Discussion

Introduction:
Examinations to identify the gender of the equine pregnancy have been shown to be highly accurate when performed in the early months of gestation. The technique has achieved acceptance and popularity particularly within the Thoroughbred breeding world. Some breeders choose to examine all mares' pregnancies routinely whereas others are more selective.
There are a variety of advantages in knowing the gender of the unborn foal. For some breeders it is simply a matter of curiosity but for those involved in commercial breeding this knowledge can yield a significant economic advantage.
Gender information can be helpful in planning future matings when, in the case of an older mare, a filly by a specific stallion would be desirable to retain a breeding line.
Due to the higher market demand for colts compared to fillies in Thoroughbred racing the value of a given mare when carrying a colt, compared to a filly, is likely to be significantly higher. This will affect sale and insurance valuations.
Whether a mare is carrying a colt or a filly may also be a factor for consideration when a breeder is choosing mares to sell in the process of upgrading a broodmare group.
By contrast, filly pregnancies are considered more valuable in polo pony breeding programmes in South America.
Currently the attitude of Thoroughbred sale companies in the UK is to avoid gender details being included in catalogue descriptions but it is common for vendors to provide details/ certificates to potential purchasers on a private basis.

Fetal anatomy and identification of gender:
There is little published information describing the development of the genitalia in the equine embryo/fetus but the stages of development are considered to parallel other mammalian species.
The precursor to the development of sex-specific external genitalia is appearance, at approximately Day 50 of gestation, of a small bud of tissue projecting from the skin between the hind limbs of the fetus - the genital tubercle. This tubercle then gradually migrates to the sex defining position.
In the filly migration is caudal and the tubercle comes to lie on the perineum, ventral to the tail, where it develops into the vulva and clitoris. In the colt the migration is cranial so that it comes to lie on the ventral abdominal wall just caudal to the umbilicus, where it develops into the penis and prepuce.
In most pregnancies the genital tubercle becomes recognisable ultrasonographically at some stage between 55 and
60 days of gestation. It is a surprisingly large and echogenic structure with a characteristic bi-lobed appearance. It is one of the most noticeable anatomical features of the fetus recognisable by ultrasound at this stage.
In order to identify gender the position of the genital tubercle must be recognised in relation to the landmarks provided by the hindlimbs.

Timing of examination:
The most convenient stage to carry out examination is between
60 and 80 days of gestation. At this stage the tubercle is relatively
easy to identify and the fetus is likely to be reached by the
standard rectal examination approach, except in a mare which
has a deep pendulous uterus.
Beyond 80 days, with enlargement of the uterus the fetus
may have moved out of comfortable reach. The increasing density
of the fetal skeleton is also likely to cause complication by acoustic
shadowing.

A later opportunity to examine the fetus per rectum occurs at approximately 110 - 120 days. However, at this stage the tubercle has progressed in development into the external genitalia i.e. clitoris or penis/prepuce and interpretation becomes more subjective. It is often difficult to achieve high confidence levels in diagnosis at this later stage.

Equipment:
Many of the currently available portable ultrasound scanners available for routine practice use will produce an image of diagnostic quality. The most important operating frequency is 5 MHz although the addition of 7.5 MHz can give improved image quality when the fetus is easily accessible. Split screen viewing and recall are useful features.

Examination technique:
The key to diagnosis is the recognition of the position of the genital tubercle in relation to the anatomical landmarks of the hindlimbs, i.e. does the tubercle lie on the perineum (filly) or does it lie cranial to the hindlimbs on the ventral abdomen (colt)?
By convention 3 standard planes of ultrasound have been described which can achieve identification. Drawing conclusions from other views can lead to errors in diagnosis. The fetus often moves during examination - patience may be required. Should the fetus lie in a nonstandard view it can be prompted to move by gentle agitation.
Mare temperament is important as the examination is likely to take longer than a typical scan and the operator will be more exposed. Sedation is often counter-productive as the relaxation causes the uterus to move forwards. In mares that strain the hyoscine (Buscopan 20 mg/ml at 0.3mg/kg bwt) can be helpful. Care must always be taken to avoid rectal injury.

Plane 1 (longitudinal)
The fetus is sectioned from 'head to tail'. In the colt the tubercle is visible between the umbilical attachment and the echogenic points of the stifles.

Plane 2 (oblique)
A cross-section of the fetal perineum reveals a triangle of echogenic points created by the tail-head and hindlimbs (tibias). The tubercle of the filly is located within this triangle.

Plane 3 (transverse)
Cross-sections of the fetal abdomen reveal the male tubercle on the ventral abdominal wall often just cranial to the stifles and caudal to the umbilicus.
Generally colts are easier to recognise than fillies because fetal landmarks allow better orientation. It is always helpful to obtain more than one standard view to assist diagnosis.

Record keeping:
Clear written records should be kept for all examinations - including mare details, gestational age, planes observed and which of these allowed identification of the tubercle. Retained images are helpful and allows confidence in the technique to develop.

Accuracy:
When confidence has developed accuracy can be absolute when the technique is performed between 60-80 days of gestation.