Stuart Fitzgerald
Dr Stuart Fitzgerald (MVB MANZCVS, University College Dublin)
Spaying Female Dog

Neutering of bitches, commonly referred to as spaying, is recommended by veterinarians to most pet owners as a matter of course. In the UK, this is accomplished by the surgical removal of the bitch’s ovaries and uterus – a procedure usually performed between six months and one year of age. This removes the source of the female sex hormones, and so eliminates the hormone-driven oestrus cycle, meaning the bitch will no longer come “into heat”.

This offers many advantages to owners in terms of managing their pet, and also drastically reduces some significant health risks in females as they age. However, recent studies in the US, where earlier neutering is regularly performed, indicate that, for some breeds at least, spaying should be delayed, and other risk factors must be carefully weighed up before performing the procedure.

Overview of Reproductive Anatomy in the Female

The vulva, which constitutes the external portion of the reproductive tract, leads into the vagina, which runs through the pelvic canal. Internally, it connects, through the fibrous cervix, to the body of the uterus. The uterus, or womb, is the site where foetuses potentially implant and are nurtured during pregnancy.

Moving further forward, the cervix divides into two branches, what are known as the horns of the uterus. At the end of each horn lies an ovary, within which are contained thousands of eggs (ova) available for maturation and release while the bitch is in heat. Mature eggs travel from the ovary in the very thin Fallopian tube to enter the nearby uterine horn.

The Oestrus Cycle

As well as producing eggs, the ovaries produce large amounts of female sex hormones in a cyclical manner. Most bitches will go through two such cycles each year, although some will only do so once. When in heat, increased blood flow and surging oestrogen levels cause the vulva to swell. Internally, the cells lining the vagina undergo rapid turnover, leading to a small amount of blood loss, which will often be noticed by the owner. This is sometimes wrongly interpreted to mean that the bitch is no longer in the fertile part of her cycle; in fact, bleeding occurs up to two weeks before she is ready for mating. This phase of the cycle is termed proestrus.

Around the time of peak oestrogen levels, any vaginal bleeding will reduce or cease, and the oestrus phase begins, during which time eggs are released from the ovaries, and the bitch will be receptive to mating. Should mating occur, pregnancy lasts, on average, nine weeks, during which time the effects of progesterone, another hormone, predominate. However, progesterone levels also rise markedly in bitches that have not been mated, which can result in external signs usually associated with pregnancy, such as mammary gland development. This is sometimes termed a “false” pregnancy, or pseudopregnancy. Many dogs experiencing a pseudopregnancy will even produce milk, a phenomenon originating in wolves, where non-pregnant females will act as wet nurses for their sisters’ pups.

Benefits of Spaying

For many people, the prime reason for spaying is population control, although there are several others, which I will outline below. Preventing unwanted breeding can be extremely difficult without neutering, as an in-heat bitch will resist all efforts at containment. Those owners wanting to avoid neutering must have a very secure garden, both from the point of view of preventing her from escaping, as well as to keep potential suitors at bay.

During oestrus, pheromones produced by the bitch will travel over a large distance, attracting male dogs from the surrounding area. For females being kept indoors, the owner should always ensure there are at least two closed doors between the bitch and the outdoors, as many a person has been surprised by the speed at which a dog can escape when an open front door presents itself, however briefly.

Medical Problems Prevented by Spaying

Mammary Tumour

Mammary Tumour (L. Mahin /

Mammary Tumours

Mammary (breast) tumours are very common in older unneutered females. Because of cycles of enlargement, and sometimes milk production, the mammary tissue has a tendency to become “lumpy” with age. Many of these lumps will be composed of fibrous tissue or cystic cavities filled with milk-like fluid, but some represent abnormal tissue growths. Benign tumours usually remain relatively small, though may exhibit growth spurts during oestrus cycles. Malignant tumours, on the other hand, will continue to grow to the point where their blood supply becomes inadequate, and tend to spread through the blood and lymphatic systems to set up secondary tumour sites. These secondary sites are most commonly the lymph nodes and lungs.

Distinguishing benign from malignant mammary tumours is not feasible without taking tissue biopsies, and so a sensible approach to these lumps is to remove them in their entirety when first noticed, and to submit the whole mass as the biopsy sample. As a rule of thumb, tumours less than 3 cm in diameter at the time of removal are either benign, or are malignancies at too early a stage to have spread. Therefore, the prognosis following such surgeries is good. Dogs with larger tumours require very thorough assessment for signs of tumour spread, including lymph node biopsy and chest x-rays at a minimum.

Spaying drastically reduces the incidence of mammary tumours – by a factor of thousands in bitches neutered at six months of age. Although this benefit still exists for those neutered later in life, the likelihood of developing a tumour increases with every oestrus cycle.

Ovarian Tumours

Tumours of the ovaries are far less common, but are seen occasionally. These can also be either benign or malignant, and may cause vague signs of abdominal discomfort or weight loss. Because of their internal location, some can reach a large size before discovery.


Another effect of the bitch’s hormone cycle is to induce gradual thickening of the wall of the uterus over her lifetime. Cysts also tend to form within the uterine wall, leading to a condition known as cystic endometrial hyperplasia. These alterations in the organ render it susceptible to infection, especially during oestrus, when the cervix relaxes and does not present as much of an obstacle to bacteria ascending from the outer genitalia. The womb provides an ideal environment for these bacteria to proliferate, and colossal volumes of infected fluid and pus (up to several litres) can be produced. This infection is known as a pyometra, and may be open or closed, depending on whether the pus can drain externally through the cervix.

Bitches with an open pyometra will be seen to lose foul-smelling fluid from the vulva, and may vomit, be inappetent, and drink markedly more than usual. Although no fluid discharge is seen in those with a closed pyometra, the other symptoms are similar, though often more dramatic. Treatment involves fluid therapy to aid kidney function, intravenous antibiotic therapy, and surgery to remove the infected womb. Anaesthesia and surgery are risky in bitches with pyometra, and around 10% of patients may not survive. An alternative approach is to use a hormone treatment to reduce the volume of pus and improve the dog’s condition before attempting surgery, but this is not always an option, and the alternatives must be weighed up by the attending veterinarian.

Vaginal Hyperplasia

In some bitches, the vaginal swelling that occurs during oestrus is highly exaggerated, leading to protrusion of vaginal tissue through the vulva. This vaginal tissue is red in colour and quite fragile, and may be as large as grapefruit. Trauma and self-mutilation of the tissue are common, which can lead to infection and blood loss. Although there are no effective treatments for this condition, the swelling abates after around two weeks with changes in hormone levels. Any bitch having exhibited signs of vaginal hyperplasia should be spayed before her next heat, as the swelling is likely to be more pronounced on subsequent occasions.


Though not strictly a medical problem, as outlined above, pseudopregnancy is not generally helpful to a bitch in the modern domestic setting. Behavioural changes, lethargy, and loss of appetite are common features of a false pregnancy, as is milk production. Changes in behaviour can be disruptive to the balance within the household, particularly if other dogs are present, when nervousness and even aggression can manifest as the bitch tries to defend her nesting area or adopted toys that act as child surrogates. Though pseudopregnancy can be treated, either with hormone medications or simply by withholding food for 24 hours, this can be a surprisingly traumatic experience, and is a common reason for owners to elect for neutering in adult females.

Common Problems after Spaying

As for male dogs that have been castrated, neutered females have a markedly lower metabolic rate than those that are not neutered. This has led to a perception that spaying causes obesity. However, it is not the neutering, but rather the inappropriate feeding that follows it, that causes weight gain, and a range of foods are available that are less calorie dense and more satiating, which have been specially developed to help owners manage their neutered pets weight. Veterinarians and veterinary nurses are ideally placed to advise owners on nutrition around the time of surgery, and it is a very good idea to schedule two or three weight checks at the veterinary practice in the weeks following surgery to pick up on any slight gain before it becomes a major issue.

The other common problem that can be linked to spaying is an increased risk of urinary incontinence. Around 5% of unneutered bitches will develop incontinence, a loss of bladder control, at some point in their lives. For neutered bitches, this risk is approximately doubled, though it may rise toward 20% in large breed dogs spayed before one year of age. For this reason, it is generally advised to allow these larger bitches to have at least one cycle before spaying, as the risk of incontinence then decreases.

When to Neuter

While vets have been quite comfortable recommending early neutering for decades, the subject of timing has come to the fore in recent years, with the emergence of some interesting research findings. Although early neutering of pups from three months of age has long been practised in the United States, six months of age is generally considered the minimum in the UK and Ireland. This age appears to be appropriate for most small- and medium-sized dogs, while larger breeds should ideally be left until around twelve months old, or even older in certain breeds.

Very recent work by researchers in the US has identified associations between spaying, particularly early spaying, and a range of health conditions in certain breeds. In Golden Retrievers and Rottweilers in particular, it seems that removing the influence of sex hormones can increase the risk of joint disease, including osteoarthritis and cruciate ligament rupture, and of certain cancers. While the influence of increased obesity rates in neutered bitches may be a confounding factor in these studies, these are troubling findings. As for any surgical procedure, veterinarians must take time to discuss with their clients the pros and cons of spaying, and consideration of all factors, including breed, is vital in deciding upon an optimal time for neutering.

Surgical Technique

Stitches after surgery

Stitches After Spaying Surgery

Spaying is an invasive procedure, requiring the surgeon to enter the dog’s abdominal cavity. Under general anaesthesia, the dog is laid on her back, and a large area of hair is clipped from her underside. The exposed skin is then disinfected, and the abdomen covered with a sterile drape to prevent contamination from surrounding hair.

The surgeon makes an incision just behind the umbilicus (the belly button), which extends through the skin, the subcutaneous fat, and the abdominal muscles to expose the abdominal cavity. The critical part of the procedure involves securely ligating (tying off) the large blood vessels that supply the ovaries and uterus. This can be challenging in large or obese dogs, and failure to achieved secure ligation can result in rapid, massive blood loss, though this is thankfully a rare complication, and one that can be recognised and remedied during the procedure.

The cervix is similarly ligated, before the surgeon excises the uterus and ovaries and then closes the abdomen using at least three layers of sutures to prevent wound breakdown occurring. Pain relief is administered to all dogs undergoing surgery; however, spaying is a major surgery, and some degree of discomfort is to be expected should the dog exert herself too much in the days following. Bitches are normally discharged later on the day of surgery, and owners will be advised to keep them rested until the time of suture removal around 10 days later.

Alternatives to Spaying

Hormone therapies are available that can be used to postpone or prevent oestrus cycling; however, these are associated with a range of negative effects. Behavioural changes, mammary tumours, and an increased risk of pyometra are all common in bitches receiving these treatments long term, and for this reason, most vets would advise against their use. Apart from medical concerns, these drugs cause pain on administration, and over the course of the dog’s lifetime become significantly more expensive than neutering.