Embryonic vesicle diameter at first pregnancy diagnosis in in vivo- and in vitro-derived pregnancies and its relationship with pregnancy loss in the mare.

Overview

• This study examined how the size of the embryonic vesicle at the first pregnancy check relates to early pregnancy loss in horses.
• It compared pregnancies resulting from natural artificial insemination and those created using in vivo and in vitro embryo production techniques.

Study Design and Groups

• Retrospective analysis involving three groups of mare pregnancies:
  • Artificially Inseminated (AI) mares (n=263)
  • Recipients of in vivo-derived flushed embryos (IVD-F) (n=312)
  • Recipients of in vitro-produced embryos via intracytoplasmic sperm injection (IVP-ICSI) (n=161)
• Embryonic vesicle diameter was measured at different time points:
  • Day 12 post-ovulation for AI pregnancies
  • Day 4 post-embryo transfer for IVD-F pregnancies
  • Day 7 post-embryo transfer for IVP-ICSI pregnancies

Key Measurements

• Mean embryonic vesicle diameters and early embryonic loss (EEL) rates for each group:
  • AI: 8.1 ± 2.0 mm diameter, 8.7% EEL
  • IVD-F: 7.2 ± 2.4 mm diameter, 8% EEL
  • IVP-ICSI: 8.8 ± 2.7 mm diameter, 13% EEL

Findings on Vesicle Size and Pregnancy Loss

• Small-for-age vesicles (defined as those below the 25th percentile in diameter for their respective group) were linked to higher rates of early embryonic loss in:
  • AI pregnancies: EEL rate of 17.6% in smallest quartile vs. 4.6% in largest quartile (75th-100th percentile)
  • IVP-ICSI pregnancies: a more pronounced effect with 31.8% EEL in smallest quartile vs. 0% EEL in largest quartile
• IVD-F pregnancies did not show a significant difference in EEL between smallest and largest quartiles (12.3% vs. 9.1%).

Pregnancy Viability and Larger Vesicle Size

• For IVP-ICSI pregnancies specifically:
  • Having a vesicle size above the 50th percentile (larger than 9 mm) was strongly associated with lower fetal loss rates beyond Day 42: 2.5% fetal loss versus 11.3% in those below the 50th percentile.
  • This suggests that a larger vesicle size at early diagnosis is a positive indicator for pregnancy viability in IVP-ICSI pregnancies.

Conclusions and Implications

• Embryonic vesicle diameter at the first pregnancy diagnosis is a useful prognostic indicator of early pregnancy loss risk in mares.
• This relationship is most significant for pregnancies derived from in vitro production techniques (IVP-ICSI), where small vesicles strongly predict higher early embryonic loss.
• Clinically, measuring vesicle size could help veterinarians and breeders identify pregnancies at increased risk, allowing for closer monitoring or intervention.
• The data indicates:
  • 31.8% early embryonic loss in IVP-ICSI pregnancies with small vesicles (<7 mm, below 25th percentile)
  • Only 2.5% early embryonic loss in those with larger vesicles (>9 mm, above 50th percentile)