How to Determine Fetal Gender in Early and Advanced Gestation

1. Introduction

A wealth of information on equine fetal anatomy and physiology has been acquired in recent years. Ultrasonographic fetal monitoring techniques have documented, in great detail, fetal growth and organ development at various stages of gestation. Ultrasonographic anatomy of fetal sex organs, from the genital tubercle to fully developed organs, has been extensively described in its entire progression.^1–7 Fetal gender determination in the mare provides a useful management tool to breeders by allowing a predelivery estimation of the value of offspring. Knowing fetal gender in advance of delivery allows for commercial strategies to be implemented, as the value of stock at sales time is often determined by the gender of the offspring. Furthermore, it is well established that some stallions have a greater proportion of quality female versus male offspring or just the opposite. Finally, culling of broodmares is easier when fetal gender is known. 

2. Diagnostic Windows for Fetal Gender Determination

First Stage Diagnosis: Early Gestation

There are two different stages when the diagnosis of fetal sex can be determined by ultrasonography. The first stage is between 57 and 70 days gestation and involves the identification of the genital tubercle (Fig. 1) by transrectal ultrasonography.¹ The genital tubercle, the precursor of the penis in the male and the clitoris in the female, appears around day 55 of gestation as a hyperechoic equal sign (=), located between the fetal hindlimbs, at an approximately equal distance between the tail and the umbilicus. As gestation progresses, the genital tubercle migrates towards the tail in the female fetus (Fig. 2) and towards the umbilical cord in the male (Fig. 3). The shape of the genital tubercle may change over time, appearing trilobed (Fig. 4) or conical (Fig. 3).

The early stage technique requires good equipment and considerable expertise, allowing consistent fetal sex determination within a small diagnostic window (optimal time days 59–68) and by a single diagnostic parameter. Best imaging will be accomplished when the fetus engages high up into a uterine horn (Fig. 5) and the fetus to transducer distance is greatly reduced. After day 70, the fetus tends to reside more deeply in the mare’s uterus/abdomen and is not consistently accessible for imaging by transrectal ultrasonography due to the disproportionate increase in fetal fluid volume compared to fetal body mass, typical of this stage of gestation. After day 100, the fetus can be reliably found within the mare’s pelvis.
 

Second Stage Diagnosis: Advanced Gestation

The second stage for fetal sex diagnosis avails of a much wider diagnostic window between 100 and 260 days gestation. Multiple parameters can be used to validate diagnosis (fetal primary sex organs) at this stage, but a combination of transrectal and transabdominal ultrasound scanning may be required.

Fetal presentation plays a substantial role in the diagnostic approach in advanced gestation. Establishing fetal orientation should be readily accomplished in the course of the examination, knowing that in the fetus in posterior presentation the hindquarters will be easily accessible per rectum and expedite transrectal diagnosis. The active nature of the equine fetus determines frequent changes in presentation up to 9 months gestation, making transrectal examination potentially diagnostic even in advanced pregnancy. The ability to change presentation decreases as gestation advances,^6,7 as fetal size and the encasement of the hindquarters within the fetal horn prevent further rotations along the short axis. After 9 months, fetal sex can still be determined transrectally in grossly undersized fetuses and when a posterior or transverse presentation persists. A transabdominal approach will be required for diagnosis when the fetal hindquarters are out of reach of the operator’s hand per rectum, as it commonly occurs from around 5 months gestation and when the fetus is in anterior presentation (Fig. 6). [...]