Modification of the embryo-maternal relationship in ewes by progesterone treatment early in the oestrous cycle.

When 25 mg progesterone/day were injected into ewes on Days 0-3 of the oestrous cycle, (i) the subsequent cycle was shortened by 4 days and (ii) on Day 6 such ewes provided an acceptable uterine environment for the survival of 10-day-old embryos. We suggest that exposure of the non-pregnant uterus to approximately 8 days of normal luteal concentrations of progesterone may be necessary to initiate luteolysis.