Incidence of spontaneous ultrasonographic regression of the corpus luteum in pregnant embryo recipients and broodmares: a field study.

Abstract

Early embryonic loss (EEL), particularly between Days 14 and 42 of pregnancy, remains a key concern in equine reproduction. This study investigates the incidence of spontaneous luteolysis-used here as a marker of maternal recognition of pregnancy (MRP) failure-in both pregnant Thoroughbred (TB) broodmares and recipient mares carrying either in vivo-derived (IVD) or in vitro-produced (IVP) embryos. Retrospective reproductive data from 2078 pregnancies in TB and 532 pregnancies in recipients were analysed to determine the incidence and characteristics of clinical luteolysis during pregnancy. Overall, the incidence of EEL was 8.0, 8.7, and 13.5&#xa0;% in broodmares, recipients with IVD and IVP embryos, respectively. Among pregnancies experiencing EEL, spontaneous luteolysis was observed in 18.7&#xa0;%, 22.2&#xa0;%, and 26.7&#xa0;% of pregnancies in broodmares, IVD, and IVP embryo recipients, respectively. Among all pregnancies, the highest (P&#xa0;<&#xa0;0.05) incidence of luteolysis (4.9&#xa0;%) occurred in pregnancies with IVP embryos (1.7&#xa0;% in TB and 1.9&#xa0;% in IVD pregnancies). Clinical diagnosis of luteolysis was primarily based on transrectal ultrasonography, assessing corpus luteum (CL) integrity (size, echo-density and vascularity) and inter-ovulatory interval (IOI) in mares with luteolysis. Progesterone or altrenogest supplementation was administered in 34&#xa0;% (18/53) of cases with spontaneous luteolysis of which 44.4&#xa0;% (8/18) maintained their pregnancy by Day 42. Risk factors identified for luteolysis in pregnancy included individual mare, type of pregnancy and embryo transfer timing. Late-cycle transfers (Day 9 or 10 post-ovulation) were associated with increased luteolysis risk in recipients carrying IVD embryos. Notably, vesicle size alone did not reliably predict spontaneous luteolysis. The findings highlight that spontaneous luteolysis is more prevalent than previously reported and may occur even with apparently morphologically normal embryos, suggesting that MRP failure is multifactorial and not solely linked to embryonic development. These results emphasize the need for earlier and more accurate diagnostics to identify at-risk pregnancies and guide treatment, especially in IVP derived pregnancies, which have the highest incidence of luteolysis.