The nature of embryo reduction in mares with twin conceptuses: deprivation hypothesis

Ultrasonography was used to determine whether there is embryo reduction in mares with unilaterally fixed twins when a major portion of the vascularized area of the wall of one of the embryonic vesicles is in apposition with the wall of the adjacent vesicle, rather than with the endometrium (deprivation hypothesis). In addition, the effect of ovulatory pattern (synchronous and asynchronous) on the incidence of embryo reduction was studied. Twin vesicles were ultrasonically detected on days 11 to 15 (ovulation = day 0) and were examined daily until there was embryo reduction or until day 40. In 31 mares with twin embryonic vesicles, unilateral fixation (71%) was more frequent (P less than 0.05) than was bilateral fixation (29%). In 28 mares with known ovulatory patterns, synchronous ovulations did not affect the type of fixation (9/17 unilateral and 8/17 bilateral); however, for asynchronous ovulators the frequency of unilateral fixation (10/11) was greater (P less than 0.01) than the frequency of bilateral fixation (1/11). The incidence of embryo reduction was greater (P less than 0.01) for unilateral fixation (14/19) than for bilateral fixation (0/9) and was greater (P less than 0.05) for asynchronous ovulators (9/11) than for synchronous ovulators (5/17). In mares with embryo reduction, the reduction was complete before detection of both embryo propers (early reduction) in 10/14 and after detection of both embryo propers (late reduction) in 4/14.(ABSTRACT TRUNCATED AT 250 WORDS)