Transrectal ultrasound of a mare's reproductive tract is commonly performed, and it is the single most important diagnostic test available to assess the health and reproductive status of the uterus, cervix and ovaries. It is often used in conjunction with rectal palpation, which is conducted during the same exam and provides complementary information.

With ultrasound, exact measurements can be taken of ovarian follicles, which allows more precise estimation of ovulation, and makes artificial insemination more successful. The uterus and ovaries may be assessed to determine the status of heat cycle and the uterus may be assessed for fluid accumulation.

Pregnancy may be diagnosed starting at 10-14 days post ovulation (after the end of the heat cycle) and continues to be of value in evaluation of the fetus through the pregnancy.

Ultrasound involves the use of pulses of sound sent through tissues by crystals on the surface of the probe. These pulses reflect from tissues at different speeds and an image is formed by computer analysis, which deciphers and visualizes this reflection pattern. In this case, an elongated, linear probe is held in the hand and carefully carried into the rectum and oriented over the reproductive tract, which lies beneath (ventral to) the rectum.