Effect of gestational age and fetal position on the possibility and accuracy of ultrasonographic fetal gender determination in dairy cattle.

The possibility and accuracy of transrectal ultrasonographic determination of foetal gender were studied by identifying and locating the genital tubercle, scrotum or mammary gland in 15 dairy cows at 49-168 days post-breeding. Ultrasound examinations were performed weekly. Each examination was made without references to the results of the examinations in the previous weeks. At each examination, an attempt was made to view the foetus in frontal, cross-sectional, and sagittal planes. The expected foetal gender was compared with that observed at birth. Gestational age affected significantly the possibility of foetal gender determination. The possibility of foetal gender determination increased from 26.7% at day 49 to 100% at day 56 of gestation. Between days 56 and 98, foetal gender determination was possible in all cases. Thereafter, the ability to determine foetal gender decreased with development of pregnancy and was impossible in all cases by days 161 and 168. The overall accuracy of foetal gender determination was 97.3%. Foetal gender determination between days 49 and 63 depended on the location of the genital tubercle in a frontal view of the foetus. By day 70, all foetal positions could be used for foetal gender determination. However, by progress of gestation, the usefulness of frontal view for sex determination decreased, while that of cross-sectional view increased. Accordingly, the best time for foetal gender determination in dairy cattle is between days 56 and 98 of pregnancy.