Findings in relation to existing literature
Our eSBT patients had a live birth rate per fresh cycle (52.1%) similar to that reported after eSBT in younger women,26 which might imply that once the blastocysts-stage was achieved, pregnancy outcomes were less effected by maternal age.27,28 In this regard, despite their age, a subset of these women can still have embryos cultured to blastocyst-stage and can benefit from policy of eSET.
The finding in the present study differs from some previous studies suggesting that eSET can result in a significant lower ongoing pregnancy rate compared with DET (21.4 versus 40.3%) in unselected IVF patients.29 Nevertheless, only cleavage-stage transfer was included in their study, which may have limited their results. The better outcomes in our work probably reflects marked improvements in IVF over the past decade. Extended culture to blastocyst-stage transfer enables more sophisticated assessment of embryo morphology, with better selection of the embryos more likely to success. Similar to the present study, Davis et al 30 recruited 45 patients older than 35s undergoing eSBT in a small retrospective study. Of them, twenty-three patients (51.1%) have an ongoing pregnancy or live delivery, demonstrating a clear role for SET in this relatively older IVF population. Another retrospective study also confirmed the feasibility of eSBT in women with a narrow maternal age (40-43 years).31
Implications for clinical practice
A growing number of women seek IVF over the age of 35 years,32 specifically as the two-child policy was fully implemented in China, ART has been a beneficial complementary technology and health service for couples with advanced age. Practical questions are encountered: if transferring one embryo at a time is associated with a diminished likelihood of a live birth in women over 35 years? If the woman had two embryos transferred, what is the probability of twin delivering and the potential risks? Providing precise information to answer these questions would help couples to decide how many embryos to transfer.
To further identify the role of blastocyst quality in practice of eSBT, we stratified our analysis into good- and fair-quality blastocyst transfer. In older women with at least one good-quality blastocyst, the live birth rate was similar between the two cohorts. However, transferring two blastocysts resulted in a large reduction in implantation rate (from 67.9% to 51.0%) and a great increase in multiple birth rate (from 1.2% to 44.1%). This observation supports the hypothesis of embryo-endometrial crosstalk that endometrium may act like a sensor of embryo quality preventing the implantation or sustainment of a low-quality or abnormal embryo.33When there was no good-quality blastocyst available, the strategy of eSBT was also feasible, since additional fair-quality blastocyst did not increase the rate of implantation or live birth. These data suggested that in case of two blastocysts were obtained, eSBT should be encouraged, irrespective of blastocyst quality.34
For women aged over 36, transfer of two blastocysts after initial fresh IVF/ICSI cycle, the most statistically significant risks are higher rates of low birth weight and preterm labor. Women with DBT are also at higher risk for cesarean delivery, though the odds are less statistically significant. In 2010, a study in Sweden involving more than 25,000 women receiving IVF, showed that the risk of neonatal death and morbidity was significantly reduced following the SET policy.35 Concerned with the worse perinatal outcomes, there is an urgent need to limit multiple births by reducing embryo transfer from two to one.36