Determination of equine fetal sex by Doppler ultrasonography of the gonads

Summary Reasons for performing the study The identification of fetal sex in horses by location of the genital tubercle between 55 and 70 days of pregnancy is hampered by the large amount of allantoic fluid, extensive fetal movements and the extremely long umbilical cord; however, reliable results have been achieved by ultrasonographic evaluation of the fetal gonads at 110–150 days of pregnancy. Objectives The aim of this study was to diagnose the sex of equine fetuses using B-mode and/or colour Doppler transrectal ultrasonography in fetuses of different ages. Study design Cross-sectional study comparing 2 methods of determining fetal sex. Methods The evaluations were performed in 86 mares at 90–180 days of pregnancy using transrectal B-mode and colour Doppler mode ultrasonography. Fetuses that had gonads with a homogeneous texture and a thin central longitudinal echogenic line were considered to be male. Females were identified by the presence of gonads with a central circular echogenic structure surrounded by a hypoechogenic external halo. Results Using B-mode ultrasonography, it was possible to determine the sex of 75% of the males, while determination of sex based on differences between the medullary and cortical layers of the ovary allowed 91.1% of females to be correctly identified. Using Doppler ultrasonography, 100% of males were successfully identified, while characteristic vascularisation of the female gonad could be detected in 98% of the evaluated female fetuses. Conclusions Colour Doppler ultrasonography combined with B-mode ultrasonography allows the determination of fetal sex with greater accuracy than B-mode ultrasonography alone, particularly for the identification of the male gonad. The use of Doppler ultrasonography enables the identification of sex in older fetuses.