# Spaying or neutering your dog: when to do it and why the right age depends on size

https://casatrufa.com/en/blog/spaying-or-neutering-your-dog-when-and-why

By Casa Trufa · Published: 2026-10-09 · For information only: it does not replace a visit to your veterinarian

There is no single age. The AAHA guidelines suggest spaying or neutering dogs that will weigh under about 20 kg (45 lb) as adults at around six months, and waiting with larger dogs until they finish growing, between 9 and 15 months. Some breed studies advise waiting longer. Decide with your veterinarian.

## In short

- Spaying or neutering prevents unwanted litters and uterine infection; done early, it reduces mammary tumors.
- In large breeds, doing it before the dog has finished growing is associated with more joint problems.
- AAHA: under 20 kg (45 lb) as an adult, at around 6 months; 20 kg (45 lb) or more, once growth is complete.
- The UC Davis study of 35 breeds gives different guidelines for the Bernese, the Border Collie and the Heeler.
- Law 2374 of 2024 created a free national program for certain groups; it does not require every dog with a home to be sterilized.

## What does spaying or neutering a dog mean?

Spaying or neutering is surgery that removes the ability to reproduce. Colombia’s Law 2374 of 2024 describes it by its technical names: orchiectomy in males, which is the removal of the testicles, and ovariohysterectomy in females, which is the removal of the ovaries and uterus. It is done under general anesthesia and may only be performed by a veterinarian with a current professional license.

Removing the ovaries or testicles also removes the sex hormones they produce. That explains almost everything that follows: both the advantages and the drawbacks of the surgery have to do with the role of those hormones, which is why the moment they are removed matters.

## What are the benefits of spaying or neutering?

The first is obvious: there will be no unwanted litters. In females it also eliminates the risk of pyometra, a uterine infection that occurs in unspayed dogs, and reduces the risk of mammary tumors when done early. The AAHA guidelines cite that benefit as the main reason not to delay it in females.

Pyometra is not a rarity. The [Merck Veterinary Manual](https://www.merckvetmanual.com/dog-owners/reproductive-disorders-of-dogs/reproductive-disorders-of-female-dogs) describes it as a bacterial infection linked to the hormonal changes of the heat cycle, mainly in females over five years old, and its usual treatment is precisely surgery, by then on a sick dog. In the UC Davis study we cite below, between 4% and 6% of intact females of several breeds had had it.

There is a collective benefit too. In a country with many dogs living on the street, every litter that is not born without a family waiting for it counts, and that is the reason behind the Colombian law.

## What are the risks?

The known risks depend mainly on age and size. According to AAHA, spaying or neutering a large dog before it has finished growing is associated with more orthopedic disease, with some types of cancer and, in females, with urinary incontinence. On top of that comes the risk that goes with any surgery under anesthesia.

The explanation proposed by the UC Davis researchers is that sex hormones play a part in closing the growth plates of the bones. If they are removed long before that closure, the long bones grow slightly more than normal and joint alignment can be altered. In their data, small breeds did not show that increase in joint problems; several of the large ones did.

It is the same reason why [hip and elbow dysplasia](https://casatrufa.com/en/blog/hip-and-elbow-dysplasia-in-dogs) is watched so closely in large breeds.

## What is the right age to spay or neuter by size?

The [AAHA life stage guidelines](https://www.aaha.org/resources/life-stage-canine-2019/spay-and-neuter-timing/) divide dogs by the weight they will reach as adults, with the line at 45 pounds, about 20 kg. Below it, they suggest spaying or neutering at around six months. Above it, waiting until the dog has finished growing.

| Expected adult weight | Males | Females |
| --- | --- | --- |
| Under 20 kg (45 lb) | At 6 months | Before the first heat, at 5 or 6 months |
| 20 kg (45 lb) or more | Once growth is complete, between 9 and 15 months | Between 5 and 15 months, at the veterinarian’s discretion |

The very wide range for large females is not an oversight. AAHA asks the veterinarian to weigh two things that pull in opposite directions: doing it before the first heat gives better protection against mammary tumors; doing it after growth is complete gives better protection for the joints.

## What do the breed-specific studies say?

That size does not explain everything. A team at the University of California, Davis [reviewed clinical records for 35 breeds, covering fifteen years for most of them,](https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00388/full) and found that most breeds showed no more joint problems and no more cancer according to the age of neutering, but some did, and differently in males and females.

This is what it suggests for five of the breeds on this site, the ones that were in the study:

| Breed | Guideline suggested by the study |
| --- | --- |
| Bernese Mountain Dog | Males: wait well beyond 2 years. Females: no clear increase in risk |
| Border Collie | Both sexes: after one year |
| Australian Cattle Dog (Blue Heeler) | Females: at 6 months or later. Males: no increase in risk |
| Cavalier King Charles Spaniel | No increase in risk at any age |
| Maltese | No increase in risk at any age |

The [Maltipoo](https://casatrufa.com/en/breeds/maltipoo) and the Cockapoo are crosses and do not appear as such. A [second paper by the same team](https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00472/full) on mixed-breed dogs serves as a reference: below 20 kg (45 lb) of adult weight it found no more joint disorders with early neutering.

It should be read with its limits in mind. These are data from a single university hospital, with only a few dozen dogs in some groups, and they describe association, not cause. The authors themselves present them as an aid to deciding, not as a rule.

> [!CONSEJO] Take your dog’s expected adult weight to the appointment — you can estimate it from the parents — and ask about its specific breed. If it is a [Bernese Mountain Dog](https://casatrufa.com/en/breeds/bernese-mountain-dog), the conversation is different from the one about a Maltese.

## What does the law say in Colombia?

[Law 2374 of 2024](http://www.secretariasenado.gov.co/senado/basedoc/ley_2374_2024.html) created the National Program for the Surgical Sterilization of Cats and Dogs (Programa Nacional de Esterilización Quirúrgica de Gatos y Perros) as an ethical method of birth control. It does not require every dog with a home to be sterilized: it organizes and funds a public service, and defines for whom it is free and for whom it will be mandatory.

The points that matter to a family:

- **Free of charge** (Article 5): for animals in the special lines and for those belonging to people in groups A, B and C of Sisbén IV.
- **Low cost** (Article 6): municipalities may offer it on those terms to people in group D.
- **Mandatory** (Article 8): from 2027, only for the animals in the special lines of Article 4, among them homeless dogs, dogs sheltered by foundations and the “special handling” dogs of Article 126 of the Police Code.
- **Identification** (Article 4, item 4.5 and paragraph 1): every veterinarian who sterilizes an animal must identify and register it; for dogs with a home, with a tattoo inside the right ear.

The Ministry of Environment reported in 2026 that [Resolution 0229 of 2026](https://www.minambiente.gov.co/gobierno-petro-reglamenta-programa-nacional-de-esterilizacion-quirurgica-de-gatos-y-perros/) regulates the program. We read the press release, not the text of the resolution. The sterilization days are organized by each mayor’s office: ask at yours.

There is also a bill, Senate Bill 002 of 2025 (Proyecto de Ley 002 de 2025), on the breeding and selling of companion animals. In its report for first debate it proposed that animals be handed over already sterilized. It is a bill, not a law, and we could not confirm what stage it is at. If it were passed in that form, it would clash with what the evidence advises for large breeds. What applies today is in our guide on [whether selling dogs is legal in Colombia](https://casatrufa.com/en/guides/is-selling-dogs-legal-in-colombia).

## How do you decide with your veterinarian?

With a specific conversation about your dog, not with a general rule. The right age comes from weighing its breed, its sex, its expected adult weight and the life it leads at home. Consult your veterinarian before setting a date: they are the one who can examine the dog and who is responsible for the surgery.

Questions that help at that appointment:

1. Has the dog finished growing, or should we wait?
2. Which risks weigh more in this breed: joint risks, or mammary and uterine ones?
3. If we wait, how do we manage heat cycles or living with other dogs?
4. Which tests are done before anesthesia, and how is pain controlled afterwards?

> [!AVISO] Waiting does not mean leaving things to chance. An intact dog needs you to prevent unplanned matings: walks on a leash, fences in good repair and special attention to a female in heat.

## Frequently asked questions

**At what age is a small dog spayed or neutered?** According to the AAHA guidelines, males at around 6 months and females before their first heat, between 5 and 6 months. This applies to dogs that will weigh under about 20 kg (45 lb) as adults.

**At what age is a Bernese Mountain Dog spayed or neutered?** Late. The UC Davis study found more joint disorders in males neutered before 2 years of age and suggests waiting well beyond that age. In females it found no clear increase, so the age is decided with the veterinarian.

**Is it mandatory to spay or neuter my dog in Colombia?** For a dog with a home, in general no. Law 2374 of 2024 makes sterilization mandatory from 2027 only for the animals in its special lines, such as homeless dogs or dogs classed as “special handling”.

**Is spaying or neutering free in Colombia?** The national program is free for the law’s special lines and for animals belonging to people in groups A, B and C of Sisbén IV. Municipalities may offer it at low cost for group D.

**Is it better for a female dog to have a litter before she is spayed?** None of the guidelines we consulted recommends it. What they point out is the opposite: the benefit against mammary tumors is greater when the dog is spayed before her first heat.

## Sources

1. [AAHA: Canine Life Stage Guidelines 2019, spay and neuter timing](https://www.aaha.org/resources/life-stage-canine-2019/spay-and-neuter-timing/). Table by expected adult weight (under or over 45 pounds) and balance of benefits and risks
2. [Hart, Hart, Thigpen and Willits (2020): neutering age in 35 breeds, Frontiers in Veterinary Science](https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00388/full). Data from a single university hospital (UC Davis); suggested guidelines by breed
3. [Hart, Hart, Thigpen and Willits (2020): mixed-breed dogs by weight category, Frontiers in Veterinary Science](https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00472/full). No increase in joint disorders under 20 kg
4. [Merck Veterinary Manual: reproductive disorders of female dogs](https://www.merckvetmanual.com/dog-owners/reproductive-disorders-of-dogs/reproductive-disorders-of-female-dogs). Pyometra in unspayed females
5. [WSAVA: guidelines for the control of reproduction in dogs and cats](https://wsava.org/global-guidelines/reproduction-guidelines/). Landing page for the guidelines; we took no figures from it
6. [Ley 2374 de 2024, Programa Nacional de Esterilización Quirúrgica de Gatos y Perros (Law 2374 of 2024, national surgical sterilization program for cats and dogs)](http://www.secretariasenado.gov.co/senado/basedoc/ley_2374_2024.html). In Spanish. Articles 4, 5, 6, 8 and 9
7. [Ministerio de Ambiente (Colombia’s environment ministry): regulation of the national sterilization program (Resolución 0229 de 2026)](https://www.minambiente.gov.co/gobierno-petro-reglamenta-programa-nacional-de-esterilizacion-quirurgica-de-gatos-y-perros/). In Spanish. Ministry press release; we did not read the text of the resolution
8. [Senado de la República (Colombian Senate): report for first debate on Proyecto de Ley 002 de 2025](https://www.senado.gov.co/index.php/documentos/comisiones/constitucionales/comision-quinta/proyectos-de-ley/proyectos-de-ley-y-ponencias-periodo-constitucional-2022-2026/proyectos-de-ley-y-ponencias-legislatura-20-julio-2025-20-julio-2026/p-de-l-no-002-2025-senado-reproduccion-cria-y-comercializacion-de-animales-de-compania/8995-ponencia-1er-debate-p-de-l-no-002-2025-senado-reproduccion-cria-y-comercializacion-de-animales-de-compania/file). In Spanish. It is a bill, not a law; we could not confirm its current status