A broodmare’s belly will undoubtedly grow as her fetus matures, but any rapid or unexpected expansion—particularly during the last trimester—is cause for concern. She might be suffering from one of two life-threatening conditions: hydrops allantois or hydrops amnion, characterized by excessive accumulations of allantoic or amnionic fluid in the uterus, respectively.

Fortunately these conditions are extremely uncommon; however, an incorrect diagnosis can quickly equal death for both the mare and her unborn foal. If detected early, veterinarians can manage these cases, so Nathan Slovis, DVM, Dipl. ACVIM, CHT, of Hagyard Equine Medical Institute, in Lexington, Ky., described how to diagnose and treat them as well as educate owners about what to expect at the 2013 American Association of Equine Practitioners convention, held Dec. 7-11 in Nashville, Tenn.

Veterinarians don’t know what causes hydrops conditions. But they do know that the disorders are life-threatening, Slovis said, because if undetected or untreated they can result in abdominal wall hernias, prepubic tendon rupture, and cardiovascular shock.

"The condition is often detected by the horse owner as a sudden onset (over a period of a few weeks) of abdominal enlargement, ventral edema (fluid accumulation in the abdomen), colic, lethargy, anorexia, tachycardia (rapid heartbeat), and dyspnea (difficulty breathing, due to pressure on the abdomen)," he said. Misdiagnoses include twins and other causes of colic or ventral edema.

To make a definitive diagnosis, Slovis said the veterinarian should perform a transrectal examination of the reproductive tract,