Clinical Outcomes of Frozen-Thawed Embryo Transfer in Natural Cycles with Spontaneous or Induced Ovulation: a Retrospective Cohort Study from 1937 Cycles.

Our objective was to assess whether there is a difference in the pregnancy outcomes in the natural cycle (NC) with spontaneous LH rise compared with modified natural cycle controlled by hCG for final oocyte maturation and ovulation after frozen-thawed embryo transfer (FET). In this retrospective cohort study, we analyzed the clinical outcomes of a total of 1937 patients undergoing FET followed by endometrial preparation with the natural cycle and modified natural cycle. The primary outcome was live birth, and secondary outcomes included miscarriage rate, clinical pregnancy rate, preterm birth rate, and ectopic pregnancy rate. The type of endometrial preparation did not impact live birth (adjusted odds ratio [aOR] 0.92; 95% confidence interval [CI], 0.69-1.23), miscarriage (aOR 0.83; 95%CI, 0.50-1.39), clinical pregnancy (aOR 0.88; 95%CI, 0.66-1.18), preterm birth (aOR 0.91; 95%CI, 0.56-1.50), or ectopic pregnancy (aOR 1.06; 95%CI, 0.29-3.94). In conclusion, in women undergoing FET, natural cycles and modified natural cycles resulted in comparable clinical outcomes.