Foetal Sexing

Equine foetal sexing allows us to determine whether a mare is carrying a colt or a filly before the foal is born.

Our experienced stud vet can perform equine foetal sexing using transrectal ultrasound, usually between approximately 60 and 120 days of gestation. This non-invasive examination can provide valuable information for breeders and owners planning for the future of their foal or breeding programme.

We offer equine foetal sexing and pregnancy ultrasound services for mares across Suffolk and Essex.

What is equine foetal sexing?

Equine foetal sexing is an ultrasound examination used to identify the sex of an unborn foal during pregnancy.

At specific stages of gestation, the developing reproductive structures can be identified using ultrasound. Their position and appearance allow an experienced stud vet to determine whether the foetus is likely to be male or female.

The procedure is usually performed transrectally in a similar way to other routine reproductive ultrasound examinations.

When can you determine the sex of a foal?

We can usually determine the sex of an equine foetus between approximately 60 and 120 days of gestation.

The most suitable timing will depend on:

  • The mare’s stage of pregnancy

  • The size and position of the foetus

  • The accessibility of the relevant structures

  • The mare’s size and conformation

  • The quality of the ultrasound view

  • The mare’s temperament and tolerance of examination

During earlier pregnancy, foetal sexing is based on identifying the position of the genital tubercle. This structure later develops into the penis in a colt or the clitoris in a filly.

In a colt, the genital tubercle moves towards the umbilicus. In a filly, it moves towards the tail and hindlimbs.

At later examinations, the vet may be able to identify more developed reproductive structures to support the diagnosis.

What is the best time for equine foetal sexing?

The early window around 60-70 days can provide a useful opportunity to identify the position of the genital tubercle.

A later window, commonly between approximately 90 and 120 days, may allow the vet to assess more developed reproductive anatomy.

There can be a period during which the foetus moves deeper into the mare’s abdomen and becomes more difficult to examine transrectally. For this reason, timing the appointment carefully can improve the likelihood of obtaining a clear view.

We recommend contacting us in advance so that we can calculate the most appropriate scanning date from the mare’s known ovulation date.

How is equine foetal sexing performed?

Foetal sexing is normally performed using a transrectal ultrasound examination.

During the procedure, the vet carefully introduces an ultrasound probe into the mare’s rectum to obtain images of the uterus and developing foetus. The vet then assesses the location or appearance of the relevant reproductive structures.

The examination is similar to the ultrasound scans used for:

  • Reproductive cycle monitoring

  • Pregnancy diagnosis

  • Twin detection

  • Embryonic heartbeat checks

  • Routine pregnancy monitoring

The mare will need to be safely restrained in a suitable environment. Stocks are preferable where available, and sedation may be recommended for some mares.

The procedure should only be performed by a suitably qualified and experienced vet.

Does foetal sexing harm the mare or foal?

Transrectal ultrasound is routinely used in equine reproductive veterinary work and is generally well tolerated.

As with any rectal examination, there is a small risk of injury to the mare. The examination must therefore be performed carefully by an experienced vet, with appropriate restraint and sedation where required.

Ultrasound itself does not use ionising radiation and does not require surgery.

Our priority will always be the safety and welfare of the mare and her pregnancy. If the mare becomes unsettled or the examination cannot be performed safely, the procedure may need to be stopped or rearranged.

How accurate is equine foetal sexing?

Equine foetal sexing can be performed with a high degree of accuracy when:

  • The examination takes place during an appropriate stage of gestation

  • The foetus is in a suitable position

  • The relevant anatomical structures can be clearly identified

  • The procedure is performed by an experienced stud vet

However, no ultrasound diagnosis should be regarded as infallible.

The foetus may be positioned in a way that prevents the vet from obtaining a clear view. It is better to record the result as inconclusive and arrange another examination than to make a diagnosis without sufficient evidence.

In some cases, a repeat scan may be recommended to confirm the result.

Why might breeders choose foetal sexing?

There are several reasons why an owner or breeder may wish to know whether a mare is carrying a colt or a filly.

Foetal sexing may help with:

  • Planning future breeding decisions

  • Managing stud numbers

  • Preparing potential buyers or owners

  • Informing sales and commercial decisions

  • Planning the foal’s registration and future

  • Balancing the number of colts and fillies within a breeding programme

  • Satisfying an owner’s personal curiosity

The result does not provide information about the foal’s future health, temperament, conformation or performance potential. Routine pregnancy care and monitoring remain important regardless of the foal’s sex.

Can the pregnancy be checked at the same appointment?

Where appropriate, the examination may also provide an opportunity to assess aspects of the pregnancy, including:

  • Confirmation of a viable pregnancy

  • The presence of a foetal heartbeat

  • Foetal movement

  • The appearance of the uterus and pregnancy

  • Any concerns requiring further investigation

Foetal sexing is not a replacement for routine pregnancy monitoring. Your vet will recommend an appropriate scanning schedule based on the mare’s age, health, breeding history and pregnancy risk.

Equine foetal sexing in Suffolk and Essex

Starling Equine Vets provides equine stud veterinary services for horse owners and breeders across Suffolk and Essex.

Alongside foetal sexing, our reproductive services include:

  • Pre-breeding examinations

  • Mare fertility investigations

  • Reproductive cycle monitoring

  • Artificial insemination using fresh, chilled or frozen semen

  • Pregnancy diagnosis

  • Embryonic heartbeat scans

  • Twin management

  • High-risk pregnancy monitoring

  • Foaling advice

  • Newborn foal checks

  • Mare and foal veterinary care

Our service is tailored to the individual mare, pregnancy and breeding programme.