Effect of P replacement during the LFT. This chart shows menstrual bleeding on each day after P withdrawal during the luteal-follicular transition. P implants were replaced at 12, 24, 30, 36 40 48 60 and 72 hours during this period (vertical gray arrows). Incidence of vaginal spotting (pink) and frank menstruation (red) on each day are shown as bars across the x axis. Each bar represents one animal. Replacement of P at 12, 24 and 30 hours blocked frank menses. Replacement of P at 36 h blocked bleeding in all but one animal, at 48 h failed to block bleeding in three out of 4 animals, and P replacement later than 48 h after P withdrawal failed to block bleeding in all animals.