or mastitis. The vet will palpate the breasts to feel for any lumps or abnormalities. Blood and urine tests may be recommended to check for pregnancy or signs of infection. An ultrasound or mammogram of the breasts may be done to look for tumors or cysts. A needle aspirate or biopsy of tissue may be taken to check for cancer cells. Bacterial culture and sensitivity testing can identify the type of bacteria causing an infection and determine the most effective antibiotic treatment.

According to VCA Animal Hospitals, “Diagnosis of mastitis is based on physical examination findings and diagnostic testing such as bacterial culture and sensitivity testing, ultrasound examination and/or biopsy of affected mammary gland tissue” (https://vcahospitals.com/know-your-pet/mastitis-in-dogs). Diagnostic tests along with a thorough physical exam are key to determining the underlying cause of mammary gland swelling and appropriate treatment.


Treatment for swollen mammary glands depends on the underlying cause. For mild cases of mastitis or pseudopregnancy, veterinarians may recommend home care such as warm compresses, massage, and antibiotics. More severe mastitis may require hospitalization, IV antibiotics, and fluid therapy. If the swelling is caused by a mammary tumor, surgery to remove the tumor and affected glands is usually necessary. The overall outlook depends on how quickly treatment begins and the specifics of the underlying cause. With prompt veterinary care, most dogs recover fully from mastitis, pseudopregnancy, and even benign mammary tumors. Regular exams help detect problems early when they are most treatable.




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