All You Need to Know About Heat Cycles in the Female Dog

Have you ever seen a dog wearing a diaper and wondered why? Pet parents are curious about all aspects of their pets’ lives, including reproduction. As a small animal veterinarian, these are some of the most commonly asked questions about heat cycles in female dogs. 

What is a Heat Cycle? 

A heat cycle is a hormonal cycle that occurs in intact (unspayed) female dogs. There are four parts to this cycle.

  • Proestrus – This is the beginning of the heat cycle and is characterized by firm swelling of the vulva and bloody vaginal discharge, lasting an average of nine days. Blood levels of estrogen rise during this phase. The female dog will not yet allow mating at this time, but may be attractive to male dogs.   
  • Estrus – The female dog is receptive to breeding by males during this phase, known as “heat”, which also lasts an average of nine days. Estrogen levels decline and progesterone levels rise. Luteinizing hormone levels peak leading to ovulation during this phase. The vulva becomes softer and discharge may decrease in volume and become straw-colored and more watery. The female may also exhibit behavioral changes to encourage male interest. Vaginal cytology (examining cells taken from a swab of the vagina) can help determine which phase of the cycle a female is in and if she is ready to be bred. 
  • Diestrus – Once again the female becomes resistant to breeding, the vulva begins to decrease in size, and discharge stops. Progesterone levels peak and then begin to slowly decline. If a dog is not pregnant, diestrus usually lasts for about two months. 
  • Anestrus – This is the non-breeding or resting phase of the cycle, and usually lasts four to five months. Near the end of anestrus, luteinizing hormone and follicle-stimulating hormone begin to increase, preparing the body to restart the cycle. 

Are Heat Cycles the same as Periods? 

Not exactly. In dogs the bloody discharge comes directly from the walls of the vagina, not shedding of the uterine lining. Also unlike people, dogs also do not go through menopause, although their cycles may become less frequent and more irregular as they age. 

How Often Does a Dog Go Into Heat?

Most dogs reach sexual maturity around six months of age, which is when they begin having an estrus cycle. After this most dogs have a heat cycle approximately every six to eight months. In general small breed dogs may begin cycling slightly earlier and have more frequent cycles than their large breed counterparts (for example three times a year versus once a year). Dogs do not go through menopause like people do, however their cycles may become more irregular and less frequent as they get older. Additionally, it can take about two years for a young female to develop regular cycles. It is important to keep track of your dog’s heat cycle and relay this information to your vet, as it is a key aspect of their overall health.

How Long is a Female Dog in Heat? 

The estrus phase of the heat cycle (during which a female may become pregnant if bred) can last from three to 21 days, with an average length of nine days. 

What Behaviors Does a Female Dog Display When in Heat?

There are both physical and behavioral changes that you may notice when a dog is in heat. As your dog approaches her fertile period her vulva will become swollen, and a bloody discharge will be produced from the vaginal wall. The color and amount of this discharge will vary among dogs and as the cycle progresses, often changing from red to straw-colored. 

A female in heat will lick her vulva and urinate more frequently (or show urine-marking behavior) to broadcast her reproductive state to other dogs. She may be more friendly with other dogs, seek out males, mount and hump, turn her tail to the side and raise her hind end (flagging), and allow males to sniff/lick her vulva and mount her. She may also be more anxious and restless than usual. 

What Happens if a Dog is Bred While in Heat? 

If a female is bred or mounted by an intact male during estrus, there is a chance that she will become pregnant, and you can expect to see puppies approximately 63 days later. Your vet will be able to diagnose pregnancy based on a physical exam, blood test, x-rays, and/or ultrasound, depending on how early it is in the pregnancy.  

How To Care For Your Dog in Heat – Diapers and More

If your female dog is in heat she will be very attractive to intact (uncastrated) male dogs, many of whom will do whatever it takes to get to her. This could include aggressive behavior directed at other dogs or people. You will need to be careful on walks, and never leave her unattended in the yard. If you have an intact male dog in your house (even a sibling or parent), they will need to be kept separate during this time. 

You will also want to put a diaper on or keep her confined to areas where it will be easy to clean up bloody vaginal discharge. Lastly, it is important to keep track of your dog’s heat cycles. If you take an intact female dog to the vet they will always ask when her last heat cycle was, as it can help with assessment of her overall health. 

When is Vaginal Discharge or Bleeding Abnormal? 

If vaginal bleeding or discharge persists, is different than usual in appearance, is accompanied by other signs of illness (such as lethargy, vomiting, diarrhea, decreased appetite), or occurs in a spayed female dog, you should seek veterinary care. It could be a sign of pyomera, vaginitis, infection, trauma, foreign body, or cancer.  

How to Prevent Puppies? Should you Spay Your Female Dog?  

The best way to prevent your dog from having puppies is to have her spayed. This is a surgery (known as ovariectomy or ovariohysterectomy) in which the ovaries plus/minus uterus are removed respectively. Most vets recommend spaying a dog around six months of age, before her first heat cycle, although some may elect to wait longer especially in large or giant breeds. If you are not planning to breed your dog, it is highly recommended to get her spayed to help increase her life expectancy and reduce risk of diseases such as mammary (breast) cancer and pyometra (infection of the uterus). Spaying your female dog will also help prevent messy heat cycles and overpopulation of puppies. Other types of birth control or treatments for unwanted mating are less accessible and may have significant side effects. 

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Dr. Liza Cahn, DVM

Liza is a veterinarian who graduated from MSU CVM in 2013 and spent five years working in small animal practice. She loved working with dogs and cats and educating owners on all aspects of veterinary medicine, especially animal behavior and dermatology. She has since transitioned to remote work to be able to spend more time at home with her husband, two young kids, and two cats. She is thrilled to be able to combine her passions for veterinary medicine and writing.