On average this occurs about twice a year or every six months, although it varies from dog to dog. When cycling first begins, there may be a great deal of variability in the time between cycles. This is normal. Some females take eighteen months to two years to develop a regular cycle.
There is no evidence that irregular heat cycles predispose the dog to false pregnancies or pyometra (uterine infection). Small breeds tend to cycle more regularly than the larger breeds. Three and occasionally four heat cycles per year can be normal in some females.
Very large breeds may only have a “heat” cycle once every 12-18 months. In most giant breeds (Great Danes, Irish Wolfhounds, St Bernard’s, etc.) an oestrus cycle every twelve months is common.
How long does a “heat” cycle or oestrus last ?
“Heat” cycles vary, but average two to three weeks for most dogs. “Heat” should be considered to begin with the first signs of vulvar discharge, or when the female begins licking or paying attention to her vulva. The vulva will begin to appear swollen. It ends when all discharge ceases and the vulva has returned to its normal size.
What are the signs of “heat”?
The most notable sign is vaginal bleeding. This may not become apparent until a few days after the female has actually come into oestrus. Vulvar swelling should be taken as the first sign in addition to the female paying increased attention (such as licking the area) to her rear end.
From the beginning of the heat period she will be attractive to male dogs, but will usually not be receptive, or allow mating, until about 7-10 days later. The discharge will usually become less bloodstained at this time.
Some females experience heavy vaginal bleeding during oestrus. If you are concerned, please consult your veterinarian.
You may also find that she is passing small quantities of urine more frequently. The urine contains both pheromones and hormones which signal any interested males that she will be receptive soon.