Female dogs experience their first reproductive cycle, or heat, when they are between six months and two years old, explains WebVet. The female dog can become pregnant during her first heat. Whitney Veterinary Hospital recommends letting a dog go through at least one heat before becoming pregnant.
Dogs in their first heat are often too physically and emotionally immature to breed successfully, explains Whitney Veterinary Hospital. Breeding too early can result in joint problems like swayback and bent legs. The hospital recommends letting smaller breeds experience one heat before breeding. Large breeds like Mastiffs and St. Bernards may not be ready for breeding until they experience four heats.