A female dog might be physically capable of producing a litter of puppies as early as 5 months and generally reaches sexual maturity by 15 months. The age at which a dog goes into its first heat can vary depending on the breed. Small dogs often mature earlier than larger ones.
Responsible breeders make sure that the male and female dogs are both healthy and in the best mental and physical state possible. This requires a balanced diet and regular exercise. Both dogs should be checked by a veterinarian to ensure they are in good condition for breeding, are up-to-date on shots, and don't have any communicable diseases or parasites.