Although it depends on the specific dog, female dogs are usually the most fertile around the 11th day of estrus, and typically cycle twice a year, but exact times vary by breed and location. Male dogs are fertile all the time, after reaching sexual maturity.
Male dogs can become fertile as early as 6 months old, but they must not be bred until they are at least 12 to 15 months old. Female dogs have their first estrus cycle, colloquially known as heat, starting between 6 months to 2 years old. Females must not be bred on their first heat cycle or required to carry litters after they reach 12 years old. Breeders test dogs for common genetic diseases in that breed and require dogs to prove themselves in the show ring or in working trials before being bred.