The heat cycle is a normal biological process that unspayed female dogs go through as their bodies prepare for mating and possibility of producing a litter. The heat cycle occurs every 6-8 months and lasts around 18-21 days. During this time, the dog’s reproductive system physically matures and they become receptive to males for breeding.
The heat cycle consists of 4 stages: proestrus, estrus, diestrus, and anestrus. In the proestrus stage, the dog’s vulva swells and they may spot blood. In estrus, the female is fertile and receptive to breeding. Diestrus is the period after mating where the fertilized eggs implant or the uterine lining is reabsorbed if no pregnancy occurs. Anestrus is the resting stage when the reproductive system is inactive.
During the heat cycle, especially around estrus, the dog’s body undergoes many changes as her hormones fluctuate. These include physical changes like vulva swelling, behavioral changes like increased affection/restlessness, and the possibility of pregnancy if breeding occurs. Understanding what to expect during this natural process can help owners monitor their dog’s health.
Signs of Heat
There are several signs that indicate when a female dog is in heat or estrus. The most noticeable sign is changes in the dog’s behavior. According to The Spruce Pets, female dogs in heat tend to become more affectionate, clingy, and nervous during this time. They may also urinate more frequently.
Another significant sign is vaginal discharge. As explained by WebMD, this discharge starts off bloody but then becomes lighter in color and texture. The vulva will also appear swollen and enlarged.
In addition to behavioral and discharge changes, the dog’s abdomen may appear swollen or enlarged. This temporary swelling occurs as the ovaries prepare to release eggs. The nipples and mammary glands may also become enlarged. All of these signs alert owners that their female dog is in heat.
Dog’s Vulva Swells
One of the most noticeable signs that a dog is in heat is swelling and enlargement of the vulva. The vulva is the external female genitalia around the vaginal opening. As estrogen levels rise at the start of a heat cycle, increased blood flow to the area causes the vulva to swell and become two to three times its normal size (1). This swelling is a natural part of estrus and prepares the dog’s body for mating and pregnancy.
According to PetMD, “When a dog is in heat (receptive to mating), her vulva becomes swollen, and a bloody discharge will be evident.” The swelling and discharge last for around 3 weeks as the heat cycle progresses (2). The swelling may appear quite pronounced compared to when the dog is not in heat. However, it is a normal part of the estrous cycle and generally does not indicate a medical issue on its own.
If the vulvar swelling persists or the area seems abnormally enlarged outside of a heat cycle, veterinary attention may be needed. Conditions like vaginal hyperplasia can cause chronic vulvar swelling unrelated to estrus. Diagnostic tests can determine if an underlying health problem is contributing to persistent vulvar enlargement (3).
Abdomen May Appear Swollen
While a female dog’s belly does not actually swell when in heat, her abdomen may appear slightly larger or swollen due to increased blood flow to the area [1]. The vulva and mammary glands enlarge, which can make the abdomen seem bigger.
However, true abdominal swelling or bloating is not normal and may indicate an underlying health issue. Some causes of actual abdominal swelling during heat include:
- Pregnancy – dogs can become pregnant during their heat cycle, which causes the abdomen to expand as the fetuses grow.
- False pregnancy – some female dogs exhibit mothering behaviors and may even lactate after heat cycles from hormonal changes, with abdominal enlargement similar to pregnancy.
- Infection – conditions like pyometra (infected uterus) can cause swelling or distension of the abdomen.
If a female dog’s belly appears disproportionately large, swollen, or distended during or after a heat cycle, it’s important to see a veterinarian to determine the underlying cause and whether treatment is needed [2].
Nipples Become Enlarged
A dog’s nipples often become enlarged and swollen during her heat cycle. This is caused by increased levels of estrogen during this time. According to PawSafe, “The hormones involved in the heat cycle, such as estrogen, make the mammary glands prepare for potential pregnancy and nursing.”
The swelling of the nipples is the breast tissue expanding in preparation for potential milk production. Even if the dog does not become pregnant during that heat cycle, the physical changes still occur due to hormonal fluctuations. According to JustAnswer, “A dog’s nipples do usually enlarge after a heat cycle as part of the ‘false pregnancy’ scenario.”
The nipple enlargement may persist for several weeks after the heat cycle ends before gradually returning to normal size. PawsGeek notes that “There’s usually no need to worry if your dog’s nipples get bigger in heat. They will return to normal size after a few weeks.” Monitoring the dog’s nipples and contacting a vet if swelling persists longer than expected is recommended.
Increased Appetite
A dog’s appetite often changes when she is in heat. This is due to the hormonal fluctuations that occur during the heat cycle. As estrogen levels rise, it can increase a dog’s appetite significantly. Some dogs may eat up to 1.5 times their normal amount during this time.
According to https://animalwellnessmagazine.com/dog-heat-2/, appetite changes are one of the most common symptoms of a dog in heat. The hormones stimulate them to eat more to provide energy for potential pregnancy and nursing. This is why spaying your dog can help stabilize her food intake.
It’s important not to restrict your dog’s diet during this time. Let her eat as much as she wants of her regular food. Just monitor that she is still eating normally and her appetite returns to baseline once the heat cycle passes.
Behavioral Changes
When a female dog goes into heat, one of the most noticeable signs are the behavioral changes. The hormonal fluctuations that occur during this time can cause the dog to act differently than normal. Some common behavioral changes include:
Increased nervousness and restlessness – The dog may seem anxious or agitated during her heat cycle. She may pace around frequently or seem unable to settle down and relax.
Increased vocalizing – Dogs in heat may whine, whimper, howl or cry more than usual. This vocalization is their way of signaling to potential mates that they are ready for breeding.
According to Preventive Vet, these behavioral changes are triggered by a surge of the hormone estrogen, which peaks right before the female dog enters the most fertile part of her heat cycle. This causes her to become more easily excitable. The behaviors tend to subside once she is out of heat.
Lasts 18-21 Days
The entire heat cycle for a female dog typically lasts between 2-4 weeks (Source). This is known as the estrous cycle. The average cycle lasts about 21 days, but can range from as short as 18 days to as long as 24 days (Source).
The heat cycle begins with the proestrus phase, when the female dog’s vulva begins to swell and she will have some bloody discharge. This phase lasts approximately 7-10 days. Next comes the estrus phase, when the female is receptive to mating. This phase lasts about 9 days. Ovulation occurs towards the end of this phase and fertilization is possible for several days after. The final phase is the diestrus phase, which lasts 55-90 days if no pregnancy occurs. The dog’s vulva will return to normal during this resting phase before beginning a new cycle (Source).
So in total, a female dog’s heat cycle generally repeats every 6 months or so. Each cycle lasts around 3 weeks from start to finish, with the female dog being most fertile during the middle estrus phase.
When to See a Vet
While most heat cycles are normal and do not require veterinary attention, there are some situations when you should take your dog to the vet:
If the heat cycle lasts longer than 21 days or the swelling does not go down after 3 weeks, this could signal an issue like pyometra (a uterine infection) and warrants a vet visit (Source).
If the heat cycles suddenly stop before your dog reaches social maturity around 18-24 months, have her examined by a vet to rule out potential health problems.
If your dog shows signs of heat exhaustion like excessive panting, lethargy, or vomiting, immediately take her to the vet for treatment to reduce body temperature and prevent lasting organ damage.
See your vet promptly if the vulva remains enlarged or swollen more than a few weeks after the heat cycle finishes.
Veterinary care is advised if you notice abnormal vaginal discharge in your dog, as this can indicate an infection or another condition requiring treatment.
Summary
To recap the main points about the physical changes that occur when a female dog is in heat:
The most noticeable change is that the dog’s vulva will swell up and become quite prominent. This allows male dogs to easily locate females in heat for mating. Some female dogs may also experience a slight swelling in their abdomen that could appear to be a swollen belly. However, this abdominal swelling is minimal and temporary.
Another clear sign of heat is enlarged nipples. Females going into heat will develop enlarged nipples in preparation for potential pregnancy and nursing. An increased appetite frequently accompanies these physical changes, as the dog’s body requires additional nutrition and energy during this time.
These physical signs of heat last for 18-21 days if the female dog does not become pregnant. If you notice any abnormalities or have concerns during your dog’s heat cycle, be sure to consult your veterinarian.
