According to VCA Animal Hospitals, the average female dog goes into estrus twice a year, which means she can have two litters of puppies a year. However, just because a dog is in estrus does not always translate to pregnancy. A number of factors influence whether a dog becomes pregnant during her heat cycle.
Age tends to play a role in a dog's estrus cycle. Many dogs do not experience regular estrus cycles for the first couple of years upon reaching sexual maturity. For example, it is normal for a young dog to only have one heat cycle a year.
Not all dog breeds have regular heat cycles. Smaller dog breeds can have a heat cycle three times a year, and certain larger breeds may have a heat cycle only once a year. A dog cannot get pregnant between heat cycles. However, a dog can become pregnant during a "silent" heat. Silent heats are simply heat cycles that go undetected.
Typically, it is not recommended for a dog to have more than one litter a year. Having too many litters a year can be detrimental to the mother's health. Before breeding a dog, it is advisable to ensure that the dog is in optimum health.
Veterinary Partner recommends waiting until a female dog is at least 2 years old before breeding. Breeding a dog before she reaches 2 years of age can lead to complication for the both the mother and puppies.