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Estrus Cycles in Dogs

When does a female dog have her first estrus cycle?

Dogs will have their first estrus cycle when they reach puberty. Estrus is the stage of the reproductive cycle in which the dog can become pregnant; sometimes a dog that is in estrus is said to be "in heat" or "in season". On average, puberty or sexual maturity is reached at about 6 months of age, but this can vary by breed. The smaller breeds of dogs tend to have their first estrus cycle at an earlier age, while the large and giant breeds of dogs may not come into heat for the first time until they reach eighteen months to two years of age.
Estrus is the stage of the reproductive cycle in which the dog can become pregnant.

How often does a female dog come into heat?

Most dogs come into heat twice per year, or about every six months, although the interval can vary between breeds, and from dog to dog. Small breed dogs may cycle three times per year, while giant breed dogs may only cycle once every 12-18 months. When young dogs first begin to cycle, it is normal for their cycles to be somewhat irregular. It can take up to 2 years for a female dog to develop regular cycles. There is no season of the year which corresponds to a breeding season for domesticated dogs.

How long does estrus last?

Although this can vary with the individual dog, on average a dog will be 'in heat' for 2-3 weeks. The onset of estrus is characterized by swelling of the external vulva. Next, the dog will develop a vaginal discharge. Estrus ends when the discharge completely disappears and the vulva returns to its normal size.

What are the signs of estrus?

The earliest sign of estrus is swelling or engorgement of the external vulva, but this swelling is not always obvious. In many cases, a bloody vaginal discharge will be the first thing that a pet owner will observe when their dog comes into heat. In some cases, the discharge will not be apparent until several days after estrus has begun. The amount of discharge varies with the individual dog; some dogs have only a tiny amount of discharge while others have heavy bleeding. Occasionally, especially in dogs with long dark hair, the discharge will not be apparent at all.
... a bloody vaginal discharge will be the first thing that a pet owner will observe when their dog comes into heat.

The vaginal discharge will change in color and appearance as the cycle progresses. At first, the discharge is very bloody, but as the days pass, it changes to a watery, pinkish-red discharge, usually about 7-10 days into the cycle. A female dog that is in heat will often urinate more frequently than normal, or may develop 'marking behavior', in which she urinates small amounts on various objects either in the home or when she is on a walk. During the estrus or heat stage of her estrus cycle, her urine contains pheromones and hormones, both of which signal her reproductive state to other dogs. This is the reason that dogs in heat will attract other dogs, particularly males. 

Male dogs can detect a female in heat from a great distance and may begin marking your property with their urine in an attempt to claim their 'territory".

At what stage of the estrus cycle is the dog able to get pregnant?

The female dog usually ovulates at about the time that the vaginal discharge becomes watery; this marks her most fertile stage and at this time she will usually become receptive to breeding. However, sperm can survive for a week in the reproductive tract and still be capable of fertilizing the eggs, so it is possible that she can get pregnant at any point while she is in estrus. Contrary to popular belief, it is not necessary for the female to 'tie' with the male dog in order to get pregnant (for further information see our handout "Breeding for Pet Owners - Estrus and Mating in Dogs").

How long does pregnancy last in a dog and when can pregnancy be detected?

Pregnancy lasts approximately 9 weeks (63 days) in the dog. However, it is not possible to detect pregnancy during the first 3 weeks. An experienced veterinarian may be able to palpate or feel the fetuses in the uterus of a relaxed dog at about 28-32 days of pregnancy. Ultrasonic pregnancy diagnosis can be performed any time after about 25 days of pregnancy. At 6 weeks of pregnancy, the fetal skeletons begin to become calcified and pregnancy can be confirmed by means of radiographs (x-rays) after this time.

How can I prevent my dog from becoming pregnant?

The best way to prevent your dog from becoming pregnant is to have her surgically sterilized (an ovariohysterectomy or 'spay'operation) before she has her first estrus cycle. Since it can be difficult to predict when this first cycle will occur, most veterinarians recommend performing an ovariohysterectomy before the dog is 6-7 months of age.
The best way to prevent your dog from becoming pregnant is to have her surgically sterilized...

Is there anything I can do if my dog has been mismated, or accidentally mates with another dog?

If this happens, you need to contact your veterinarian as soon as possible. There are mismating injections that can be used within the first 1-2 days after mating occurs, but there are risks associated with their use. Your veterinarian will discuss your options and any risks associated with them.

Should I let my dog have an estrus cycle or a litter of puppies before spaying her?

There are no valid reasons for letting a dog have an estrus cycle or have a litter of puppies before being spayed. Dogs can become pregnant on their very first estrus cycle, increasing the chance that an accidental breeding may occur. Dogs are indiscriminant, so a brother may breed with its sister, a father may breed with his daughter, and a son may breed with his mother.

A common myth is that female dogs will become more friendly and sociable if they are allowed to have a litter of puppies. This is not true, and only serves to contribute further to dog overpopulation.

This client information sheet is based on material written by: Cheryl Yuill, DVM, MSc, CVH
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