What Are the Signs of a Female Dog in Heat?

If your dog isn't spayed, you'll learn to recognize the signs she's in heat -- and so will every male dog in the neighborhood. Formerly known as estrus, heat is the time in a dog's reproductive cycle when she's receptive to mating. Generally, dogs experience two heat cycles a year, lasting an average of 18 days.

Dog, close-up
Close-up of a dog's face.
credit: Chris Amaral/Digital Vision/Getty Images

Growing Up

You went through puberty, and your dog is no different. Dogs mature at different rates, with smaller breeds maturing faster than larger breeds. Eventually, all dogs reach sexual maturity, which generally occurs around 6 months of age, though for large dogs, it may be two years before she experiences her first heat cycle, compared to four months for a toy dog.

What's Going On?

The timing and stages of a dog's reproductive cycle is marked by hormones. Every dog goes through four cycles of oestrus, usually twice a year, whether she becomes pregnant or not. During the first stage, proestrus, her body prepares for the possibility of mating. Her levels of estrogen begin to rise in proestrus, peaking toward the end of the phase, when her progesterone levels begin to climb. This phase lasts an average of 9 to 11 days. During the second stage, estrus, she's actively looking to mate. Her progesterone continues to rise during this time, which lasts an average of 9 to 11 days as well. Progesterone dominates during the third stage of diestrus. The length of diestrus depends on whether the dog is pregnant; a pregnant dog spends about 63 days in diestrus, while one who is not is in diestrus between 60 to 90 days. The final stage is anestrus, lasting four to fivw months while the sex hormones remain at low levels.

Time to Mate

A dog in estrus shows physical and behavioral signs of her condition. Physically, you may notice a blood-tinged vaginal discharge, accompanied by some vulval swelling. As she progresses through estrus, the color of the discharge changes color, transforming to a pinkish-red around a week or 10 days into her cycle, which is about the time she becomes receptive to mating. During estrus, a dog often appears nervous or easily distracted and urinates, or marks, more than normal. As she becomes more interested in male attention, she may engage in flagging behavior, tensing her rear legs, elevating her rear end toward a male dog and waving her tail to one side.

Ongoing Cycles

If your dog isn't spayed, you can expect her to go into heat twice a year, roughly about every six months, unless she's a very large or very small dog. Giant breed dogs, such as Great Danes, usually have a heat cycle every 12 to 18 months. Some toy breeds experience three cycles a year. You can expect to have male suitors for about 18 days during her heat cycle, as male dogs are highly attracted to females in estrus. Your dog can become pregnant only for about nine days during each heat cycle.

Cooling Off

Unless you plan to breed your dog responsibly, consider having her spayed. WebMD notes spaying not only prevents accidental pregnancy, but helps guard against breast cancer and reproductive system diseases. A dog may be spayed as young as 2 months old, however your vet can discuss the best time to spay your dog. Many people believe allowing a dog to have a litter of puppies will make her a better pet, however, VCA Animal Hospitals note that is a myth and that giving birth has no impact on how friendly a dog is.