The efficacy of embryo recovery on consecutive days from Day 6.5 to obtain small embryos for vitrification.

Background: Vitrified embryos ≤300 μm give better pregnancy rates following warming and transfer than larger ones. Embryo recovery undertaken close to when the embryo enters the uterus (Day 6-6.5) helps in the recovery of embryos ≤300 μm. However, flushing early can mean missing an embryo not yet in the uterus, whereas later can result in embryos >300 μm.

Objectives: To evaluate if repeated embryo flushing on consecutive days from Day 6.5 would increase the number of embryos ≤300μm recovered.

Study design: Retrospective case series.

Methods: Four hundred and ninety-six inseminations with cooled (n = 339) or frozen (n = 130) semen were undertaken in 91 mares over a three-year period; pre-ovulatory for cooled and post-ovulatory for frozen semen, with mares checked for ovulation daily or every 6 h, respectively. At a presumed Day 6.5 (cooled semen) or 7 (frozen semen), mares' uteri were flushed, with flushing repeated 24 and 48 h later if no embryo was recovered. Linear mixed-effects models and likelihood ratio tests were used to investigate factors potentially influencing embryo diameter, embryo grade, and oviduct transport time (e.g., semen type, ovulatory drug, stallion age, mare age, season and flush number).

Results: Using repeated flushing, 98.8% (243/246) of embryos recovered were ≤300 μm. Second and third flushes yielded 61 embryos (24.8% of recovered embryos). The only variable that influenced embryo diameter was flush number (p = 0.003); for embryo grade, no variable was significant, whereas oviduct transport time was affected by semen type (p = 0.003), ovulatory drug (p = 0.029) and stallion age (p = 0.005).

Main limitations: Noncontrolled clinical study.

Conclusions: Embryo flushing on consecutive days from the time of expected entry of the embryo into the uterus was effective in collecting embryos ≤300 μm and resulted in additional embryos being retrieved compared with a single flush. Oviductal transport time in some mares is >156 h from ovulation.