“Don’t neuter him early or he’ll stay small” is one of the most widely repeated pieces of dog advice, and it is backwards.
The confusion is understandable, because two different studies answer two different questions and get results that sound contradictory until you notice they are not measuring the same thing.
Two questions, two answers
| Question | What the evidence shows | Source |
|---|---|---|
| Does neutering change adult weight? | Barely. Shifts under one centile. | Salt et al. 2017 |
| Does neutering change adult height? | Yes — early neutering produces longer limbs. | Salmeri et al. 1991 |
Almost every article online cites one of these and generalises it to “size.” That is where the myth comes from.
Why early neutering makes dogs slightly taller
Long bones lengthen at the growth plates — bands of cartilage near each end, called physes. When those plates ossify, the bone can no longer get longer and adult height is fixed.
Growth plate closure is triggered in part by sex hormones. Remove the source of those hormones before the plates have closed, and closure is delayed. The bones carry on lengthening for a while longer than they otherwise would.
As Today’s Veterinary Practice summarises the orthopedic literature: the timing of long bone physeal closure “is controlled, in part, by gonadal hormones, particularly estrogen, in both sexes,” and gonadectomy before physeal closure “is associated with significantly greater final length of associated long bones.”
The foundational work is Salmeri et al. (1991), which measured skeletal development in dogs gonadectomised at different ages and found delayed physeal closure with correspondingly greater bone length.
So the direction is the opposite of the folklore. Early neutering does not produce a smaller dog. If anything it produces a slightly taller, longer-limbed one.
The effect is modest — this is a matter of a small difference in leg length, not a different-sized dog.
Why weight barely moves
Salt et al. (2017) built veterinary growth standards from over six million weight records and looked specifically at whether neutering warranted separate charts.
It did not. Neutering produced slight shifts in growth trajectory — upward if performed before 37 weeks, downward after — but the shifts were under one centile and never large enough to cross neighbouring centile lines. The resulting WALTHAM charts are explicitly validated for use in both neutered and intact dogs.
A slightly longer-limbed frame carrying much the same weight is a coherent result, not a contradiction.
The timing question that actually matters
If size is not a good reason to time the surgery, what is?
Growth plate status. In an average-sized dog the last major plates — the distal radius and ulna, and the proximal humerus — close around 10 to 13 months. In very large and giant breeds they can remain open to 18–20 months. Neutering before or after that window is the fork in the road, and it matters mainly for joint development rather than height.
Breed and sex specific risk. Research over the past decade associates early neutering in some large breeds with higher rates of cruciate ligament rupture and hip dysplasia — but the effect varies substantially between breeds, and between males and females of the same breed. There is no single correct age.
Everything else. Mammary tumour risk, pyometra, prostate disease, roaming and behaviour all enter the decision, and they do not point the same direction.
This is genuinely a conversation for your vet, who knows your dog’s breed, sex, size and household. What it is not is a decision about how big your dog will be.
Where the size myth probably came from
Myths this durable usually have an observation behind them, and this one has three.
Neutered dogs do often look different. Body composition shifts after neutering — a somewhat softer outline, less muscle definition. A dog that reads as “less impressive” gets remembered as “smaller,” even at the same frame size.
The comparison group is wrong. Working and show dogs are frequently kept intact, and they are also selected for size and conditioned hard. Comparing the neighbourhood’s neutered pet Labrador to a field-trial Labrador is not a neutering comparison.
Very early spay/neuter had a genuine historical association with poor outcomes in some shelter contexts, where puppies were also underweight, parasitised or malnourished. Attributing the resulting small adults to the surgery rather than the circumstances is an easy mistake.
None of that survives a controlled measurement. When you actually radiograph the bones, early-neutered dogs come out slightly longer-limbed.
What to actually ask your vet
Since size is not the useful question, here are the ones that are:
- “When do you expect this dog’s growth plates to close?” This anchors the timing conversation to something physical rather than an arbitrary age.
- “For this breed and sex, what does the joint disease evidence look like?” The research is genuinely breed- and sex-specific — a recommendation for a female Golden Retriever does not transfer to a male Rottweiler or either sex of a small terrier.
- “What are the risks of waiting, for this dog?” Delaying is not a free action. It carries its own considerations — unwanted litters, mammary tumour risk in females, roaming and marking behaviours — and they weigh differently in different households.
- “How should I change feeding afterwards?” This is the change that will visibly affect your dog, and it is the one owners are least often given a number for.
A vet who knows your dog can weigh those against each other. An article cannot, including this one.
The weight gain everyone actually notices
There is a real effect people are picking up on, and it gets misattributed.
Neutering lowers a dog’s energy requirement. Feed the same portions after surgery as before and the dog gains weight — not more frame, more fat. That is a feeding adjustment, and a straightforward one.
Adjust portions in the weeks after surgery and monitor body condition rather than the scale alone: you should be able to feel the ribs under a light press and see a waist from above.
What this means for predicting adult size
Our puppy size calculator applies no adjustment for neuter status, and that is a deliberate decision rather than an omission.
The weight effect measured in the largest available study is smaller than the honest error bars on any size prediction. Adding a numeric correction for it would imply a precision the evidence does not support. We would rather leave it out and say so than tune the model on an effect that cannot be seen at this resolution.
The methodology page lists the other things we deliberately do not model, and why.