Dog Heat Cycle Changes
The typical dog heat cycle lasts about three weeks. During this time a female goes through physical and emotional changes that prepare her body for the possibility of fertilization.
One of the earliest signs that a dog is coming into heat is the swelling of the vulva.
After swelling has begun, many females protect their vulvas by tucking their tail between their legs. This usually occurs in the early stage of the heat cycle before the fertile period ensues. Many females also seem to feel a little depressed at the beginning of the cycle.
Swelling of the vulva is quickly followed by the onset of spotting. Blood is discharged and sometimes found on furniture and flooring. Many owners miss the early swelling and only realize their female is in season once they find spotting.
A female in heat feels compelled to clean herself frequently. Depending on how frequently she cleans, you may see little to no sign of spotting.
Many females experience a small amount of breast tissue development caused by hormones circulating in their bloodstream. This is especially so during the first heat cycle.
Hormonal changes also lead to a distinct change in a female's scent. Male dogs often pick up on this scent before the female presents any other sign of being in heat. They tend to follow the female around, frequently sniffing at her vulva and generally pestering her.
During the second stage of the heat cycle, a female typically becomes more flirtatious with other dogs. Her spotting decreases as her body begins to release eggs for fertilization.
Tail flagging and standing firmly in place is usually a sign that the female is ready to breed. She may even make strong advances toward the male dog to let him know she's interested.
End of the Dog Heat Cycle
As the female enters the third stage of the cycle, her spotting temporarily increases, and she seems less interested in breeding behavior. The vulva begins to return to normal size and discharge trails off and ends. If there was no successful breeding, hormone levels return to normal. In most cases it will be another six to twelve months before the next heat cycle begins.
To learn more about your dog's health, see this dog health issues slideshow.