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 moni tored by transrectal ultrasonography until the outcome was determinabl e. One fetus (2 mares) or both fetuses (8 mares) died (cessation of he artbeats) during Months 2 and 3. Loss of fetuses was externally observ ed 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 o ccurred in only 1 of 15 mares (7%). First day of detected apposition b etween 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 fir st 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 th at was consistently detected ultrasonically during all examinations wa s the common membrane resulting from the area of apposition between th e 2 allantochorions. The common membrane is herein designated as the t win membrane because of its apparent diagnostic potential even in late pregnancy when the presence of the 2 fetuses may not be detected dire ctly. thickness of the twin membrane reached approximately 3 mm at Mon th 7 and thereafter gradually increased to 15 mm near the end of pregn ancy. Two layers of the membrane (1 for each allantochorion) became di stinct at Month 6 and Month 8 with 5.0 MHz and 3.5 MHz transducers, re spectively; the 2 layers were separated by a hypoechogenic layer.