Embryonic loss in mares: Incidence and ultrasonic morphology

Pregnancy was determined by ultrasound on Days 11, 15, 20, 25, 30, 35, 40, 45, and 50 in 154 ponies and 27 horses. In ponies, the embryonic loss rate for Days 11 to 15 (28 154 , 18.2%) was greater (P<0.01) than for any of the subsequent five-day intervals (0% to 3.3%). There were no losses during Days 11 to 15 in horses (0 27 ), and the difference between ponies and horses was significant. The loss rates for the seven periods encompassing Days 15 to 50 were not significantly different among periods. Pseudopregnancy occurred more frequently (P<0.01) following embryonic loss after Day 20 (Days 11 to 15, 26%; Days 15 to 20, 33%; after Day 20, 100%). Embryonic vesicles that were lost during Days 11 to 15 were smaller on the average than control vesicles. However, most of the vesicles grew at an apparently normal rate. Two of five vesicles that were lost between Days 15 and 20 and three of four that were lost between Days 20 and 25 were undersized during preceding examinations. Undersized vesicles were found during 13 415 (3%) examinations during Days 11 to 20 in mares that maintained the embryo and in 21 106 (20%) in mares that lost the embryo. Embryonic vesicles that were lost during Days 11 to 15 usually disappeared without previous indications, except in three mares in which the vesicle was floating in a small collection of fluid. Ultrasonic indications of impending loss at later stages included failure of fixation, an echogenic ring (vesicle) or mass floating in a collection of fluid, an echogenic area in the dead embryo, absence of heart beat, and a gradual decrease in volume of placental fluids with disorganization of the placental membranes. The solid remnants and at least some of the fluids resulting from late embryonic and early fetal death were retained sometimes for weeks or months until the debris was apparently expelled through an open cervix.