Table 1. Selected studies and their protocols
Study	Design	Applied Protocols	Results	Conclusion
Natural cycle cryo-thaw transfer may improve pregnancy outcomes. Morozov et al., 2007.	Retrospective Cohort	Group 1: Artificial Cycle (n=174) From day 2 of the cycle, estradiol started at 4 mg/day. If endometrial thickness at 13 days was greater than 8 mm, progesterone IM 50 mg/day was introduced and the transfer was done within 3 days. Group 2: Natural Cycle (n = 68) Ovulation monitoring with US and serum LH. Transfer carried out 3 days after. Progesterone 800 mg per day was initiated.	Pregnancies: Group 1: 41 Group 2: 25	The use of hormonal supplementation showed a drop in pregnancy rates compared to the natural cycle.
Use of the natural cycle and vitrification thawed blastocyst transfer results in better in-vitro fertilization outcomes, Chang et al., 2011.	Retrospective Cohort	Group 1: Natural Cycle (n = 310) Monitoring with US. Transfer performed approximately 5 days after ovulation. Initiated progesterone 600mg/day on the day of the transfer. Group 2:Natural Cycle + hCG (n=134) When the endometrium> 8mm and the dominant follicle> 20mm, ovulation was induced by 10,000 IU of hCG. (Results not assessed in the comparison of the present study) Group 3 Artificial Cycle (n=204) From day 3 of the cycle, estradiol valerate started at 4 to 6 mg/day orally. If endometrial thickness at 14 days was not greater than 8 mm, the dose increased to 8 mg per day. If minimal thickness was reached, vaginal progesterone 600 mg per day was initiated.	Pregnancies: Group 1: 130 Group 2: 80 Group 3: 85	There was no significant difference in pregnancy rates between the artificial and the natural cycles.
Natural cycle is superior to hormone replacement therapy cycle for vitrificated-preserved frozen-thawed embryo transfer. Xiao et al., 2012.	Retrospective Cohort	Group 1: Regular Natural Cycle (n = 380) Monitoring with US and serum LH. After ovulation progesterone, IM 40 mg was introduced. After 3 days, the transfer was made. Group 2: Artificial Cycle (n=646)From day 3 of the cycle estradiol valerate was introduced at 2 mg/day for 3 days, at 4 mg for 3 days, and 6 mg from the 10th day onwards. If the endometrium was then trilaminar with a thickness greater than 8 mm, progesterone IM 40 mg/day was started and the transfer was done in 3 days. Group 2: Artificial Cycle (n=646) From day 3 of the cycle estradiol valerate was introduced at 2 mg/day for 3 days, at 4 mg for 3 days, and 6 mg from the 10th day onwards. If the endometrium was then trilaminar with a thickness greater than 8 mm, progesterone IM 40 mg/day was started and the transfer was done in 3 days.	Pregnancies: Group 1: 144 Group 2: 228	The results suggest that natural cycles are superior to hormonal cycles in certain circumstances and in a certain population of patients.
Pregnancy loss after frozen-embryo transfer-a comparison of three protocols. Tomás et al., 2012.	Retrospective Cohort	Group 1: Regular Natural Cycle (n = 1168): Monitoring with US and serum LH. Transfer was performed approximately 3-5 days after ovulation. It was then initiated 600 to 800 mg/day of vaginal progesterone, or Crione 90 mg twice daily on the day of transfer. Group 2: Natural Cycle + hCG (n=444): After 10 days of spontaneous menstruation, when the endometrium was greater than 8mm and the dominant follicle reached 16-17mm in diameter, ovulation was induced by 5,000 IU of hCG, and embryo transferred 5 days later. (Results not assessed in the comparison of the present study)Group 3: Artificial Cycle (n=2858) From day 1 of the cycle, estradiol started at 6 mg per day. If endometrial thickness at 10 days was greater than 7 mm, the transfer was made and vaginal progesterone 600 mg per day was started 4 days earlier. Group 3: Artificial Cycle (n=2858) From day 1 of the cycle, estradiol started at 6 mg per day. If endometrial thickness at 10 days was greater than 7 mm, the transfer was made and vaginal progesterone 600 mg per day was started 4 days earlier.	Pregnancies: Group 1: 248 Group 2: 95 Group 3: 691	A higher pregnancy rate was obtained in the artificial cycles, however, due to the increase of preclinical and clinical pregnancy loss, comparable clinical pregnancy and birth rates are reported for all three protocols.
The artificial cycle method improves the pregnancy outcome in frozen-thawed embryo transfer: a retrospective cohort study Zheng et al., 2015	Retrospective Cohort	Group 1: Regular Natural Cycle (n = 654) Monitoring with USG and serum LH and progesterone until the endometrial thickness is greater than 8 mm, after the transfer 60 mg of progesterone was initiated. Group 2: Artificial Cycle (n=2506) From day 1 of the cycle, estradiol valerate of 2 mg per day was started for 4 days, 4 mg for another 4 days and 6 mg of 9th to 12th day thereafter. If endometrial> 8 mm, progesterone IM 40 mg/day was started and the transfer was done in 4 days. The luteal support was maintained with 60 mg of progesterone.	Pregnancies: Group 1: 323 Group 2: 1469	The study suggests superiority of the hormonal protocol.