‘Fun’ ultrasound in horses

De beste periode om een rectale echo, vergelijkbaar met het scannen voor ovulatie, te maken is als het ongeboren veulen ongeveer 70 dagen oud is.

March 14, 2021

One day a curious breeder took his Friesian mare to veterinarian Matthijs van Waart’s equine clinic De Drie Hoeves in Willemsoord, Overijssel. The mare was in foal and the breeder was keen to find out whether his mare was carrying a filly- or colt foal. Van Waart states: ‘In horses it’s technically possible to determine the sex of the unborn foal. The owner of the Friesian mare had a prospective buyer in America for his pregnant mare. The potential new owner wanted to know if the mare he might purchase would give him a filly or a colt, indicating that he preferred a filly.’

Ultrasound examination

The sex of the unborn foal can be determined by means of a genetic examination or by performing an ultrasound. When the foal was flushed out of the mare in an early stage then genetic examination makes it possible to determine if it is a male or female embryo. In the case of the pregnant Friesian mare Matthijs established the sex of the foal by ultrasound. According to Van Waart this can be done at different stages of gestation, if the quality of the equipment is sufficient.

70 days

He explains: ‘The best period to carry out a rectal ultrasound, comparable to scanning for ovulation, is when the unborn foal is about 70 days old. When the foal is in a favourable position, meaning that the umbilicus and hind legs can be seen clearly, then it is possible to establish the foal’s sex. The reliability of sex determination on the basis of ultrasound scanning depends on the quality of the ultrasound equipment and the expertise and experience of the veterinarian who carries out the ultrasound.’


Matthijs van Waart told the owner of the Friesian mare that the reliability of ultrasound examination fluctuates between 75 – 95%: ‘He could not give the overseas buyer 100% certainty. This more or les implies that this type of ultrasound should be considered something of a ‘fun’ ultrasound. The ultrasound I carried out on the Friesian mare gave me the impression that she was carrying a filly foal. The happy owner was elated and offered to treat me to cake if my prediction would prove to be right. The mare travelled overseas to America and months later the owner popped by for a cup of coffee. And he had brought a delicious cake with him.’

Read the full real-life story by veterinarian Matthijs van Waart in the April issue of Phryso

The above diagram shows a distinct difference between a filly foal and a colt foal. When highlighting the tail, upper legs and umbilicus, the genital tubercle, the early developmental stage of clitoris and penis, also shows up. In a filly foal the tubercle is situated between the tail and the upper legs and in a colt foal this structure can be seen between the upper legs and the umbilicus.

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