Strengths and limitations
The main strength of this study is in that the female age and quality of blastocysts was taken into account which could introduce bias into results if not adjusted for. Since adverse perinatal outcomes tend to become more common with increasing maternal age, we also assessed the potential effect of eSBT on incidence of preterm birth, low birth weight, and congenital anomalies. Finally, all cycles were performed at the same center, the study population has the advantage of being very homogeneous concerning blastocyst assessment and transfer policy.
Limitations of the present study are its retrospective design, including potential biases in patient selection. Despite that the women received eSBT were chosen much less selectively, good prognosis women in eSET cohort whereas less good prognosis women in DBT group, as evident by having a lower rank of previous IVF cycles. In addition, analysis restricted to individuals with at least two viable blastocysts and may not apply to patients with mandatory SBT. It should be noted that women who intended to proceed to blastocyst transfer but failed to have an embryo replaced on day 5 were not included in the present study and we do not practice a mandatory policy for offering all older patients an SBT. We did not assess the direct cost of the IVF treatment itself or the overall costs associated with multiple births. Cost analyses performed in European countries have been in favor of strategy of eSET.24,25 A prospective study including health economic analysis has been initiated in our center. Further research on the policy of SBT in women of advanced reproductive age is needed.