A prospective study by Fatemi et al. (2010) revealed a significantly higher ongoing pregnancy rate after transferring frozen-thawed embryos in natural cycles with a spontaneous LH peak compared with natural cycles controlled by hCG for final oocyte maturation and ovulation (31.1% vs. 14.3%, respectively). In this trial, FrET (frozen embryo transfer) was planned 5 days after the LH surge or 5 days after the administration of 5000IU of hCG.