After a mare is bred, waiting to determine if the mare returns to heat before having her examined for pregnancy may appear to be a way to save money by avoiding "needless" pregnancy evaluations, but may ultimately result in greater costs. The use of ultrasound enables us to diagnose pregnancy at an early stage, 12 to 14 days after ovulation. Ovulation usually occurs approximately one day before the mare goes out of heat. The advantages of early pregnancy diagnosis are numerous, whereas the disadvantages are negligible. In this article the principle reasons why early pregnancy examinations are so important to a well-managed breeding program will be discussed.

By knowing that a mare is not pregnant before the time she is due to come back into heat, plans can be made to prepare for another breeding. Although the length of time a mare is in estrus or "heat" varies somewhat between mares and according to the time of the year, the length of time a mare is out of heat is fairly consistent, approximately 15 days. By checking a mare for pregnancy two weeks after she goes out of heat, if she is not pregnant, plans can be made to either take her back to the stallion for rebreeding or prepare for another shipment of semen for artificial insemination. This time frame allows for arrangements to be made before she comes back into heat, thereby providing ample time so that the next cycle is not missed and valuable time is not lost. This advanced warning is also helpful to the manager of the stallion who may need to work the mare into the breeding schedule. If pregnancy examination is delayed until 18 or 21 days after breeding, the mare may be too close to ovulation (or even just past ovulation) for rebreeding during that cycle.

Because the length of time between heats is fairly consistent, if a mare is showing strong signs of heat at the time of pregnancy examination 12 to 14 days after the last breeding, it is an indication of some problem causing her to "short cycle." Short cycling, or coming back into heat before expected, may result from a uterine infection. Uterine infections cause the release of a hormone, prostaglandin, that results in a mare returning to estrus. This will result in her having a shorter than normal interval between estrus periods. Mares that come back into heat before they are due should be examined for abnormal conditions of the reproductive tract, including poor conformation, urine pooling, and endometritis, which could be responsible for the abbreviated diestrus. Likewise, examination for pregnancy in late diestrus may reveal fluid in the uterus, another abnormal condition that should be further evaluated.

An ultrasonographic image of the vesicles of a mare pregnant with twins. The mare is 17 days pregnant. The ultrasound provides the only currently effective method of evaluating for twins in the early part of gestation, when it is most feasible to eliminate one of the vesicles.