Female dogs typically go into heat every six months, with the first heat occurring between the ages of 6 and 24 months. However, these intervals vary with different breeds and individual dogs.
The first heat, or estrus, can occur in small-breed dogs as young as 5 months old and in larger breeds at 14 months or older. Estrus lasts for an average of 18 days, but it can end within a few days or last as long as four weeks. During this time, the dog may appear nervous and more alert than usual and urinate more frequently. She may also initiate sexual encounters with other dogs by lifting her hind quarters, tensing her back legs and pulling her tail to one side.