A female dog will have her first heat between 6 and 12 months of age, but that doesn't mean she's ready to mate. Depending on her size, genetics and breed, she will continue to go into season every four to 12 months. A dam theoretically can produce a litter of puppies each time she's in season, but it's not a good practice.
Although a female dog's first heat might signal physical maturity, a dog this young is still a puppy. Smaller breeds become reproductively mature as early as 6 months of age but finish growing between 12 and 15 months of age; larger breeds may be reproductively mature by age 1 but they may grow until age 2. When a puppy is in season, her body remains immature -- it's not ready to gestate and deliver a litter. A female dog who is still a puppy mentally is more prone to reject her litter or refuse to nurse. A dog should not be bred the first time she comes into heat. After her first heat, she will cycle again at least once a year if not twice. The estrus cycle interval varies from dog to dog.
The Heat Season
A female dog will be in heat for roughly 21 days at regular intervals throughout her life or until she is spayed. She will have some pinkish discharge the first few days; it will turn red as she reaches fertility in about nine days. She is fertile not for all three weeks but only for a period of time known as estrus, which usually lasts for the middle five to nine days. A veterinarian can perform progesterone blood testing to pinpoint the exact period of fertility. The dog's discharge will take on a brown tinge for the final days of the cycle. If she is bred and whelps a litter, expect her to resume heat seasons 4 to 12 months after delivery, at the same interval she had them before pregnancy.
Ready to Breed
The accepted guideline for breeders is that a dog should not be bred before her second heat season. In most breeds, this occurs between 1 and 2 years of age. Gestation takes 61 to 65 days from conception, so a typical midsize female dog could whelp her first litter around age 2. A larger dog's first litter would be slightly later; a small dog might deliver a first litter not long after her first birthday.
Prior to breeding, your dog should be tested by a veterinarian for genetic conditions common to her breed and undergo other appropriate tests for normal hips and eyes. A dog prone to genetic faults or a poor temperament should not be bred, as her faulty traits can pass on to puppies.
Plan Future Litters
Some breeders and reproductive veterinarians recommend breeding a dog in back-to-back heat seasons, while others believe a mother dog should skip a season between litters to rest her body. You should make the decision on an individual basis in consultation with your veterinarian, who can assess your dog's physical health and recovery after each litter.
The American Kennel Club will not accept the registration of a litter from a dam less than 8 months old or more than 12 years old.
Breeding Too Often
A responsible breeder differs from a puppy mill operator by employing carefully planned breeding practices and by making the health of the dam and puppies, and the improvement of the breed, priorities. You can breed a dog on each heat, but doing so will impact her health and the health of her litters over time. The number of puppies in a litter will decrease after the dam reaches age 6. Puppies may be smaller, less healthy and more prone to illness. A breeding dam's body will not recover as quickly from the stress of pregnancy as she ages.