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When Reanne Crowley's puppy turned six months old – the standard age at which dogs are neutered – she let Chief alone, reproductive organs intact. The Vancouver Island waitress waited almost another year before taking her St. Bernard to be neutered, when he was around 18 months old.

"A friend of mine who is a dog trainer had told me that waiting until the dog is fully grown is best before neutering him, because neutering a male dog at a young age can stunt their growth," says Ms. Crowley. "I asked my vet about it and he agreed."

Ms. Crowley is one of a growing number of dog owners who are waiting longer to have their female dogs spayed or males neutered. While common practice says six months is the preferred age to have the surgery done, some owners are wondering whether common practice is always the best practice.

Not necessarily, says Jim Berry, vice-president of the Canadian Medical Veterinary Association and companion animal practitioner at the Douglas Animal Hospital in Fredericton, N.B. New, yet-to-be published research may show that neutering before 12 months of age can lead to body-development issues – such as longer and disproportioned legs – in larger, male dogs, says Dr. Berry.

As neutering changes hormone levels, some believe that neutering large, male dogs before they are fully mature will affect their growth.

These new studies would add to previously published research, which hypothesized that early neutering or spaying might put dogs at a slightly higher risk for certain diseases, such as bone cancer. These risks depend on the sex, size and breed of the dog.

The case for neutering and spaying at all is founded on a number of issues.

Neutering and spaying is key to population control, an issue that Dr. Berry says is still relevant in Canada. When cat and dog populations grow too large at animal service centres and humane societies, a percentage of animals must be euthanized. Dr. Berry says neutering and spaying helps keep euthanasia levels to a minimum.

For some, especially in crowded urban centres, it can make pet ownership more manageable, as owners of intact female dogs have to deal with clean-up when their pet is in heat, and unneutered male dogs exhibit aggressive behaviours such as biting and mounting other pets in the dog park.

Cathy J. Gartley, an assistant professor at the Ontario Veterinary College at the University of Guelph, agrees that the six-month benchmark age may not be the ideal age after all. "There is data to suggest that spaying before 5 1/2 months increases the chance of developing urinary incontinence, compared to spaying after 5 1/2 months," says Dr. Gartley. She also noted that "osteosarcoma [bone cancer]in giant breeds is supposedly more common in spayed or neutered animals." Dr. Gartley says the data pertains to larger dogs, not small or toy breeds, and suggests leaving large and giant breed dogs intact until fully grown.

Dr. Berry says he has noticed an increase in clients who, like Ms. Crowley, are waiting to spay or neuter their dogs.

Ms. Crowley thinks she has made the right decision. She says her boyfriend had his chocolate labrador, Champ, now six years old, neutered when he was under a year. "He does not have any joint or bone problems so far, which is good," says Ms. Crowley of the lab. "But [her boyfriend]noticed a while back that his head is a bit smaller than other chocolate labs and this could be because he wasn't fully developed when he was neutered."

However, based on the data currently available, both Dr. Berry and Dr. Gartley believe that for most dogs, the benefits of early neutering and spaying outweigh the potential risks. "Spaying at six months has been shown to decrease the risk of mammary cancer to almost zero," says Dr. Gartley. "Neutering prevents common prostatic diseases, prevents testicular cancers and decreases sexual behaviours."

Until more conclusive studies are done, most vets approach the issue on a case-by-case basis. "To me, it's all about informed consent," says Dr. Berry. "If the pet owners know the pros and cons, then it's up to them to make a decision about what they want to do, what's best for them, and what's best for their pet."