Morphology of twin and triplet equine conceptuses during Weeks 3 and 4 of pregnancy Morphology of twin and triplet equine conceptuses.

Twin ovulations are common in horses, but twin pregnancies are rarely carried to term. Theories of how one or both twins is/are naturally eliminated in early pregnancy, termed 'embryo reduction', have been based on ultrasonographic, not morphological, studies. Here we describe conceptuses recovered transcervically between Days 15 and 28 from 31 twin and two triplet pregnancies. Signs of contact between conceptuses were deduced from those seen in one pair that remained attached by their capsules on Day 18. Signs were found on capsules in two of 10 pairs before or during fixation (immobilisation) at Days 16-17 even though contact had not been seen by ultrasound. After fixation, the signs became stronger in seven of nine unilateral pregnancies, indicated adhesion between pairs and included effects on the vitelline circulation and/or degeneration of one twin. Conceptuses recovered from five of seven unilateral twin pregnancies after the time of capsule disruption (~Day 21) evidenced embryo reduction; in the two surviving pairs, attachment between twins was near the trilaminar/bilaminar yolk-sac wall border. The findings are consistent with the notions that: (1) the capsule plays a role in initiating adhesion between twins; and (2) twin survival depends on an unencumbered trilaminar yolk-sac wall and a functional vitelline circulation. Twin births are rare in horses because contact between twin conceptuses in utero usually eliminates one by 'embryo reduction', as determined previously ultrasonographically. The present morphological study of 33 sets of multiple conceptuses reveals how contact before 'fixation' (immobilisation; Day 16) can progress to mutual adhesion and reduction before and after loss of the capsule (Day 21), with the site of adhesion affecting the severity of the effects on the conceptus undergoing elimination. [ABSTRACT FROM AUTHOR]