Why Does My Dog Have Lumps Under Her Nipples After Heat?

When a female dog goes through her heat or estrus cycle, her body undergoes many hormonal changes. These changes prepare her body for potential pregnancy and nursing. As a result, the mammary glands enlarge and it is common for small lumps to develop under the nipples. While this is usually a normal part of the cycle, it can cause concern for dog owners who notice these lumps forming on their dog. Understanding the reasons behind lump development, how to check them, and when veterinary advice is needed is important information for any dog owner.

This article will provide an overview of the canine estrus cycle, mammary gland development, and the hormonal changes that lead to temporary lump formation. Diagnostic procedures, potential treatments, and prevention methods will also be covered to fully inform dog owners about this common occurrence.

The Estrous Cycle in Dogs

The estrous cycle in dogs refers to the reproductive cycle that female dogs go through. There are four main stages of the estrous cycle in dogs:

Proestrus – This initial stage lasts approximately 9 days. The vulva begins to swell and there is some bloody vaginal discharge. However, the female dog is not yet receptive to breeding during this stage [1].

Estrus – This is the period where the female dog can become pregnant, known as being “in heat.” It lasts around 9 days. The discharge becomes less bloody and the female will allow mating. The uterus is preparing for pregnancy [2].

Diestrus – The next stage lasts 60-90 days if pregnancy does not occur. Hormone levels decrease and the discharge becomes cream or white. The female will no longer allow mating [3].

Anestrus – This quiescent stage means the female is not in heat. It lasts 4-5 months until the cycle starts again [1].

The estrous cycle explains when a female dog can become pregnant during the estrus stage. Understanding the stages helps owners know what to expect as their dog goes through her heat cycles.

Mammary Glands and Development

Dogs have 5 pairs of mammary glands, although some dogs may have 4 or 6 pairs. The mammary glands are located along two parallel rows extending from the chest to the groin area. There are 2 thoracic mammary glands (M1 and M2), 2 abdominal mammary glands (M3 and M4), and 1 inguinal mammary gland (M5) in each row. The inguinal mammary glands extend into the groin area near the hind legs.

Mammary gland development occurs in distinct stages. Newborn puppies are born with simple ducts that resemble human breast buds. Significant mammary development occurs during puberty and estrus cycles under the influence of hormones like estrogen and progesterone. Dogs spayed before their first heat cycle often have underdeveloped mammary glands due to lack of hormonal exposure.

During estrus cycles, the mammary glands enlarge and swell in response to rising levels of reproductive hormones. This temporary gland development prepares the mammary tissue for potential pregnancy and nursing. If conception does not occur, the mammary glands regress until the next cycle. This enlargement during estrus followed by regression when not bred is considered physiologic and normal.

Development, Anatomy, Histology, Lymphatic Drainage, and Diseases of the Canine Mammary Gland

Hormonal Changes

During the heat cycle, there is a surge of estrogen that causes the initial swelling of the vulva and begins to prepare the dog’s body for pregnancy. According to the Cornell University College of Veterinary Medicine, as the cycle progresses, estrogen begins to decrease as progesterone levels start to rapidly rise (source). This shift in hormones triggers ovulation. The high levels of progesterone continue through the diestrus phase to help maintain pregnancy if conception occurs.

The fluctuations in these reproductive hormones, especially the large amounts of progesterone, also cause the mammary glands to enlarge and develop tissue in preparation for potential nursing. According to the Proceedings of the North American Veterinary Conference, progesterone receptors in the mammary tissue bind to progesterone, stimulating growth and development. This is why it’s common for female dogs to develop temporary lumps under their nipples during and after their heat cycle as the mammary glands swell and get ready for milk production (source). The size increase is temporary in dogs that do not get pregnant.

Why Lumps Form

The lumps that form under a female dog’s nipples after a heat cycle are generally non-cancerous mammary hyperplasia caused by hormonal changes. Mammary gland hyperplasia occurs when the mammary glands proliferate and swell in response to circulating hormones like estrogen and progesterone. This proliferation causes fluid-filled lumps or nodules to form in the mammary tissue.

When the veterinarian palpates the lumps, they will often feel like a ‘stud chain’ formation – a series of small, rounded masses under the skin along the mammary chain. The masses are not usually painful or problematic for the dog unless they become overly enlarged or infected.

According to the Ohio State Veterinary Medical Center, about 85% of canine mammary masses are non-malignant hyperplasia caused by hormonal stimulation of the mammary glands. Intact female dogs and dogs that were spayed after their first heat cycle have the highest risk.

Susceptible Dogs

Though mammary gland tumors can occur in both male and female dogs, unspayed female dogs are at greatest risk. Studies show that 1 in 4 unspayed female dogs will develop mammary tumors, with around 50% being malignant. The risk decreases significantly if a dog is spayed before her first heat cycle begins. That’s because reproductive hormones like estrogen and progesterone stimulate mammary gland tissue growth, increasing the chances of abnormal cell changes.

Certain breeds are more prone to developing mammary tumors as well, including Poodles, Spaniels, Retrievers and small Terriers like Yorkshire Terriers. Genetics play a role in susceptibility, though the specific genes involved are still being researched. Overweight female dogs also have a slightly higher incidence rate. All dogs should receive regular vet exams to check for lumps or other abnormalities, but paying close attention to at-risk breeds is especially important.

Diagnosing the Lumps

Veterinarians will perform a thorough physical exam of the mammary glands and surrounding tissue to check for lumps or abnormalities. They will look at the size, shape, texture and location of any lumps and note if they seem adhered to deeper tissues. Cytology, which is the microscopic examination of cells, may be done by aspirating cells from the lump with a needle. This can help determine if the lump is cystic fluid, a benign mammary hyperplasia or a potential cancer [1].

However, the only definitive way to diagnose mammary tumors is to surgically remove them and send them for histopathology, which is examination under a microscope. A biopsy allows the veterinarian to determine if the lump is benign or malignant. Benign lumps tend to have well-defined borders and be movable, while malignant lumps often have irregular borders and feel firmly attached to underlying tissues [2]. Staging of malignant mammary tumors will also be done to determine how far any cancer may have spread [3].


There are several treatment options for dogs diagnosed with mammary lumps after heat:

Observation – Some benign tumors that are small and slow-growing may be observed over time rather than removed immediately. Your vet will monitor the lump at regular visits to ensure it is not growing or changing. Observation is generally only an option for older dogs that are not good candidates for surgery.

Anti-hormonal medication – Drugs that block estrogen production may help slow tumor growth and prevent new tumors from developing. Common medications prescribed include spaying/ovariohysterectomy and anti-estrogen drugs like tamoxifen. These may be used alone or along with surgery.

Spaying – Spaying removes the ovaries and uterus, eliminating the source of estrogen that stimulates tumor growth. It is considered the most effective way to prevent recurrence of mammary tumors. Spaying is recommended even if existing tumors are successfully removed by surgery. [1]

Surgery – Surgery to remove the tumor(s) is the primary treatment. The surgeon will remove the tumor with clean margins to ensure no cancerous cells are left behind. In some cases, one or more mammary glands may need to be removed. Dogs generally recover well from mammary tumor surgery.


The most effective way to prevent mammary lumps is to spay female dogs before their first heat cycle, according to research cited at Hill’s Pet Nutrition. Spaying removes the hormones estrogen and progesterone that stimulate mammary gland development and lead to tumor formation. Studies show that dogs spayed before their first heat have just a 0.5% risk of developing mammary tumors later in life.

For dogs already past their first heat cycle or those who cannot be spayed, it’s important to monitor their mammary glands carefully for any lumps or changes. Perform regular at-home checks and point out any concerns to the veterinarian right away, as early intervention greatly improves outcomes. Benign masses can be surgically removed, while malignant tumors may require chemotherapy or radiation. With prompt veterinary care, many dogs can live normal lives despite a mammary lump diagnosis.


To briefly summarize, female dogs commonly develop mammary lumps and swellings when they go through heat cycles due to hormonal fluctuations. The lumps are caused by hypertrophy and hyperplasia of mammary tissues. While most regress on their own after heat, some may persist or develop into mammary cancer over time.

This highlights the importance of being aware of any lumps forming under the nipples after heat. Routinely check the mammary glands and notify your veterinarian if you find any suspicious lumps that don’t regress. Early detection and treatment are key to successful outcomes. Spaying your dog at an appropriate age reduces the risk. Overall, vigilant monitoring and proactive vet care helps safeguard your dog’s mammary health.

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