Dog Menstrual Cycle: What Owners Need to Know!

5 min read 0 Comments

Small brown pug wearing a blue menstrual diaper on a sidewalk in front of grass

Dr. Erica Irish author of Dog Menstrual Cycle: Everything You Need to Know

In humans, a woman’s uterus is prepped for pregnancy each month. However, when a pregnancy does not occur, the uterus sheds its lining and releases blood and soft tissue. On average, this occurs once every 28 days and is known as menstruation. 

It can be difficult to deal with monthly bleeding as well as the fluctuations in associated hormone levels. Female dogs maybe a little bit luckier in that respect because they do not cycle as often as humans do! What are some of the other differences and similarities between human and dog “menstrual” cycles?  


Dog “Menstrual” Cycles

In veterinary medicine, we do not refer to dog’s cycles as menstruation like in humans. Instead, dogs go throughestrous cycles, and these cycles are broken down into four phases, each lasting different amounts of time:  

  • Anestrus – the longest phase; the time of least reproductive/hormonal activity (between heats)
  • Proestrus – estrogen levels peak and the vulva enlarges; usually lasts 9 days
  • Estrus – the most fertile time for breeding, when the female is receptive to males; lasts 9-14 days
  • Diestrus – female no longer receptive and progesterone peaks, regardless of being pregnant or not being pregnant; lasts for 2 months  

Owners typically see signs that their dog is going into estrus or “heat” when it is really proestrus that is starting. Your dog’s vulva may appear enlarged, and you may begin to see bloody vaginal discharge.

These female dogs may not be receptive to male dogs yet, and they may have an increased appetite or a change in their personalities. Dogs in proestrus may also seem nervous, and they may urinate more frequently in order to signal male dogs for their readiness via hormones and pheromones present in her urine.

In true estrus, which is when a dog is ready to be bred, the vulva begins to shrink and the bloody discharge may start to have a pinkish, watery appearance. Your dog may be more receptive to male dogs at this time, and you may notice her start to plant her feet on the ground and raise her tail high into the air or move her tail to the side. This position is known as “flagging.”

Diestrus occurs directly after estrus and lasts for about two months. Progesterone, which is the hormone that is responsible for maintaining a pregnancy, is present in high amounts, even if an actual pregnancy did not take place. This may explain why some female dogs can go through “false” pregnancies where they gain weight, eat more, and can appear pregnant but aren’t really.

Anestrus is the time period when your dog is not experiencing any of these cycles. The average female dog can go into heat or estrus twice a year. Small breed dogs can go into heat as often as every four months while large and giant breeds dogs can go up to two years before they have a heat cycle. Basenjis and Tibetan Mastiffs are known for having an estrus cycle only once a year.


Sold out



When does a dog have her first heat cycle?

The average dog can go into her first heat cycle by the time she has reached puberty. Some dogs can experience estrus as young as six months of age while others may be closer to a year of age. Large and giant breeds may not see their first heat cycle until they are two years old. Like in humans, initial heat cycles may occur irregularly. Adults, more than two years of age may experience more frequent cycles. 


What to expect and when to breed

Since dogs in heat may experience vaginal bleeding for at least two weeks, you may need to consider how to handle this. Many pet owners opt to purchase small reusable diapers for their dogs because they can put absorbable pads in them similar to the feminine napkins that humans use during menstruation. 

You should also be wary of any changes in behavior as some females (particularly those in proestrus) can become anxious or even aggressive in some cases. 

If you are planning on breeding your dog, you will need to ensure that an accidental mating does not occur. This is because intact male dogs are indiscriminate when it comes to mating, and so a brother might try to mate with his immediate relative. 

This kind ofinbreeding can result in genetic problems if pregnancy results. If an accidental mating does occur despite your best efforts to avoid it, your veterinarian can provide an injection containingdinoprosttromethamine which can decrease progesterone levels. 

To help with knowing when to breed your dog, your vet can collect swab samples from your dog’s vagina in order to check the cells present. If there are lots of cells but mostly red and white blood cells, then your dog may be in proestrus and not quite ready to mate. If there are lots ofcornified epithelial cells and not many red or white blood cells, then your dog may be in estrus. Multiple checks provide the most accuracy. 

There is also a blood test available that can detect when a female dog is ready to be bred. This test quantifies the amount of progesterone present. Like with vaginal cytology, multiple repeated tests may be necessary to help pinpoint the exact time of readiness.


Should I Spay or Should I Go?

Pregnancies can often be complicated, especially if it’s your dog’s first pregnancy, and certain dog breeds like French Bulldogs often require scheduled Caesarean section surgeries due to the high likelihood of birthing complications ordystocia.

No matter your dog’s size or breed, dystocia can be life-threatening and requires expensive emergency intervention or surgery. Therefore, if you are not planning on breeding your dog, it is best to consider having her spayed.  

Spaying is also known as anovariohysterectomy or the surgical removal of the uterus and ovaries. Spaying not only eliminates the chances of an accidental mating/pregnancy, but it also prevents semi-annual estrus and vaginal bleeding. Also, if your dog is spayed before her first heat cycle, it dramatically decreases her chances of developing mammary cancer when she is older.


Female Dog Menstrual Cycle 

The average dog’s estrus or heat cycle occurs at least twice a year, but the associated vaginal bleeding indicates readiness for pregnancy whereas menstrual bleeding in humans indicates the beginning of a new cycle of readiness for pregnancy. Also, humans cycle monthly compared to dogs.

If you are planning on breeding your dog, your vet can help you get ready and tell you when your dog is ready to be bred. If you do not wish to breed your dog, consider having her spayed before her first heat.


For More Articles Check Out


Dr. Erica Irish

Erica has worked in the veterinary field since 2006, starting out as a veterinary technician before graduating from the UF College of Veterinary Medicine in 2013. As a general practitioner in an animal hospital, she has many interests and is especially interested in dermatology, cardiology, internal and integrative medicine.