Spaying Dogs When and Why?

The decision on when to spay your dog depends on many factors including your dog’s breed, genetic predispositions/disease risk, size, lifestyle, and how you feel about managing her through heat cycles. The American Animal Hospital Association (AAHA) provides a simple spay/neuter age chart shown below that can help you decide. It’s based on AAHA’s 2019 Canine Life Stage Guidelines, which recommends small-breed dogs that are expected to be under 45 pounds as adults be spayed prior to their first heat (at 5-6 months). The recommended window for large-breed dogs is 5 to 15 months depending on the factors mentioned above.

A recent study from the University of California, Davis provides more detailed guidelines for deciding on the age to spay 35 dog breeds based on risk of joint problems, cancers, and urinary incontinence. Your veterinarian can also help guide you in making your decision.

Canine Spay Neuter Aaha Graphic

Spaying a female dog involves removing her reproductive organs (ovaries, uterine horns, and uterus) under general anesthesia. Without them, she’s unable to get pregnant and have puppies. Recommendations around spaying dogs continue to evolve as new information becomes available. This document explores the benefits and risks of spaying.

At SOVH, dog spays are performed laparoscopically to ensure a minimally invasive, quick, safe, surgery and a comfortable and speedy recovery. For more information, see our handout “Laparoscopic Spays.”

Female Dog Anatomy Graphic

Preventative Benefits of Spaying

Spayed dogs tend to live longer than their unspayed counterparts. But lifespan aside, there are a number of conditions that can be avoided by having your dog spayed.


Not all dogs are good candidates for breeding, particularly if they carry undesirable genetic traits affecting the skin, heart, respiration, immune or other systems, and even behaviour that can be passed on to their offspring or that make birthing risky (a too-narrow birth canal, for example). Meanwhile, despite pet owners’ best intentions to avoid a pregnancy, accidental pregnancies do happen and can result in large litters for which homes need to be found. Our humane society shelters are already overpopulated with dogs that have been surrendered or abandoned and need a home.

If you’re thinking of breeding your female dog, it’s important to research what’s involved so you’re prepared to ensure a safe gestation and delivery. (There’s a lot more to it than simply letting nature take its course.) It’s also best to wait until your dog has had 2 to 3 heat cycles (by then she’ll be about 2 years old). Most female dogs will go into heat when they’re 6 months to 1 year old and can get pregnant before they’re fully developed themselves, which isn’t the best thing for her or her puppies’ health.

If you don’t plan to breed or spay your dog, you’ll have to make an effort every seven months or so to keeping her isolated from intact male dogs for the duration of her heat cycle. (See “Heat Cycles”)


Pyometra is a serious bacterial infection of the uterus that can quickly become life-threatening. The body of the uterus and uterine horns become distended with bacteria and pus which can lead to a fatal rupture into the abdomen if left unattended. Pyometra is quite common in unspayed female dogs and can present at any time, though it’s seen more often in middleaged to older dogs as the uterus undergoes changes over time that predispose it to infections. In most cases, the recommended treatment for pyometra is surgical removal of both ovaries and the infected uterus – only now, that surgery is more difficult and riskier, and consequently more expensive than a regular spay in a healthy dog.

Female Dog Pyometra Graphic


Mammary gland (breast) tumours are common in middle-aged to older female dogs, and the most common tumour seen in unspayed female dogs. About 50% are malignant/cancerous (locally invasive, fast-growing, and spread to other regions of the body). Spaying at a young age significantly reduces the risk of mammary tumour development later in life. Dogs spayed before their first heat cycle have a 0.5% risk of developing mammary tumours while those spayed after one heat cycle have an 8% risk, and dogs spayed after two heats have a 26% risk. Females spayed after 2 years of age are seven times more likely to develop mammary tumours than those spayed before age 6 month of age. Spaying later in life can still reduce the risk of breast cancer, just not to the degree as spaying early on.

While ovarian and uterine tumours are uncommon in female dogs, they do occur, and their development can be prevented altogether by having a dog spayed.

Dog Breast Tumor

Heat Cycles

A heat cycle or estrus is a period of hormonal change during which a female dog is receptive to mating and can become pregnant – even during her first heat cycle before she herself is full grown, and even into her senior years.

In dogs, the first heat starts at about 6 months of age but can start earlier in toy and small-breed dogs or later in large (and later still, in giant breeds). Dogs average 2 heat cycles a year, but toy and small-breed dogs can have 3 while large and giant breeds may have only one every year or two. These cycles can take up to two years to become regular and predictable. Each heat lasts 2-3 weeks (shorter in some individuals, longer in others), and they continue throughout the life of a dog.

Canine Heat Cycle Stages

During the week or two before estrus (proestrus), a female dog undergoes physical and behavioural changes as her body prepares to become receptive to pregnancy. These changes typically include:

  • A swollen vulva.
  • Bloody vaginal discharge.
  • Persistent licking of the genital area.
  • Clingy, skittish, or agitated behavior.
  • Aggression toward male dogs. (Male dogs are attracted to females in proestrus, but females typically won’t tolerate their attentions until estrus.)

During estrus, a female dog undergoes further changes:

  • Vaginal discharge lessens in volume and becomes more pink or yellow in colour and watery in consistency. (This should not be mistaken for her cycle ending – in fact, within a few days of entering estrus, a female will be at her most fertile for several days.)
  • Urinating more often than normal and marking her environment to signal her readiness to breed with another dog. (Male dogs can pick up her scent from a good distance and will come calling, invited or not.)
  • A strong urge to mate, and as a result, potentially run off in search of a mate.
  • Mounting behaviour. (Yes, females do that too.)
  • Tail ‘flagging’ – holding her tail to the side to signal her receptiveness to mating.

Some female dogs develop cysts (fluid-filled structures) within an ovary that prolong proestrus, estrus, and a female’s attractiveness to males. Cysts also contribute to changes to the uterus that will predispose a female to pyometra.

False (Pseudo) Pregnancy

A dog that doesn’t become pregnant during estrus can still experience a false pregnancy (pseudopregnancy) a few weeks later (during late diestrus, the next stage in her cycle). Signs of pseudopregnancy may include:

  • Maternal behaviour – nesting and mothering inanimate objections (toys, pillows, etc.).
  • Mammary gland growth and mild milk production. (Her body mistakenly thinks it has puppies to feed.)
  • Licking or suckling the abdomen, a behaviour that prolongs the condition because it stimulates mammary glands to continue to produce milk.
  • Distended abdomen.
  • Nervousness, aggressiveness, increased thirst and urination, variable appetite, and diarrhea.

These signs are usually mild and will typically resolve in a few weeks, but if they don’t, treatment may be needed. Meanwhile, dogs that experience pseudopregnancies seem to have a higher risk for developing mammary tumours (breast cancer), and the more pseudopregnancies they have, the greater the risk.

Risks of Spaying

While spaying serves to prevent a number of health issues (see “Preventative Benefits of Spaying”), it can also predispose a dog to other issues depending on the breed and the age at which a dog is spayed. Certain breeds are at greater risk of developing urinary incontinence (leaking urine) if spayed very young. (The condition can be managed with medication in those individuals.) Large and giant breed dogs that have been spayed before they’re fully grown may be at greater risk of developing certain orthopedic conditions and cancers. On the whole, reasons to spay outweigh reasons not to spay. And minimizing one risk or another can be a matter of timing.

What about the surgery itself – isn’t that a risk? We don’t take any procedure that involves general anesthesia lightly. But anesthesia in veterinary medicine is on par with that in human medicine, and we take every precaution to keep our patients safe. Moreover, our dog spays are performed laparoscopically to ensure the best outcome. (Read more on “Laparoscopic Spays.”)