Summary: Late pregnancy check ups in mares during advanced gestation by transabdominal ultrasound bring reliable results on fetomaternal and fetal well-being. Fetal gonads can be scanned as well. Aim of this study was to evaluate accuracy of fetal gender determination by using B-Mode and Doppler ultrasonography video images analyzed by different raters. The accuracy of these diagnoses was compared among the raters. Pregnancy checks were performed on different stud farms in the mid-west of Germany. The day of gestation was derived from breeding records, and the mares were examined between 70 -240 days pregnant. Fetal sex was determined as well by B-Mode and Doppler sonography. Examinations were videotaped. Gonads with homogenous structure and a hyperechogenic line along the longitudinal axis, or an intense Doppler signal along this axis were assigned to be male. Gonads with bizoned echogenicity and a strong circular Doppler signal were assigned female. To review the practicability and repeatability of gender determination 100 videotapes were selected and shown to 20 voluntary raters who correlated videotapes and fetal gender. Confirmation of gestation and fetal well-being during advanced gestation (90 -240 days) by transabdominal ultrasound examination is practicable. Gender determination can be accomplished by a combination of B-Mode and Doppler sonography. The best results can be accomplished during 4th (75 %) and 5th (62 %) month of gestation, especially by using Doppler ultrasonography. Practitioners who are unexperienced in gender determination can complete the task based on videotapes showing fetal gonads. Characteristic optical features can be identified better referring to vascularisation detected by colour Doppler ultrasonography. In conclusion gender determination by transabdominal sonography can be conducted by using a combination of B-Mode and Doppler sonography. Optimal working conditions and videotaping the examination can increase the accuracy. Using colour Doppler helps to find distinctive features and determine the gender with greater accuracy. The accuracy of the diagnoses is optimal between 4-5 month of gestation.Keywords: mare / gender determination / transrectal ultrasonography / transabdominal sonography / reproduction Citation: Tönissen A., Martinsson G., Pricking S., Otzen H., Ertmer F., Rau J., Sielhorst J., Rohn K., Sieme H. (2016) Transabdominal ultrasonographic determination of fetal gender in the horse during mid-gestation -a comparative study using randomized video images to investigate variation in diagnostic performance among raters, and the effect of month of gestation. Pferdeheilkunde 32,[29][30][31][32][33][34][35]

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