Transuterine migration of the fetus in the mare between day 42 and parturition.

A total of 2187 Thoroughbred and Standardbred mares was examined over a 4-year period to determine the location of the fetus at 42 days, the location of the previous fetus as determined by examination 5-15 days after foaling, and pregnancy location the next season. Maiden mares showed 44 and 56% of pregnancies in the left and right horns respectively. No transuterine migration was observed in 139 maiden mares examined after their first foal. Lactating mares showed 62% implantations and full-term pregnancies on one side of the uterus with the next season's pregnancy being in the opposite uterine horn. No ready explanation could be found for 19% of the group which showed unilateral location of pregnancy at all 3 examinations. Evidence of transuterine migration after pregnancy examination at Day 42 was found in 18%. Further, in 50% of these mares, the placental cord was attached to the entrance of the non-pregnant horn. Mares that had a fetus in the same horn in two consecutive seasons had a significantly higher rate of abortion (P less than 0.01) when compared to mares with a fetus in the opposite horn the next season.