Identifying the sex of the fetus by ultrasound depends on identifying the genital tubercule - which is located under the tail in females and behind the umbilical cord in males - and appears first after 50 days and is basically a bilobed white structure generally seen fleetingly - you need a cooperative cow and to be able to visualize both the abdomen and butt. It is also helpful if the fetus is positioned so that the head is not the first thing you seen. Once the fetus is over the pelvic brim (generally after day 90 sometimes longer) they are difficult to impossible to sex depending on their position