Dogs in Heat

Posted by  on June 24, 2010

How do you know when to breed a dog?  When is it “safe” to allow males around your female?  What can you do to prevent unwanted litters?  Read on for the answers to these and other questions regarding your dog’s heat cycles.


Yes, it’s true.  Dogs go through puberty just like people do, and prior to this milestone, your bitch cannot become pregnant.  Although large dogs tend to have a later puberty than small dogs, anywhere between six and 24 months is considered a normal age for the dog to go through puberty.  However, the signs of puberty in your dog will not be anywhere near as easy to spot as they are in your children.

During the dog’s first cycle, bleeding may be irregular or split into two separate and distinct periods.  The dog may be fertile during the entire time, so care must be taken during this entire time to prevent unwanted litters.  Although your dog may be fertile during her first few cycles, it is recommended that you wait until she is about 1-1/2 or 2 years old before breeding her so that her eggs and reproductive organs will be fully mature.  The general rule of thumb is to wait to breed until at least the dog’s third cycle.

Following the first cycle, your dog will come into heat about twice each year.  Some dogs are fertile for as long as 7 – 9 days before estrus and 7 – 9 days after it ends.  The dog must be isolated from male dogs for the entire 3 – 4 weeks if you want to be sure to prevent pregnancy.

Does a dog go through a cycle like a human female does?

A dog absolutely has a cycle, but it is much longer than a human female’s 28 days.  The good thing?  Dogs do not experience PMS!  The dog’s cycle is divided into four stages:  diestrus, anestrus, proestrus, and estrus. Diestrus lasts about 60 days, while anestrus lasts for a variable length of time after diestrus.  The two stages together are called interestrus.

dog in heat

Following interestrus, a dog enters the stage of proestrus, which lasts about 9 days and is when the male will begin to show interest in the female, even though the female wants nothing to do with him at this point.  Estrus, when the female is fertile and will allow a male to mount her, lasts about 9 days, as well.

The average duration of interestrus ranges from 6-1/2 months for large dogs like German Shepherds, to 12 months for smaller dogs like Basenjis.  Also, the interestrus period becomes longer as the dog becomes older, starting to lengthen at about eight years of age.


Diestrus is the stage that occurs right after estrus.  The uterus and ovaries are under the effects of hormones, but there are no outward signs of this stage.  If the dog was successfully fertilized during the preceding estrus cycle, she will be pregnant for 60 – 64 days.  If she remains unfertilized, she will stay in diestrus for 60 – 90 days.

During anestrus, the dog’s endometrium is repairing itself, either from the pregnancy or the build-up to an unrealized pregnancy.  If the dog does not remain in anestrous for at least 90 – 150 days, she is unlikely to be successful in producing a litter during the following heat cycle because the uterus will not have been repaired enough to support a pregnancy.

During these two periods, known together as interestrus, neither the male nor the female will show any interest in each other.  The female’s external sexual organs will appear normal.  As anestrus draws to a close, follicle stimulating hormone is elevated, which “ripens” the eggs to be let out of the ovaries and into the fallopian tubes.  The eggs will actually be released during proestrus.


Proestrus may last anywhere from 3 – 17 days, but is most likely to last about 9 days.  The male will become interested in the female, but the female will not be ready for him yet.  Ovarian follicles secrete estrogen, which causes swelling of the vulva around the bitch’s vaginal opening, which is a key sign telling you that your dog is entering her fertile period and should be isolated from male dogs to prevent pregnancy. Because of this swelling, your dog will likely begin to lick her privates excessively.

Another major sign is that the dog will begin to leak vaginal blood during proestrus.  Some dogs bleed a lot and require a diaper to protect your furnishings and carpets; some dogs will barely bleed at all.  If you don’t want to go to the expense of dog diapers, a useful substitute is men’s or boys’ underpants with a fly opening.  Place a thin sanitary napkin in the front of the underwear, then put the underwear on the dog with her tail sticking out through the fly.  To estimate the size your dog will need, use this rule of thumb:  a 30-pound dog will generally need children’s size 6 underwear.  Don’t forget to take off the underwear when you let your dog outside to relieve herself and to leave it off long enough for her to clean herself when she comes back in.  Change the pad regularly to prevent odor.

The color of the blood will be dark red in the beginning, but it will fade to a peach or salmon color by the end of proestrus.  Bleeding is your cue to isolate your dog from males for the next 3 – 4 weeks if you wish to avoid pregnancy.


At the end of proestrus, the follicles begin to slow their estrogen secretions, causing the dog to begin to show the signs of true estrus.  Estrus may last from 3 – 21 days, but averages 9 days.  Male and female will both be interested in each other and the bitch will “flag” her tail, carrying it to the side and inviting the male to mate.

About 1 – 2 days into estrus, luteinizing hormone is released, causing the eggs to drop from the ovaries into a position when they can be fertilized.  The dog will be most fertile on days 3 and 4, i.e. two days after the LH peak.  When pregnancy is desired, LH levels can be tested by a simple blood test to determine when mating should occur.

If the eggs remain unfertilized, they will simply die in 5 – 6 days, marking the end of estrus.

The only 100% effective way to keep your dog from becoming pregnant is to spay her.  However, if for some reason you do want to breed her at a later time, you must be vigilant in keeping her away from males during her fertile period.

When should I spay my dog?

If you are not planning on breeding your dog, it is best to have her spayed prior to her first heat cycle.  Doing so is protective against many types of cancer later in life.  Many shelters insist on spaying all dogs before placement so spaying may be done as early as six to eight weeks of age.  At traditional veterinary clinics, it is much more common to spay at six months.

One common myth is that a dog will somehow mature better if she is allowed to go through one heat cycle prior to being spayed.  In fact, there is no such advantage, and waiting to spay your dog actually puts her at a disadvantage because she is at a much higher risk of developing mammary cancers later in life.

Another myth is that your dog should be allowed to have one litter before spaying so she will be a better pet or so your children can experience “the miracle of life.”  Again, not true.  There is no advantage to having your dog reproduce before spaying her, and your children might be much better served by spending time working at a shelter to experience the misery of abuse and neglect suffered by unwanted puppies.

Here is a sad fact from the Atlanta Humane Society, one which is mirrored at most other shelters in large cities: “20 tons of dead animal bodies produced in one year in the Atlanta Area alone.”  The moral of the story?  Don’t breed your dog unless you have good reason to, such as furthering the breed for show or competition purposes by conscientiously and responsibly pairing dogs who are prime examples of the breed standard.