A female dog can go into her first heat cycle as young as 6 months old, but they can happen as late as 12 to 24 months of age depending on the breed of the dog.
If your dog does get pregnant, you should take her to the vet to get her checked out and to discuss your options. The vet can help you determine how far along your dog is and advise you on the best course of action. If you decide to have the puppies, your vet can help you monitor your dog's health and provide guidance on how to care for the puppies.