Frozen-thawed blastocyst transfer in natural cycle increase implantation rates compared artificial cycle

Objective: To analyze global outcomes in frozen-thawed embryo transfer according to endometrial preparation with natural cycle and artificial cycle taking into account the developmental stage of the embryo.Methods: Retrospective cohort study, held in a tertiary-care university hospital with 1265 cycles for frozen-thawed embryo transfer of 860 patients, performed between January 2014 and December 2017. A total of 1097 embryo transfers were performed: 163 transfers in natural cycle (N), 531 in artificial cycle: transdermal estrogens and 403 with oral estrogens.Results: Demographical factors were similar in the three groups studied. When comparing the number of embryos transferred, the quality and the stage of embryos within the three groups there were no differences. Implantation rates were significantly higher in natural cycle. When adjusted depending on the developmental stage, blastocyst embryos transferred in natural cycle preparation had the highest implantation rate. Multiple pregnancies were also higher in natural cycle preparation. There were no significant difference between the groups in the clinical pregnancy rate, ongoing pregnancy, live birth, and miscarriages.Conclusions: Our results suggest that a natural cycle endometrial preparation assessing the disappearance of the dominant follicle by ultrasound increases implantation rates, especially when transferring blastocysts.