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The canine estrus cycle is divided into four stages: proestrus, estrus, diestrus, and anestrus. Although the first three stages of the cycle are referred to as active heat, the period during the cycle when a non-spayed female dog is receptive to breeding and capable of getting pregnant (in season) usually only lasts from 18 to 21 days. Males will travel several miles to get to a female in season, so to avoid unintended litters, you must be extra vigilant in keeping her away from intact males during this period.
Proestrus (3 to 17 days): This first stage of the heat cycle lasts an average of nine days. The female will emit an odor (pheromones) that attracts males, but in this first stage of the cycle she will reject their advances by jumping, snapping, and growling at any interested males. Physically, you may notice a slight bloody discharge and a notable swelling of her vulva. A few days before estrus begins the bitch may let a male mount her, but she wont stand for breeding. Proestrus ends as soon as she is receptive to the male.
Estrus (7-9 days): The second phase of the heat cycle is called estrus, or standing heat. This is when the female is willing to accept a male for breeding. During this time the discharge changes to a light pink or straw color and the female will often lift her pelvis and raise her tail to the side to signal her readiness to breed. Because she can get pregnant during this time, keeping her safely away from intact male dogs is critical. Estrus lasts an average from seven to nine days (the range is 2 to 20 days) and ends when the female once again refuses to stand for the male.
Diestrus (60-90 days): This final stage in the active part of the cycle begins when the female refuses to stand for breeding. Most males lose interest now, too. The discharge may become redder for a few days before stopping completely and the vulva slowly returns to its normal size. When the discharge and swelling have ended, the heat cycle is considered complete. Diestrus lasts about 60 days, and then merges with anestrus. If the female was allowed to become pregnant during estrus, diestrus lasts until the she gives birth, approximately 63 days after conception.
Anestrus (130-150 days): The fourth and final phase of the estrous cycle consists of little to no ovarian activity. This is the time between the end of one heat cycle and the beginning of a new heat cycle. The length of anestrus varies, lasting on average 130 to 150 days.
Facts about the canine heat cycle:
Females dogs (bitches) become sexually mature when they have their first heat period. This usually occurs between 6 and 12 months of age, although there can be considerable variation among individual dogs and breeds. Toy breeds become sexually mature several months earlier than giant breeds. Sexual maturity does not correspond to physical maturityachieving an adult height and weightwhich generally happens at about 14 months of age in medium-sized dogs.
Ovarian activity and the ability to conceive begin to decline after 6 years of age and ceases (in most females) at around 10 years of age.
Most female dogs will come into heat twice a year until they are spayed. Some may come into heat every five to nine months. Basenjis are noted for only having only one heat per year. The heat cycle is specific for each individual and not influenced by external factors such as hours of daylight. Many females have their first heat cycle at approximately the same age as their mother did.
Although a female dog may be able to physically conceive as young as 6 months, she should not be bred until she is at least 2 years old. There are two reasons for this: 1) a young dog is not yet emotionally mature enough to deal with a litter, which may cause her to harm or abandon her puppies and 2) the common tests used to screen for health abnormalities prior to breeding (hips, elbows, heart, eyes) will not provide accurate results until she is 2 years old.
Some dogs heat cycles get messy, while others are hardly noticeable. Doggie diapers and are available to protect your house, and you may also want to clean her vulva area once or twice a day with a damp cloth to prevent skin irritation. Be prepare for some behavioral changes, too. Your your dog may appear tired, cranky, agitated, attempt to mount other dogs, or try to escape and breed.
Source: Dog Owner's Home Veterinary Handbook, Fourth Edition; The Merck Veterinary Manual, Seventh Edition; Personal Experience.
By EllenB 
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Here are questions related to How Long is a Dog in Heat?.
How long will dogs be in heat?
By Aryana from CA
By Sherri 06/30/2011
Are you a breeder? If not, you can have her spay.
How long will a dog be in heat? I'm very new at this.
By Annette from Ontario, Canada
There are many reasons why someone would wait to spay or neuter their dog or cat. As long as they are responsible and do not allow their pet to mate, then it should not be an issue until the owner finds the right time to alter.
For us, we need the hormones of my dog's uterus for proper bone and mental growth for what her job is. I also want to wait because OFA testing usually happens at 2 years and I only want her sedated once. I will spay three months after her last heat closest to her second whelp day.
There has been research and documentation against pediatric altering.
Studies have shown more instances of joint damage in larger breed dogs with those that were altered before adolescence. Waiting until adulthood increases your chances of healthy bone and joint growth. Usually with cats there is not a reason to wait, but I will right up until he/she starts going into puberty, about 4 to 5 months.
I am all for altering, especially if you are unable to make sure your dog will not breed while waiting for the right time, but I did want to say that there are reasons why someone would wait to spay or neuter their dog.
By Todd from Phoenix, AZ
By Melanie Jackson 01/12/2010
All pets should be spayed or neutered. It's healthier for the pet, and less stressful for you having to worry about these things. :-)