Early pregnancy loss in brood mares.

During 1985, linear-array ultrasonography was used to study early pregnancy loss in commercial brood mares: 600/1115 (54%) of the cycles resulted in detected pregnancy at Week 2 after ovulation and 80 (13%) of these pregnancies resulted in early pregnancy loss. The pregnancy loss rate was significantly higher (P less than 0.05) for twin pregnancies (10/41) than for singleton pregnancies (70/559). The pregnancy loss rate was significantly higher (P less than 0.05) at 2-4 weeks (29/60) than at 6-8 weeks (12/60). The pregnancy rate was significantly less (P less than 0.05) for post-partum mares inseminated at the foal heat (157/302) than for those inseminated at a subsequent oestrus (203/334). Mares with a history of endometritis had a significantly higher (P less than 0.05) per cycle pregnancy loss rate (7/26) than did mares with a history of not having endometritis (64/498). When mares that had lost pregnancies were re-mated, 37/75 (49%) detectable pregnancies resulted and 7 (19%) of these pregnancies were again lost. The per cycle pregnancy rate was 56% (153/273), 55% (177/319), 60% (130/216), 51% (72/142), 45% (34/76) and 33% (12/36) for mares aged 2-5, 6-9, 10-13, 14-17, 18-21 and greater than 21 years, respectively. The corresponding per cycle pregnancy loss rates were 12%, 14%, 9%, 14%, 24% and 33%.