When should I spay or neuter my pet?

The answer used to be easy, six months for dogs, six months for cats.   No thought required.   Then the shelters, concerned with the number of unwanted pets, began to promote spaying and neutering at younger ages.  Their problem was real, but they didn’t make this decision based on medical evidence.   Problems are now emerging.

What do we know?

For many years we have known that spaying a female dog or cat before the first heat cycle almost eliminates their life long risk of mammary tumors (breast cancer).   In dogs 50% of tumors are malignant.  In cats 90% are malignant.  But by spaying dogs before the first heat cycle the risk drops to less than ½ of 1/10 of 1% (0.05%).   Spaying after the first heat cycle but before the second also reduces the risk, but not by as much.  The first heat cycle generally occurs around 7 months for small dogs, and a little later in large breed dogs.   We are forced to euthanize dogs and cats for mammary tumors several times per year. Pets not receiving regular examinations often have advanced cancer on presentation, making treatment more expensive and sometimes too late.

We know from human and dog studies that pregnancy, as a young adult, carries less lifetime risk for mammary tumors than delayed pregnancy, so delaying spaying for a dog intended for breeding is not necessarily bad. However you will have to find homes for all of those puppies.

It is also well established that dogs and cats cycling in and out of heat over and over again, but not getting bred, start to develop changes to the uterus (endometriosis) that often lead to a severe life threatening uterine infection. This can generally be cured by surgery, but is much more expensive and far riskier than a routine spay as a young adult. If caught late, you may face over $1000 of medical bills. It can be fatal, especially if you procrastinate on seeking veterinary care. We see this condition a few times a year at Urbana Veterinary Clinic. It is 100% avoidable by spaying.


These facts are the origin of the 6 month recommendations. But the early spay neuter ‘experiment’ has taught us new lessons. The findings are still being refined, and research is ongoing.

In a recent study of 759 golden retrievers at the UC Davis veterinary school were evaluated for orthopedic and cancer risks.   Neutering before one year of age was associated with an increased lifetime risk of hip dysplasia (also affected by genetics, diet and exercise), cruciate ligament tears (a knee injury requiring expensive surgery) and the cancer lymphosarcoma.  However neutering after one year of age was associated with an increased risk of mast cell tumors and hemangiosarcoma (malignant blood vessel cancer) in female dogs.1

There is also a 2002 study2 (one of Dr. Cole’s classmates was the lead author) that found Rottweilers spayed or neutered before one year of age had an increased risk of the malignant bone tumor osteosarcoma compared to dogs that were not spayed or neutered, and dogs spayed and neutered later in life.   This study did not look at other breeds.

There is even a study3 showing that cats neutered before 4 months of age have an increased risk of the rare orthopedic disease Legg Calve Perthes disease.  This disease is life changing, surgery is necessary to use the leg again.


What does this mean?

  • Cats should be spayed and neutered around 6 months of age.
  • Female dogs should be spayed between 6-12 months of age.
    • We consider her size, breed and any known family history of disease to determine the best time, as well as the risk of unwanted pregnancy.
    • Generally small dogs should be spayed around 6 months of age, large dogs maybe a little older.
  • Male dogs should be neutered between 6-12 months of age.
    • Large breed dogs should be neutered at a year of age unless they are in danger of causing an unwanted pregnancy or developing aggression problems.
  • Spaying or neutering a dog or cat less then 6 months of age is no longer recommended.
    • It is very unwise to spay or neuter them before 16 weeks of age.
  • These recommendations will continue to evolve as new evidence is collected.

1. Vet Practice News Vol 25 No 4, pp 1 and 8
2. Cancer Epidemiology, Biomarkers and Prevention, Nov 2002, 11; 1434
3. April 2010 Central Veterinary Conference, Baltimore proceedings. Gayle H Jaeger, DVM, MspVM, DACVS