A variety of problems can cause swelling at the lowest part of the belly. The most common cause in late pregnancy results from the weight of the pregnancy interfering with the mare's circulation, causing fluid to seep out into the tissue planes (edema). It is a more common finding in the last weeks of pregnancy in large mares with very pendulous bellies.

Ventral edema in late pregnancy is usually not painful, and will indent and retain its form momentarily if you push your thumb into it. It has the texture of closed-cell foam. Ventral edema usually resolves quickly after a mare foals. Rarely, however, rupture of the abdominal wall can cause excessive swelling here too.