An Objective Look At The Female Spay

Updated: Nov 15


In the UK, canine castration is one of the most common surgical procedures (Bushby & White, 2019). The majority of these are elective meaning the owner has chosen to have this procedure done.

In other countries in Europe, this is less common with countries such as Norway, the procedure is actually illegal unless it is deemed medically necessary. As a result, 93% of females & 99% of males remain intact in Scandinavian countries.

Here in the UK, we have a choice and the time of writing, more than half of owners are choosing to castrate their pet. Anecdotally, whilst talking to owners, many opt for this procedure because it is "the done thing" or because the vet suggested it. Others avoid the procedure as they feel a dog should "experience being a parent" for example. There are hundreds of reasons why people choose to/choose not to castrate their pet and it can be a minefield when making these decisions.

The aim of this blog post is to outline all the scientific research that has looked into the female spay. This blog will only focus on females, with the male version already published in a separate post.

Disclaimer: I am not a veterinary professional; therefore, I can only comment on the findings of the research to hand. Whilst I have tried to use up-to-date papers, I may also reference some older papers if I feel this is appropriate. I cannot give a conclusive statement as to whether you should neuter or not or at what age to do this but I can present the current available literature on the subject. If you are interested in the research papers, a reference list is provided at the end.

NOTE: Numbers in brackets denotes the reference which can be found in the reference section at the end of the article.

For the purpose of the article, I use the terms "spay", "neuter", "intact" & " gonadectomy" interchangeably.

I have not been able to cover every consideration and topic in this area but have selected the main areas which come up in discussion and research most frequently.

A point to note before we start:

For anybody who has studied academia, you will know the pitfalls of research. Research has flaws due to methodology, samples and other variables. Throughout the article, I will attempt to pinpoint some obvious issues with particular research, but I will discuss this in general at the end.

Scientific research vs Anecdotal evidence

Here in the UK, we have a choice and at the time of writing, more than half of owners are choosing to castrate their pet. Anecdotally, whilst talking to owners, many opt for this procedure because it is "the done thing" or because the vet suggested it. Others avoid the procedure as they feel a dog should "experience being a parent" for example. There are hundreds of reasons why people choose to/choose not to castrate their pet and it can be a complete minefield when deciding what to do. any biases or problems with the way the study was carried out and then approve this for publications. These are then published in scientific journals for other researchers, medical & veterinary professionals and others who are interested in academia to read and understand the latest knowledge in the field. Scientific research is not always conclusive, there are often limitations such as sample size, variables that could not be controlled for or something else may have affected the results which hasn’t been accounted for.

Anecdotal evidence means it relies on the info from individuals rather than scientific study. For example, this may be someone’s personal experience. This is what is often seen in regards to castration on forums, website articles and Facebook pages. This will often sound like “well my dogs have never been neutered and …..” or “well my dogs have always been neutered and….”. We need to take great care with this kind of information. Because this is more about personal experience, it is not clear whether these dogs would have experienced anything different if their castration status had been different. These samples are usually only a few dogs and it is impossible to assess what other factors were involved in these dogs lives to lead to the expressed outcome. Anecdotal evidence has its place. It can often be the basis that scientific research is based on. Enough people expressing a perceived link can often prompt a researcher to look into this in a more controlled way to see if such a link exists. The problem with anecdotal evidence is there is simply no way to know which anecdotal reports are accurate and which are not. It is not a reliable way to gain information about a medical & behavioural topic because there is no control of other potential factors when the observations were made. This means we cannot be sure what the cause of the observed outcome is.


Overpopulation is always an important consideration when considering a spay surgery but something I am not going to dwell on for too long here as this is something that can be prevented with some forethought and management. Accidental litters are something that needs to be avoided to prevent overpopulation and the overflow of rescue centres but this is not an unavoidable outcome with intact animals.

Missing out on an experience

Many people worry that they are taking something away from their dog by not allowing them to be a parent. This isn’t how animals think. They have little concept of what could have been and for a dog who has never had a litter or been given an opportunity to tie with a bitch, they do not think that they are missing anything. Considering to breed has a huge financial consideration. Responsible breeding includes ensuring your dog will not pass on genetic conditions onto their offspring. Parents must have sound temperaments and a clean bill of health. Homes should be lined up and breeder contracts should include returning the dog if the buyer can no longer look after them. This allows breeders to keep track of their dogs and prevent them entering the rescue system.

I won’t touch on the many different cost factors of breeding in this blog BUT there is no denying that neutering completely eliminates the risk of any unexpected litters and therefore the financial fallout from this.

For those who do not intend to responsibly breed from their dog, the advice has always been to neuter to prevent the risk of such accidental litters. For multi-dog households which may contain an intact male & a bitch, careful management may be something that owners just don’t want to/are unable to deal with.

In this blog, I have not factored in the risks associated with mating, pregnancy, birth or anaesthesia risks of the spay itself. The blog will specifically focus on health & behaviour pros and cons after the procedure.


I am going to begin with the biggest medical considerations to spay/neuter.


Mammary Neoplasia

Mammary Neoplasia is the most common forms of cancer seen in female dogs(1) with an annual incident rate of 16.8%(2). It has been found to affect intact females significantly more than spayed females(3) and is therefore a significant consideration when questioning whether (or when) to spay your bitch.

In a more recent studies, 72%-74% of cases of Mammary Neoplasia occurred in intact dogs and had normal oestrous cycles(4,5) with approximately 50-75% of these cases being malignant(2,5) .This figure did vary from study to study, but in each case Malignant tumours made up at least 48% of all diagnosed cases.

It was found that dogs spayed before the first heat, had a 0.5% risk of developing Mammary Neoplasia. This figure rose to 8% if spayed between the first and second heat and even higher to 26% if spayed after the second(6 ).

Age also played a significant factor in Mammary Neoplasia with frequency much lower for dogs under 6 years of age (2,5,7) with an increased risk for dogs over the age of 8(2,8). One researcher found that 60% of intact dogs over the page of 8 would be affected (4,5) . It was found however that neutering would have a beneficial effect on the risk for dogs up to the age of 9 years old(6). This means that if you chose to leave your dog intact until aged 9, they would still potentially benefit from the procedure in regard to neoplasia risk.

Why would spaying have a link to Mammary Neoplasia?

The reason for this likley lies in the role of oestrogen receptors in these kinds of tumours. 80% of mammary tumours have oestrogen receptors, progesterone receptors, or both(9).

In a study looking at both spayed and intact females, it was found that the oestrogen in intact females worked as a carcinogen for these tumours carrying oestrogen receptors. What is interesting is that for the tumours that didn’t carry oestrogen receptors, the oestrogen appeared to act as a protective factor. This may mean that whilst remaining entire may increase the risk of tumours possessing oestrogen receptors, it may add a protective factor for other forms of cancer.

Purebred dogs were significantly more at risk than crossbreeds across several studies with figures suggesting 80% of affected dogs were purebred. It should be noted here that it is possible more purebred dogs are simply being kept intact due to breeding or showing and this is leading to a skew in the results. In regard to specific breeds, each study had a list of breeds which had recorded greater incidents but these differences between studies. German Shepherds, Dobermanns, Staffordshire Bull Terriers & Rottweilers did appear across multiple studies (2,5,7,10.)

One final point to note here is that whilst the majority of studies did show an increased risk of mammary neoplasia i