Why Would A Female Dogs Breasts Swell?

Female dog’s mammary glands may swell for a variety of reasons. This article will provide an overview of the most common causes of swollen breasts in female dogs, including pregnancy, pseudopregnancy, uterine infections like pyometra, mammary gland diseases like mammary hypertrophy and mammary tumors, and mastitis. It will also cover diagnosis and treatment options for swollen breasts in female dogs.

Mammary Glands

Female dogs have five pairs of mammary glands located along the abdomen that extend from the chest area to the groin. The mammary glands produce milk to feed puppies after giving birth. The glands consist of milk-producing tissue, milk ducts that transport milk, and a nipple for milk release.

The mammary glands develop during puberty under the influence of hormones like estrogen. During pregnancy and nursing, the glands will enlarge and swell as they prepare to produce milk. Once nursing ends, the mammary glands will gradually shrink and return to normal non-lactating size.

According to the Veterinary Cancer Society[1], the primary function of canine mammary glands is to supply milk for newborn puppies. Outside of pregnancy and nursing, the mammary glands remain small and undeveloped.

[1] https://hospital.cvm.ncsu.edu/services/small-animals/cancer-oncology/oncology/canine-mammary-tumors/


One of the most common reasons for breast swelling in female dogs is pregnancy. As a dog’s pregnancy progresses, her mammary glands will begin preparing for the production of milk to feed her puppies. This natural preparation causes the mammary tissue to swell and enlarge. According to 5 Signs Your Dog Is Pregnant, swelling of the nipples and breasts is one of the earliest signs of pregnancy, typically occurring 2-3 weeks after conception.

During pregnancy, the dog’s nipples and mammary glands will become pinker in color and more swollen as they fill with milk. The area around the nipples also darkens. According to Normal Dog Nipples vs Pregnant: What Are the Changes, the nipples themselves will become more pronounced and erect as the due date approaches. All of these changes prepare the mammary glands to produce milk for nursing once the puppies are born.


Pseudopregnancy, also known as false pregnancy, is a common condition that can occur in unspayed female dogs. It happens when a female dog experiences hormonal changes that mimic the signs of pregnancy after her heat cycle without actually being pregnant. According to the UC Davis Veterinary Medicine, canine pseudopregnancy is a normal physiological process in dogs.

One of the most noticeable signs of pseudopregnancy is mammary gland enlargement. The mammary glands swell as they prepare to produce milk. This is caused by increased levels of the hormones progesterone and prolactin. These hormones cause the mammary glands to enlarge, even though the dog is not actually pregnant (VCAA).

The swelling of the mammary glands can range from slightly enlarged to voluminous, tight, and even painful. Pseudopregnant dogs often exhibit mothering behaviors like nesting and mothering toys or other objects. The enlarged mammary glands and milk production usually go away on their own within 2-4 weeks (PetMD).


Pyometra is a bacterial infection of the uterus that typically affects unspayed female dogs during diestrus, the period after estrus when progesterone levels are high.[1] One of the most common clinical signs of pyometra is an enlargement or swelling of the mammary glands.[2],[3] This occurs because the infection causes an increase in serum prolactin, which stimulates mammary development and milk production.

The bacteria Escherichia coli is often the cause of pyometra. As the uterine infection progresses, toxins released by E. coli enter the bloodstream and stimulate prolactin secretion. Prolactin then acts on the mammary glands, resulting in swelling as the glands fill with milk.[1]

Other signs of pyometra that may occur along with mammary swelling include lethargy, loss of appetite, increased thirst, and vaginal discharge. It is a serious medical condition that can be fatal if left untreated. Therefore, any intact female dog displaying mammary gland enlargement along with systemic signs of illness should be evaluated by a veterinarian immediately.

Treatment involves surgically removing the infected uterus and ovaries, along with supportive medical care. Spaying prevents recurrence of this potentially life-threatening infection.[2] The mammary gland swelling will subside once the infection is treated and prolactin levels return to normal.[1]

[1] https://vcahospitals.com/know-your-pet/pyometra-in-dogs
[2] https://oathall-vets.co.uk/wp-content/plugins/vetstream_swagger/pet-health/pet/pet_info_print.php?vetstream-type=Y2FuaXM=&nodeguid=bb54f654-c3ae-41b9-9455-d2de30184f31
[3] https://www.merckvetmanual.com/dog-owners/reproductive-disorders-of-dogs/reproductive-disorders-of-female-dogs

Mammary Hypertrophy

Mammary hypertrophy refers to abnormal tissue growth in the mammary glands of female dogs that is non-cancerous. It typically presents as enlarged or swollen mammary glands. The excessive tissue growth is caused by an increase in the number of mammary epithelial cells (hyperplasia).

Some common causes of mammary hypertrophy in dogs include:

  • Hormonal influences – Exposure to progesterone during estrus or from exogenous sources can stimulate mammary cell proliferation and lead to mammary gland enlargement.
  • Pseudopregnancy – The hormonal changes associated with a false pregnancy can induce mammary gland hypertrophy.
  • Chronic inflammation – Mastitis or other chronic inflammatory conditions of the mammary tissue may result in hypertrophic changes.

Mammary hypertrophy often regresses after estrus or completion of pseudopregnancy. However, the enlarged tissue can persist long-term in some dogs. Surgical removal of the excess mammary tissue may be recommended for severe or problematic cases. Recurrence is possible if the underlying cause is not addressed.




Mammary Tumors

Mammary tumors, or breast tumors, are a common cause of mammary gland swelling in female dogs. Mammary tumors develop due to abnormal and uncontrolled replication of cells in the mammary glands. According to the VCA Animal Hospitals, approximately 50% of mammary tumors in dogs are malignant or cancerous.

As mammary tumors grow, they can cause visible swelling in the breasts and mammary glands. The degree of swelling depends on factors like the tumor size, rate of growth, and location. Rapidly growing masses in the mammary glands are more likely to cause noticeable swelling and firmness. Swelling may start small and become more pronounced over time as the tumor enlarges.

According to the Ohio State University Veterinary Medical Center, mammary tumors often feel like discrete masses within the breasts, though in some cases they can involve the entire mammary gland area. Larger tumors may be associated with more diffuse swelling. Mammary gland swelling is just one clinical sign of potential mammary tumors in dogs.


Mastitis refers to inflammation of the mammary glands (breasts) in female dogs. It most commonly occurs due to a bacterial infection. The most common pathogens are Staphylococcus, Streptococcus, and Escherichia coli, which typically infect the mammary glands through the teats. Mastitis can occur in female dogs regardless of their reproductive status, but it is most common in lactating mothers. Symptoms of mastitis include swelling, redness, and pain in the mammary glands, fever, lethargy, loss of appetite, and milk discharge from affected glands. If left untreated, mastitis can lead to abscess formation in the mammary tissue. Diagnosis is made through physical examination of the breasts, evaluating milk discharge, and bacterial culture. Treatment involves antibiotic therapy, warm compresses, pain medication, and sometimes surgical drainage of abscesses. Preventing mastitis involves proper nursing and hygiene practices like cleaning the mammary glands and teats. With prompt veterinary treatment, most dogs recover well from mastitis (VCA Hospitals, PetMD).


There are a few ways vets will diagnose the cause of swollen dog breasts:

Careful examination may reveal slight swelling or inflammation of the affected mammary gland. The vet will palpate the breasts to feel for abnormalities. They may express fluid from the nipples to look for pus or blood, which would indicate infection. The presence of white blood cells (pus) or bacteria confirms a diagnosis of mastitis (https://vcahospitals.com/know-your-pet/mastitis-in-dogs).

Vets will take the dog’s temperature to check for fever, which often accompanies infection. They may also run bloodwork looking for increased white blood cell count. A complete blood count and serum biochemistry profile can help rule out or identify issues like cancer (https://wagwalking.com/condition/mastitis).

Diagnostic imaging like x-rays or ultrasound allow vets to visualize the internal mammary tissue and look for masses or abnormalities indicating tumor or mastitis (https://www.pawlicy.com/blog/mastitis-in-dogs/). Fine needle aspirates of masses can be examined under a microscope to differentiate between mastitis, cancer, and other causes.


Treatment for swollen dog breasts depends on the underlying cause. For mastitis, vets often recommend a course of antibiotics along with warm compresses applied to the affected mammary gland several times a day to keep the milk ducts clear and drain fluid buildup. Pain medication may also be prescribed. In some cases, the affected mammary gland may need to be surgically removed. If the cause is pregnancy or pseudopregnancy, no treatment is usually needed as the swelling will subside on its own after a few weeks. However, it’s important to avoid stimulating lactation which can prolong pseudopregnancy symptoms. Spaying can prevent recurrence of pseudopregnancy. For breast tumors, surgical removal of the tumor is generally recommended along with follow-up chemotherapy or radiation if cancerous. If the swelling is due to mammary gland hypertrophy, surgical reduction may be performed for cosmetic reasons or to prevent discomfort, skin irritation, and infection (VCA Hospitals, Pawlicy, WagWalking).

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