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.