A female dog can become pregnant as soon as she comes into her first heat, which differs from dog to dog. Some dogs enter their first heat as soon as 6 months of age, while others are 18 months old. For this reason, veterinarians advise spaying a female dog when she is 6 months old in order to avoid any unwanted pregnancies.
When She Shouldn't Be Bred
If you are planning to breed your dog, wait until the dog has come into her second or third heat. She will be far too young to be bred during her first heat and the pregnancy could hurt her health, according to Dog Owner's Home Veterinary Handbook (see Resources).
Small breeds the sizes of cocker spaniels or smaller can sometimes come into their first heat when they are only four months old. They can get pregnant.
Large and Giant Breeds
Very large breeds like Saint Bernards tend to be slower to come into their first heat, but should have their first heat by the time they are 2 years old. If your Saint Bernard hasn't had her first heat by this time, take her to the vet.
The Older Female Dog
A female dog will continue to have heat periods all through her life, but they will become more erratic when she is over 8 years old. Geriatric pregnancies can sometimes happen.
Female dogs given canine contraceptive drugs like Ovaban (megestrol acetate) can still become pregnant if they mate when taking the drug for the first three days of the eight-day dosage period.