Dogs are capable of giving birth up until their death because, unlike humans, dogs do not experience menopause in old age. Different breeds have different life spans, so the number of litters possible over a lifetime depends in part on the type of dog.
Female dogs go into heat every six months from the onset of their first heat. The gestation period is approximately 63 days, with an additional six weeks of nursing the puppies. Theoretically, a dog can give birth three times per year over its lifetime, but this puts tremendous strain on its body. Professional breeders generally breed their dogs twice in the animal's lifetime between the ages of 6 and 8 years.