When does heat start?

Dogs can go into heat as young as four months in smaller breeds, but averages about six months old. Some giant breeds may not go into their first heat until they’re 18-24 months old. It is strongly advised not to breed young female dogs during their first and second cycle. Their eggs are not yet mature and the dog hasn’t reached full maturity. If you’re planning on breeding your dog, your vet will be able to tell you when the dog is mature enough to be bred.

How long is each cycle?

Heat usually lasts between 2-4 weeks. Early in the cycle, a female dog may not be receptive to male dogs, although some are receptive through the entire cycle. It can be shorter or longer and you’ll know the cycle is over when all her vulva returns to its normal size and there’s no more bleeding or discharge. There’s a relatively small window when your dog is most fertile during the heat cycle; it may begin about nine or ten days after she goes into heat and lasts about five days. However, she can become pregnant until the end of the cycle.

Is this a lifetime thing?

Once estrus begins, it may take awhile for the cycle to become regular. Some dogs can take up to eighteen months until their cycle becomes regular. It’s a good idea to keep a record during these early days. Once it does, the average is about every six months. Smaller breeds may go into heat more frequently, as often as 3-4 times a year. Larger dogs, like Irish Wolfhounds, St. Bernards and Great Danes may only go into heat every 12-18 months. Unlike humans, female dogs experience estrus throughout their lives, although the time between cycles will get longer.

This article is provided by AKC.org