Female dogs typically have their "periods," or estrus cycles, about twice a year on average, approximately every six months. However, this varies with size and breed: small dogs can have heat cycles up to three or four times a year, while large breeds may only go into heat once a year. The estrus cycle usually lasts about two to three weeks. Dogs begin their first heat cycle around six months old, though this can range from four months up to two years depending on the dog. Older dogs tend to have longer intervals between cycles as they age. Unlike humans, dogs do not menstruate monthly but experience this estrus cycle twice a year or as per breed-specific variation throughout their lives unless spayed.