Results

The demographic characteristics of the patients/cycles included in the study are presented in Table I. The variables were compared using Student’s t-test, and despite the infertility time, the number of total and metaphase II oocytes collected had statistically significant differences, the groups were clinically comparable.

Clinical outcomes

For the first FET, we compared the clinical outcomes of the eDET and eSET groups (Table II). The eDET group had a significantly higher multiple PR and a significantly lower implantation rate, as expected, and the ongoing PRs were similar between the two groups. Conversely, when a second SET was performed in patients who did not become pregnant in the first frozen-thawed eSET (eSET + SET, n = 60), 16 ongoing pregnancies (26.7%) were achieved in this group. Thus, the estimated cumulative ongoing PRs of the eSET + SET subgroup were calculated according to the previously described formula, and became significantly higher from those of the eDET group (Figure 2).
The multiple linear regression model evaluated the association of transferring two embryos in two sequential transfers (eSET + SET), compared with the eDET protocol, with the chance of embryo implantation. The model was adjusted for confounders such as the women’s age and the number of cryopreserved embryos. We obtained a statistically significant model, in which the transfer of two embryos in two sequential transfers (eSET + SET) led to a significantly higher chance of implantation than did the transfer of two embryos together (eDET) in freeze-only cycles (coefficient: 0.142, p < 0.001), adjusted for women’s age and number of cryopreserved embryos available (Table III).