Effect of mare's age and embryo quality on pregnancy and foaling rate in a clinical ICSI program

Abstract

There are few studies on the quality of in vitro produced (IVP) equine blastocysts related to the mare age. A retrospective study on the outcome of a clinical equine IVP program using donor mares of different ages and comparing their embryo quality and pregnancy potential is presented. Pregnancy, foaling, and pregnancy loss rates are reported for the transfer of fresh ICSI embryos derived from oocytes collected from mares aged 2 - 26 years old. ICSI blastocysts were classified as: Grade A – D6, 7, or 8, good quality; Grade B – D6, 7, or 8, moderate quality and D9 or D10, good quality; Grade C – D9 or D10, moderate quality (adapted from Lewis.et.al. Reproduction, Fertility and Development. 2023; 35: 338-351). Donor mares were divided on the basis of age: young (2-10y), mid-age (11-17y), and old (18-26y). Differences concerning groups were analyzed by Fisher's Exact Test (significance level of P<0.05). Results are presented in Table 1. For grade A embryos, pregnancy and foaling rates were similar among ages, but old mares showed higher pregnancy loss than young mares. For grade B embryos, old mares showed lower pregnancy rates than other groups. For grade C embryos, no difference in pregnancy, foaling, or pregnancy loss was seeing between ages. Embryo grades in each age interval were also analyzed. For young mares, pregnancy rate for different embryo grades was similar. However, foaling rate of grade A embryos was higher, and pregnancy loss of grade A embryos was lower than grade B. In mid-age mares,grade C embryos showed a lower pregnancy rate than grade A and B (tendency, P=0.09). Similarly, grade A embryos showed a higher foaling rate than the others. In old mares, grade A embryos showed higher pregnancy and foaling rates than grade B embryos. Thus, ICSI program outcomes for pregnancy, foaling, and pregnancy loss are related to the mare age. Special attention must be given to the quality of the embryo since pregnancy and foaling rates may vary within the same embryo grade, due to the mare's age.