Modified natural protocol seems superior to natural and artificial protocols for preparing the endometrium in frozen embryo transfer cycles.

Background: Although several studies claim higher success rates in natural cycle (NC) and modified natural cycle (mNC) protocols, currently, there is no consensus on the most effective method of endometrium preparation prior to frozen embryo transfer (FET) cycles. We aimed to find out the best protocol by comparing three different protocols for preparing the endometrium in FET cycles.

Methods: This retrospective cohort study was conducted at a private in-vitro fertilization (IVF) center. Medical records of all patients enrolled in frozen embryo transfer cycles between November 2017 and February 2019 were reviewed. Group I (N.=94) included patients who underwent artificial endometrial preparation (AC), group II (N.=23) confined patients enrolled in mNC and group III (N.=12) included patients who had NC protocol. Main outcome parameters were clinical pregnancy rates, implantation rates and miscarriage rates.

Results: There was a tendency towards higher clinical pregnancy rate in mNC group. Clinical pregnancy rates of the three groups were 54.3%, 65.2% and 33.3% respectively (P=0.199). Implantation rate was significantly higher in group II (34%, 50% and 12% respectively, P=0.006). Miscarriage rates were similar for the three groups.

Conclusions: Although not reaching a statistically significant level, there is a tendency towards higher implantation rate and pregnancy rate in mNC protocol compared to true NC and AC protocols.