['To see if progesterone support has a beneficial effect on live birth rate after frozen embryo transfer in natural cycles.', 'Prospective randomized controlled trial.', 'University-based hospital.', 'Four hundred thirty-five women undergoing embryo transfer in natural cycles.', 'The women received either vaginal progesterone, 400 mg twice a day from the day of embryo transfer in natural cycles, or no progesterone support.', 'Live birth rate, biochemical pregnancy rate, pregnancy rate, and spontaneous abortion rate.', 'Live birth rate were significantly greater in women receiving vaginal progesterone as luteal phase support after frozen-thawed embryo transfer in natural cycles compared with those who did not take progesterone. There were no differences in biochemical pregnancy rate, pregnancy rate, or spontaneous abortion rate.'] Does luteal phase progesterone increase live birth rate after frozen embryo transfer?