The answer- It depends. It depends on the species (dog or cat), the breed of the dog, and the lifestyle of the dog.
Spaying is the procedure performed on female animals to remove the ovaries and usually the uterus as well. We (veterinarians) recommend to spay animals to prevent certain diseases such as mammary gland tumors/cancers and life threatening infections in the uterus called pyometra, as well as preventing unwanted litters of puppies/kittens.
Neutering is the procedure performed on male dogs to remove the testicles. We recommend neutering to prevent infections in the prostate, prostate enlargement, and certain behavioral traits predominant in male (non-neutered) animals such as marking (urinating on household items), aggression and roaming behavior, and to prevent unwanted litters of puppies/kittens.
The age that we recommend these procedures at is changing. There is no longer a “one size fits all” approach. We used to spay/neuter everything at 6 months of age, but some new data suggests there are more than just the above-mentioned issues to consider.
Cats are more standardized as there isn’t as much variation between the breeds. Female cats have an extremely high rate of cancerous mammary gland tumors if not spayed early. Male cats will mark if not neutered early enough. We recommend spaying or neutering of all cats by 5 months of age.
For dogs we generally recommend to wait a bit longer for larger breeds and sooner for small breeds. This is mainly to allow for more orthopedic development before removal of the sex hormones produced by the ovaries and testicles. For small breed dogs we still recommend spay/neuter at 6 months of age, for medium dogs generally 7-10 months, large breed dogs between 12-14 months and giant breeds between 15-18 months. This means some patients will have a heat cycle, but its not necessarily the goal for the patient to go through a heat cycle.
In addition to the breed guideline, we must consider lifestyle factors and behavior. If the pets are living with siblings then we may need to spay or neuter at least one of them earlier to prevent accidental mating. Intact males are more likely to roam which does pose a risk of being hit by a car so we have to consider the likelihood of being able to securely contain them. As mentioned above some females will go through a heat cycle. If they can’t be strictly contained to prevent accidental breeding from neighboring dogs, or if the owner does not want to deal with the mess of a heat cycle, we may decide to spay them sooner. Some pets will develop aggressive behaviors after 6 months of age due to hormonal influences, so we must monitor behavior closely. If aggressive tendencies are noticed we may decide to spay/neuter at that time and not wait for full orthopedic development. For males specifically marking behavior is very common after about 8 months of age, if these behaviors are noted we may decide to neuter the patient before full orthopedic development.
In summary, there are many factors to consider when determining when to spay or neuter your pet. We are happy to discuss the specific needs of your pet, should you have questions please let us know.