Natural outcome and ultrasonic identification of equine fetal twins.

The natural outcome of bilateral twins (one in each uterine horn) that were viable on Day 40 was studied in 15 pony mares. Fetuses were monitored by transrectal ultrasonography until the outcome was determinable. One fetus (2 mares) or both fetuses (8 mares) died (cessation of heartbeats) during Months 2 and 3. Loss of fetuses was externally observed in 4 additional mares during the late fetal stage (Months 8 to 11); 2 mares lost 1 fetus and 2 lost both fetuses. Birth of 2 live foals occurred in only 1 of 15 mares (7%) First day of detected apposition between portions of the 2 allantochorions was studied daily on Days 40 to 51 in 7 of the 15 mares. Apposition was first detected on mean Day 46.3 (standard deviation: +/-3.1). In these 7 mares, death of 1 fetus (2 mares) or both fetuses (2 mares) occurred 1 or 2 days after the first day of apposition of allantochorions; no deaths occurred before the day of apposition. Each mare with loss of one fetus during this time gave birth to a living foal. The only structure indicative of twins that was consistently detected ultrasonically during all examinations was the common membrane resulting from the area of apposition between the 2 allantochorions. The common membrane is herein designated as the twin membrane because of its apparent diagnostic potential even in late pregnancy when the presence of the 2 fetuses may not be detected directly. Thickness of the twin membrane reached approximately 3 mm at Month 7 and thereafter gradually increased to 15 mm near the end of pregnancy. Two layers of the membrane (1 for each allantochorion) became distinct at Month 6 and Month 8 with 5.0 MHz and 3.5 MHz transducers, respectively; the 2 layers were separated by a hypoechogenic layer.