Factors affecting pregnancy rates and early embryonic death after equine embryo transfer

In the present study, 638 embryo transfers conducted over 3 yr were retrosp ectively examined to determine which factors (recipient, embryo and transfe r) significantly influenced pregnancy and embryo loss rates and to determin e how rates could be improved. On Day 7 or 8 after ovulation, embryos (fres h or cooled/transported) were transferred by surgical or nonsurgical techni ques into recipients ovulating from 5 to 9 d before transfer. At 12 and 50 d of gestation (Day 0 = day of ovulation), pregnancy rates were 65.7% (419 of 638) and 55.5% (354 of 638). Pregnancy rates on Day 50 were significantl y higher for recipients that had excellent to good uterine tone or were gra ded as "acceptable" during a pretransfer examination, usually performed 5 d after ovulation, versus recipients that had fair to poor uterine tone or w ere graded "marginally acceptable." Embryonic factors that significantly af fected pregnancy rates were morphology grade, diameter and stage of develop ment. The incidence of early embryonic death was 15.5% (65 of 419) from Day s 12 to 50. Embryo loss rates were significantly higher in recipients used 7 or 9 d vs 5 or 6 d after ovulation. Embryos with minor morphological chan ges (Grade 2) resulted in more (P < 0.05) embryo death than embryos with no morphological abnormalities (Grade 1). Between Days 12 and 50, the highest incidence of embryo death occurred during the interval from Days 1 7 to 25 of gestation. Embryonic vesicles that were imaged with ultrasound during t he first pregnancy exam (5 d after transfer) resulted in significantly fewe r embryonic deaths than vesicles not imaged until subsequent exams. In the present study, embryo morphology was predictive of the potential for an emb ryo to result in a viable pregnancy. Delayed development of the embryo upon collection from the donor or delayed development of the embryonic vesicle within the recipient's uterus was associated with a higher incidence of pre gnancy failure. Recipient selection (age, day after ovulation, quality on D ay 5) significantly affected pregnancy and embryo loss rates. (C) 2000 by E lsevier Science Inc.