Spaying does not stop a dog from growing. In fact, it does the opposite: removing a dog’s ovaries before she finishes growing can cause her bones to keep growing for longer than they otherwise would, often resulting in a slightly taller or longer-legged dog. This happens because the sex hormones that normally signal bones to stop lengthening are no longer present.
How Spaying Affects Bone Growth
A dog’s long bones (the legs, primarily) grow from soft areas near each end called growth plates. These plates contain cartilage cells that divide and multiply, gradually adding length to the bone. Over time, those cells lose their ability to keep dividing, and the growth plate hardens into solid bone. Once that happens, the bone can no longer get longer. This process is called growth plate closure.
Estrogen plays a key role in this process. It doesn’t directly fuse the growth plates shut. Instead, it speeds up the natural aging of those cartilage cells, wearing out their ability to divide sooner. When their capacity to multiply is exhausted, the plate closes. Research published in the Proceedings of the National Academy of Sciences found that estrogen essentially accelerates a built-in countdown clock in growth plate cells, causing them to burn through their divisions faster and fuse earlier.
When you spay a dog, you remove the ovaries, which are the primary source of estrogen. Without that hormonal signal pushing the growth plates toward closure, the cartilage cells keep dividing for longer. The result is that the bones continue lengthening past the point where they would have stopped in an intact dog. This typically makes the dog slightly taller or gives her proportionally longer legs, though the difference is usually modest.
When Dogs Normally Finish Growing
How long a dog keeps growing depends heavily on her size. According to the American Kennel Club, small breeds (under 20 pounds) are roughly 75% of their adult size by six months and fully grown by about 8 months. Medium breeds (21 to 50 pounds) reach adult size around 12 months. Large breeds (51 to 100 pounds) keep growing until about 15 to 18 months, and giant breeds over 100 pounds may not be fully grown until 18 to 24 months.
This timeline matters because a dog spayed at four months still has a lot of growing left, especially if she’s a large or giant breed. The earlier the spay relative to the dog’s natural growth plate closure, the more additional growth can occur. A small breed spayed at six months, when she’s already 75% grown, will see less impact than a giant breed spayed at the same age, when she’s only about 50% of her final size.
Joint and Ligament Risks From Extra Growth
The concern with this extended bone growth isn’t just cosmetic. When bones grow longer than the body was designed for, it can change joint angles and put stress on ligaments. In some breeds, particularly German Shepherds and Golden Retrievers, early spaying has been linked to higher rates of hip dysplasia and other joint problems.
One of the most studied consequences is damage to the cranial cruciate ligament, the dog equivalent of the ACL in humans. A 2025 systematic review and meta-analysis found that female dogs spayed at one year or younger had roughly 2.4 times the odds of developing a cruciate ligament rupture compared to intact females. Male dogs neutered at the same age had about 2.1 times the odds. That’s a meaningful increase in risk, though it doesn’t mean every early-spayed dog will have ligament problems.
The mechanism makes intuitive sense: longer leg bones create slightly different leverage on the knee joint, and the ligaments and muscles may not be proportioned to handle the altered geometry. The effect varies by breed, which is why blanket recommendations about spay timing have fallen out of favor.
Weight Gain Versus Actual Growth
Many owners notice their dog getting bigger after spaying and assume she’s still growing. Often, what’s actually happening is weight gain, not skeletal growth. Spaying removes the hormones that help suppress appetite and maintain resting metabolic rate. Without estrogen, dogs tend to eat more and burn fewer calories at rest, creating a perfect setup for fat accumulation.
A study published in the Journal of Animal Science tracked body composition in dogs after spay surgery using full-body scans. By 24 weeks post-surgery, spayed dogs fed a standard diet had significantly more body fat than intact dogs. Bone mineral content and lean mass also changed over time, but the most dramatic shift was in fat percentage. So if your dog looks bigger a few months after spaying, the most likely explanation is added fat rather than continued bone growth, especially if she was already close to her full adult size at the time of surgery.
Bone Density After Spaying
Beyond growth plates, estrogen also helps maintain bone density throughout a dog’s life. Long-term studies in beagles have found an 8% to 10% reduction in spinal bone density after ovary removal. This is similar to what happens in postmenopausal women, where declining estrogen leads to weaker bones over time. The clinical significance of this in dogs is still debated, as some studies have found no measurable change in bone mineral density after spaying. For most pet dogs, this modest reduction in density is unlikely to cause noticeable problems, but it’s part of the broader picture of how removing ovarian hormones affects the skeleton.
Breed-Specific Spay Timing Guidelines
Researchers at UC Davis have spent over a decade studying how spay and neuter timing affects joint disease and cancer risk across different breeds. Their most recent guidelines, covering 40 popular breeds, emphasize that there is no single right age to spay every dog. The risks vary dramatically by breed and sex.
For example, female Newfoundlands spayed early showed heightened risks for joint disorders, while Siberian Huskies showed no significant effects from early spaying on joints or cancer. Pointer breeds of both sexes had elevated joint disorders and increased cancer rates when spayed or neutered early. The takeaway from this research is that the decision should be tailored to your specific dog’s breed, size, and sex rather than following a one-size-fits-all rule.
For large and giant breeds, many veterinarians now recommend waiting until the dog is at least 12 to 18 months old, allowing most or all growth plate closure to occur naturally before removing the hormonal signals. For small breeds that finish growing much earlier, the window of vulnerability is shorter, and early spaying carries less orthopedic risk. If you’re unsure about timing for your dog’s breed, the UC Davis guidelines offer breed-specific recommendations that are freely available online.