Pregnancy rates from natural and artificial cycles of women submitted to frozen embryo transfers: a metanalysis

OBJECTIVE: To compare pregnancy rates from natural and artificial cycles of women submitted to frozen embryo transfers. METHODS: A systematic review was performed by PubMed search using the following algorithm: (endometrial [All Fields] AND preparation [All Fields]) AND (("freezing"[MeSH Terms] OR "freezing"[All Fields] OR "frozen"[All Fields]) AND thawed [All Fields]) and (natural cycles) AND (artificial cycle). Inclusion criteria: prospective and retrospective cohort studies. Exclusion criteria: use of hCG in the natural cycle, oocyte donors, and use of disused freezing techniques. Data were analyzed with the SPSS v.23 software and with a significance level of 5%. The meta-analysis was performed with RevMan 5.3 software. I² was calculated. RESULTS: 709 papers were retrieved. Five studies fulfilled the inclusion and exclusion criteria. From these studies, we analyzed 8,968 natural or artificial cycles. A contingency table compared the results of the natural and artificial cycles and the number of clinical pregnancies obtained in each selected paper. The I(2) test resulted in high statistical heterogeneity (I(2)=77%). Studies by Morozov et al. (2007) and Zheng et al. (2015) obtained statistically significant results (p<0.03 and p<0.001): Morozov et al. (2007) found a higher pregnancy rate within natural cycles, and Zheng et al. (2015) found more positive outcomes when analyzing artificial cycles. The remaining selected studies did not show any statistical significance. CONCLUSION: There is insufficient scientific evidence to state that the artificial cycle yields better pregnancy rates than the natural cycle in women submitted to frozen embryo transfer. Limitations of this study include a small number of papers and heterogeneity among the studies.