Edema, or fluid swelling, is often a sign that something’s wrong. Pulmonary edema in horses’ lungs, for instance, can be fatal. In certain eye structures it can lead to vision loss. In mares in estrus, however, some uterine edema in response to estrogen secretion is commonplace. But is it also benign?

Researchers from the University of Kentucky’s Gluck Equine Research Center and Rood & Riddle Equine Hospital, both in Lexington, recently studied uterine edema at ovulation to determine whether it affected pregnancy outcome. Ed Squires, PhD, Dipl. ACT (hon), professor at the Gluck Center, presented their results at the 2014 American Association of Equine Practitioners (AAEP) Convention, held Dec. 6-10 in Salt Lake City, Utah.

When examining a mare’s uterus and ovaries during estrus, many veterinarians report seeing uterine edema as the mare approaches ovulation. The edema typically decreases just prior to ovulation. Some mares, however, continue to experience edema during or after ovulation, and no studies have been performed to determine whether this adversely affects fertility. Squires’ team hypothesized that prolonged or excessive uterine edema would lower mares’ pregnancy rates and increase incidences of early embryonic loss.

In the study, the team used ultrasound to examine 920 Thoroughbred mares for a total of 1,127 estrus cycles during the 2014 breeding season. Two days before ovulation as well as the day of, they recorded each mare’s edema score on a scale of 0 to 4, with 0 indicating no edema and 4 being excessive, along with all other findings. They then classified the mares into three age groups—young (2-9 years), middle-aged (10-16 years), and old (17 and up)—and also noted their reproductive status (foaling, maiden, or barren) at the time of breeding