Prompted by the study demonstrating higher ongoing pregnancy rate following the transfer of frozen-thawed embryos in natural cycles with spontaneous LH rise compared with natural cycles controlled by hCG for final oocyte maturation and the reports showing improved pregnancy rate in patients who received a mid-luteal injection of a GnRH-agonist (0.1 mg triptorelin), the investigators started beginning at June 2014 to offer ovulatory patients with regular menstrual cycles a natural FET with modified luteal support. Starting on the day of spontaneous ovulation, patients received daily vaginal progesterone, supplemented by a single injection of HCG and GnRH-agonist, on day of ET and 4 days later, respectively.