The American Society for the Prevention of Cruelty to Animals states that a female dog is ready to breed during the estrus, or heat stage, which generally occurs between six and twenty-four months of age. Smaller breeds tend to experience their first heats sooner than larger breeds.
An unspayed female dog typically experiences a heat period twice yearly, according to the ASPCA. During heat, the dog's estrogen levels increase then decrease, allowing for mature eggs to be released from the ovaries. During this time, the dog may exhibit signs such as nervousness, irritability, more frequent urination and higher alertness. Vaginal discharge may also occur. When in contact with other dogs, the female may try to initiate sexual contact by elevating her hind quarters or shifting her tail to the side.
The ASPCA maintains that unless the dog is to be used for breeding purposes, it is important to have her spayed before she experiences her first heat. Spaying prevents accidental pregnancy while protecting the dog against breast cancer and other diseases of the reproductive system. A dog that has been spayed prior to her first heat cycle is also at reduced risk of developing tumors of the mammary glands. A veterinarian can spay a dog as young as two months old.