Animal Reproduction Science, 19 (1989) 217-227 217 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Determining Sex of the Bovine Fetus by Ultrasonic Assessment of the Relative Location of the Genital Tubercle SANDRA CURRAN, J.P. KASTELIC and O.J. GINTHER Department of Veterinary Science, University of Wisconsin-Madison, 1655 Linden Drive, Madison, W153706 (U.S.A.) (Accepted 14 November 1988) ABSTRACT Curran, S., Kastelic, J.P. and Ginther, O.J., 1989. Determining sex of the bovine fetus by ultra- sonic assessment of the relative location of the genital tubercle. Anim. Reprod. Sci., 19: 217- 227. The objectives were to characterize the ultrasonic morphology and location of the genital tu- bercle (forerunner of penis and clitoris) in cattle fetuses and to determine whether assessment of the relative location of the tubercle is useful for predicting fetal sex. Daily ultrasound examinations were done on Days 48 to 60, sex was diagnosed, and the degree of certainty of the diagnosis was scored ( 1 = not determinable; 2, 3 and 4-- determined with minimal, intermediate, and maximal certainty, respectively). The fetuses were recovered by induced abortion on Day 60 and sex deter- mined by gross inspection of the external genitalia. In both sexes, the genital tubercle was a hy- perechogenic bilobar structure and each lobe was elongated and oval shaped, enabling it to be recognized by ultrasound. On the first day that the certainty level for diagnosis of sex reached its highest value per heifer, sex was correctly diagnosed in 16 of 16 heifers (confidence interval for correct diagnosis, 83 to 100% ). The mean certainty level increased from 1.2 _+ 0.2 on Day 48 to 4.0 _+ 0.0 on Day 58. The tubercle was located between the hind limbs on Days 48 and 49 and then moved toward the umbilical cord in males and toward the tail in females. In males, the tubercle was immediately caudal to the umbilical cord on mean Day 56.0_+0.8 and in females was under the tail on mean Day 53.8_+0.2. In conclusion, ultrasonic identification and estimation of the relative location of the genital tubercle by an experienced operator was a reliable technique for determining sex of the bovine fetus beginning on Day 55 and extending through the last day of the experiment (Day 60). INTRODUCTION The ultrasonic appearance and location of differentiated external genitalia have been used to determine fetal sex in cattle and humans. In cattle, fetal sex was determined with 94% accuracy between 73 and 120 days, postinsemina- tion, by ultrasonic visualization of the scrotum and mammary glands of male and female fetuses, respectively (Mfiller and Wittkowski, 1986). In humans, 0378-4320/89/$03.50 © 1989 Elsevier Science Publishers B.V.