Transuterine migration and spacing of bovine embryos during gonadotropin‐induced multiple pregnancy

Unilaterally ovariectomized and intact cattle were injected with 1,500 i.u. gonadotropins to induce superovulation, inseminated at estrus and autopsied 30 to 60 days post ovulation. The allantochorion was dissected in situ and the viability and position of embryos in utero measured. There was no difference between the intact and unilaterally ovariectomized animals in the number of corpora lutea. One to ten embryos were recovered per cow; intact animals had more embryos than the ovariectomized ones. One to two embryos migrated per uterine horn; the percentage of transuterine migration was lower in the intact animals than in the ovariectomized ones. In unilaterally ovariectomized and intact animals, a single uterine horn could sustain only one or two embryos. When the uterine horn contained one embryo, the embryo occupied the middle or upper two‐thirds of the horn. With the increase of the number of embryos the sites of embryos were unevenly spaced and overcrowded. This phenomenon led to a high rate of fetal mortality especially when the uterine horn contained more than two fetuses and transuterine migration failed to occur. The results are discussed in relation to endocrine mechanisms and species differences as they affect uterine contractions.