There are many ways to diagnose pregnancy in a mare, however ultrasound stands out as the most useful and versatile. While manual palpation can diagnose pregnancy and potentially twins, it can’t show a fetal heartrate or consistently diagnose placentitis. Only ultrasound can show fetal health, birth defects, placentitis, and allow for accurate fetal aging. It also allows for the monitoring of cervical tone, uterine edema, and follicular development while preparing to breed a mare.
Depending on the reason for the ultrasound and its timing, it may be done transrectally or transabdominally. As a rule, I prefer a linear probe for most of the transrectal examinations as they provide minimal distortion of the reproductive structures for accurate measurement and scoring. While a curvilinear rectal probe can provide satisfactory measurement and scoring, the fixed structures within a mare’s abdomen make it easy to locate the ovaries and fetus without the need for a broader scan field, and if one does a lot of reproductive ultrasound in mares it isn’t worth the trade-off in accuracy. Prior to sonographic examination, it is extremely important to clean out the fecal material from the rectum.
For transabdominal ultrasound, as with many large species, a curvilinear rectal or curvilinear t-handle probe are both appropriate. Unless the mare has a particularly heavy winter coat, trimming isn’t typically needed, and alcohol will be adequate for contact.
As with any procedure, safety of the examiner and the mare are extremely important. For this reason, many mares may require sedation and possibly to be restrained in stocks. Sedation can be especially beneficial in maiden mares who may otherwise be very tense during examination and can help to reduce the risk of injury to the examiner, and to reduce the risk of rectal tears.
For timing, most transrectal examinations will be performed from days 16-70, as for most practitioners in larger mares, the fetus will begin to descend into the abdomen after this point and become difficult to reach. Taller practitioners and smaller mares may still allow for transrectal examination up to 90 day after which examinations of the fetus will be performed transabdominally. Examination of the placenta is typically still done transrectally, typically in late gestation.
Ideally for confirmation of pregnancy, an exam should be done between 16 and 18 days after breeding. At this point, the embryo fixes in place at the base of a uterine horn and will appear to be a circular hypoechoic structure with a hyperechoic rim. This is also the ideal time to identify and eliminate a twin pregnancy. An additional exam should be done at about 1 month of gestation to confirm the pregnancy, as most pregnancies will be terminated in this window. High-risk mares with historic conception loss may need more frequent ultrasounds to follow and monitor the pregnancy. These exams should be further supplemented by bloodwork to check progesterone levels, and a breeding soundness exam prior to being covered that includes uterine culture and cytology, ultrasound, and potentially biopsy. Maiden mares may also benefit from more frequent observations as well.
By day 21, the vesicle takes on a shape similar to a triangle with a rounded base where the embryo will be visible as a structure at the base. From day 28-36, the vesicle will be gradually filled by the embryo, which will begin to show a heartbeat. After this point, the allantoic sac starts to fill and the vesicle is about 50% fluid and 50% embryo. At this point, the endometrial cups will have also formed and will appear on ultrasound as rounded structures along the wall of the vesicle. The embryo has now transitioned into a fetus.
The next crucial exam is at 6 months to observe fetal viability and placental health; however, many owners may request an exam to sex a foal. Fetal sexing is best done at 60 days of gestation transrectally and is extremely similar to sexing calf fetuses. It can be done after day 90 transabdominally but obtaining an adequate view can be difficult. The 6-month exam should observe a heartbeat and should be able to diagnose any major physical deformity. In high-risk mares later in gestation, mares with symptoms such as early milk let-down, and mares on fescue pasture, transrectal ultrasound is beneficial to assess the placenta. This is typically done at just distal to the cervix and is assessed by combined thickness uterus-placenta (CTUP). These measurements will vary based on gestational age and breed, and more thorough explanations can be easily found.
As in cattle, once the fetus drops into the abdomen aging is more of an art than a science. While there are published values for the size of fetal structures at different gestational age, there is a great deal of variability between breeds and individuals. The most reliable aging is done by 60 days of gestation. Should one be inclined to attempt it, width of the vitreous body of the eye is one reliable measure. It is 2 cm at approximately 150 days of gestation, 2.5 cm at approximately 200 days of gestation, 3 cm at approximately 250 days of gestation, and will remain between 3 and 3.5 cm for the remainder of the pregnancy.