Although more ova usually ovulate from the (1) ovary than from the (2), the embryos migrate and end up being evenly spaced in both uterine horns. Early death of some embryos occurs between days (3). A minimum of (4) viable embryos are necessary (prior to implantation) for the pregnancy to continue. If there are less than (4), the embryos are re-absorbed following luteolysis of the corpora lutea. The luteolytic effect of a comparatively empty uterus (i.e. less than four viable embryos) is greatest between days (5), but greatly reduced after day (6).