Do Dogs Mammary Glands Swell After Heat?

A dog’s heat cycle, also known as estrus, is a normal biological process that unspayed female dogs go through. During this time, dogs experience physical changes in preparation for pregnancy and motherhood. One of the most noticeable changes is enlargement and swelling of the mammary glands. This is a common occurrence as the mammary glands prepare for potential breastfeeding. Understanding why dogs’ mammary glands swell during heat can help owners know what to expect and when to be concerned.

What Happens During a Dog’s Heat Cycle?

A dog’s heat cycle consists of four distinct stages. The typical duration for a heat cycle is 18-24 days. According to the Cornell University College of Veterinary Medicine, the stages are:

1. Proestrus – This initial stage lasts approximately 9 days on average. The dog’s vulva will swell and a bloody vaginal discharge will be present. Male dogs will be attracted to her but she will not be receptive to mating.

2. Estrus – Also known as the “fertile period”, this stage lasts about 9 days. The discharge lightens in color and the female will now allow mating. Ovulation occurs during this time.

3. Diestrus – The discharge stops and the vulva shrinks during this stage lasting 60-90 days. The dog’s body prepares for pregnancy, even if mating did not occur. The dog will no longer allow mating.

4. Anestrus – This final stage is a “resting” period of 120-150 days where the reproductive system shuts down before the next cycle. There are no signs of heat during this stage.

According to WebMD, the typical signs of heat include swelling vulva, bloody discharge, frequent urination, restlessness, moodiness, and the urge to escape or run away.

Mammary Gland Changes During Heat

It’s very common for a dog’s mammary glands to swell and enlarge during heat. This is caused by increased blood flow and fluid retention in the breasts in response to elevated progesterone levels. According to Veterinary Partner, “the mammary glands increase in size and the nipples become erect.”1 The swelling can cause the nipples to appear enlarged and more pronounced. PawSafe notes that “of course, physical changes can include swelling of the mammary glands and increased blood flow to the area, resulting in enlarged nipples.”2 The mammary glands will often feel firmer or fuller during this time.

The changes are in preparation for potential pregnancy and nursing puppies. However, dogs that are spayed or not bred will still experience mammary changes and swelling during heat cycles. The enlargement is temporary and the breasts will return to normal size after heat ends.

Why Mammary Glands Swell

The primary cause of mammary gland swelling in female dogs is hormonal changes associated with the estrous cycle. When a female dog goes into heat, her body releases increased levels of estrogen and progesterone. These hormones stimulate development of the mammary glands in preparation for pregnancy and nursing puppies.

According to the AKC, the rise in progesterone levels during estrus leads to mammary enlargement and swelling, even if the dog does not become pregnant The mammary glands react to these hormonal cues, but since no puppies are present to nurse, the glands swell up with milk. This is known as a “false pregnancy” and the swollen glands may even leak milk.

Essentially, the dog’s body responds as if it is pregnant and preparing to nurse even if conception did not occur. The hormonal fluctuations of estrus prompt the mammary tissues to enlarge and fill with milk. This is a normal physiological response, but can cause discomfort and issues if the swelling becomes excessive.

Duration of Swelling

The swelling of a dog’s mammary glands after heat usually lasts around 1-2 weeks, though it can persist a little longer in some cases. According to veterinarians, the enlargement and swelling begins right after the luteinizing hormone peak, which triggers ovulation. The mammary glands remain swollen due to the increased levels of estrogen and progesterone during the heat cycle.

As the heat ends and hormones return to normal levels, the swelling gradually goes down. Most dogs see their mammary glands return to normal size within 2-4 weeks after the end of peak estrus. However, for some dogs it may take up to 8 weeks for the tissue to completely deflate and nipples to shrink back down.

If the swelling lasts significantly longer than 8 weeks, or seems to get larger instead of going away, it’s a good idea to have a veterinarian examine your dog. Persistent mammary gland enlargement outside of the heat cycle could potentially indicate an underlying health issue.

So in summary, expect mammary swelling for 1-2 weeks typically, up to 4 weeks on the longer end of normal, and consult a vet if it lasts beyond 2 months after heat.


Other Signs of Heat

A dog’s heat cycle results in both physical and behavioral changes. Two of the most noticeable signs of heat are vaginal discharge and changes in behavior.

As estrogen levels rise at the start of heat, most female dogs will develop a bloody vaginal discharge. This discharge starts off bloody but then changes to a watery pink or straw-colored fluid during peak fertility. The discharge may have a distinct odor that can attract male dogs. It usually lasts for around 9 days but can persist for up to 3 weeks or more [1].

Behavioral changes are also common when a dog is in heat. Female dogs may become more affectionate and clingy, or they may show signs of restlessness and nervousness. They are easily distracted and may pace persistently. Many female dogs in heat develop a strong urge to escape or roam, looking for a mate. They may whine, bark, or cry persistently. Some dogs lose their appetite during this time. Territorial marking and mounting behavior are also common [2].

Paying attention to these physical and behavioral changes can help owners identify when their dog is entering her fertile period. Restricting a dog’s roaming and contact with males at this time is important to avoid unwanted pregnancies.

Health Risks

There are two main health risks associated with a dog’s heat cycle – pyometra and mammary tumors. Pyometra is a bacterial infection of the uterus that can be life-threatening if left untreated. It occurs because the hormones of the heat cycle stimulate the uterine lining to thicken and become prone to infection. According to the American Kennel Club, nearly 25% of unsprayed dogs will develop pyometra by age 10.[1]

Mammary tumors are also strongly linked to heat cycles. Dogs spayed before their first heat have just a 0.5% risk of mammary tumors, but after one heat that rises to 8%, and after two heats it jumps to 26%. [2] The hormones of heat cycles stimulate cell proliferation in the mammary glands, which can lead to tumor formation. Aggressive mammary cancer is unfortunately common in unspayed female dogs.

To minimize these risks, veterinarians advise spaying female dogs before their first heat, usually around 6 months of age. Doing so virtually eliminates the chances of pyometra and mammary tumors later in life.




When to See the Vet

In most cases, swelling of the mammary glands after a heat cycle is normal and will subside on its own. However, it’s important to monitor your dog closely during this time. If the swelling is extreme or does not start to subside within a week or two, you should make an appointment to see your veterinarian.

Signs that may indicate a more serious issue requiring veterinary attention include:

  • Mammary glands that are extremely enlarged, inflamed, hot, or painful to the touch
  • Skin over the mammary glands that is red, ulcerated, or oozing
  • Presence of a wound or puncture
  • Difficulty walking or reluctance to move due to pain
  • Loss of appetite or lethargy
  • Fever

These signs may indicate mastitis, an infection of the mammary glands, which requires prompt veterinary care. Mastitis can occur after heat or at other times. Without proper treatment like antibiotics and anti-inflammatories, it can lead to abscess or sepsis. Veterinary attention is crucial.

It’s also a good idea to have your vet examine any significant or abnormal mammary changes after heat. They can rule out masses, tumors or other concerning conditions like pyometra. Don’t hesitate to call your vet if you have any concerns about mammary changes in your dog.

Managing Swollen Mammary Glands

If your dog’s mammary glands are swollen during heat, there are some steps you can take to help manage the swelling and keep your dog comfortable:

Apply cold compresses – Applying a cold compress or ice pack wrapped in a towel to the swollen area can help reduce inflammation and discomfort. Do this for 10-15 minutes a few times a day. Just be careful not to overcool the tissue.

Don’t let dogs breed – It’s important not to breed a dog when her mammary glands are swollen, as this increases the risk of mastitis. Wait until after the heat cycle has completed and the swelling goes down.

Gentle massage – Gently massaging the mammary glands can help stimulate drainage and reduce swelling. But be very gentle, as the tissue is inflamed.

NSAIDs – Your vet may recommend a non-steroidal anti-inflammatory drug (NSAID) like Rimadyl to help reduce swelling and discomfort. Follow your vet’s dosage instructions.

Antibiotics – If mastitis develops, antibiotics will likely be needed to clear the bacterial infection causing the inflammation. Be sure to finish the entire course as prescribed.

Cone collar – To prevent your dog from excessively licking or irritating the mammary glands, an Elizabethan collar may be needed. This prevents them from causing more trauma to the area while it heals.

Monitor for changes – Keep an eye on the mammary glands throughout your dog’s heat cycle and contact your vet if you notice signs of infection or the swelling persists longer than normal.


In summary, it is common for a female dog’s mammary glands to swell during and after her heat cycle. The swelling is caused by hormonal fluctuations that stimulate development of the mammary tissue. While usually benign, the duration of swelling can vary. Other signs of heat may include increased urination, bleeding, and behavioral changes.

Though typically not a cause for concern, it’s recommended to monitor the mammary glands and note any abnormalities or extreme enlargement. Some swelling should subside after heat, but mammary tissue remains developed in preparation for potential pregnancy and nursing. Consult a veterinarian if the swelling is severe, painful, or accompanied by abnormal discharge. Overall, mild to moderate mammary gland enlargement in an intact female dog around the time of her heat cycle is considered normal.

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