To compare the clinical outcomes of modified natural cycle (mNC) and artificial cycle (AC) protocols for frozen embryo transfers.

A total of 490 frozen–thawed autologous embryo transfer cycles, performed in a single tertiary IVF center, between January 2015 and September 2017, were retrospectively analyzed. Of these, 214 cycles were performed after mNC and 276 cycles were performed after gonadotrophin-releasing hormone (GnRH) agonist plus sequential estrogen and progestin priming protocol. The primary outcome was live birth and secondary outcomes were clinical pregnancy, implantation and miscarriage rates. Multivariate regression analysis was used to adjust covariates on clinical outcome.

The rates of live birth (33.6 % vs. 29.3 %, respectively), clinical pregnancy (40.2 % vs. 36.6 %, respectively), implantation (32.3 % vs. 28.5 %, respectively), and miscarriage (5.1 % vs. 6.9 %, respectively) were not different between the mNC and AC groups. Multivariate analysis also showed that the method for endometrial preparation had no significant effect on clinical pregnancy and live birth. The adjusted odds ratios (OR) of live births and clinical pregnancies were 0.97 (95 % CI 0.64–1.48) and 0.98 (95 % CI 0.65–1.46) for the AC compared to mNC group. However, there was a significant difference between mNC and AC in cycles in which double embryo transfer was performed. The live birth (48 % vs. 31.4 %P= 0.01) and clinical pregnancy rates (53.9 % vs. 38.8 %, P= 0.02) were significantly higher in the mNC group than the AC group for double embryo transfers.

The live birth and clinical pregnancy rates are comparable between mNC and AC with GnRH agonists in frozen thawed embryo transfer cycles. In ovulatory patients with planned double embryo transfer, mNC can be considered. Further well-designed prospective studies are needed to confirm our results.