The phrase "uterine horns" refers to the parts of a female's reproductive system called the uterus. It specifically describes the two elongated structures where babies develop and grow before birth. Full definition
But other than the effects of uterine horn position that I discussed earlier, the whole litter, not just one, will be small. (2ndchance.info)
Ten to fifteen injected blastocysts were transferred to each uterine horn of 2.5 dpc pseudo-pregnant ICR females. (journals.plos.org)
For you who don't know what is spaying, it is a surgical removal of a female cat's internal reproductive structures and this includes its ovaries or the place where egg cells are produced, Fallopian tubes, uterine horns or the two long tubes where a kitten soon to be develop and grow, and a small part of its uterine body or small parts of uterus which is merged with uterine horns and become one body. (safehavenforcats.com)