Introduction
Although there has been a gradual decrease in multiple births after assisted reproductive technology (ART) across the world, that decrease is mostly in developed countries.1Twin delivery rates in China were 27.9% and 27.2% after in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) respectively,2 which are higher than that of many other countries. Elective single embryo transfer (eSET) has been appeared to be the most effective approach to reduce multiple births associated with IVF/ICSI.3-5 eSET policy has primarily been recommended in women of younger reproductive age with a good prognosis, which is not routinely offered to women aged beyond 35 years old.6-8 There is fear that the adoption of eSET in unselected patients will lower pregnancy rates per embryo transfer and extend the time needed to achieve live birth.
Advanced maternal age is associated with a decreased chance of pregnancy after IVF and a higher risk of fetal loss, but older women are still at risk for multiples.9,10 Increasing age aggravates the risks of unfavorable obstetrics outcomes (e.g. preterm delivery, low birth weight and perinatal mortality), and this is further so in cases of multiple pregnancy.11,12 However, patients at advanced age and physicians are still hesitant to embrace eSBT and wish to maximize their chance of pregnancy with the transfer of more than one embryo. Several trials that did recruit older women aged over 36 to make such a comparison possible.13,14 These two studies, however, were restricted to cleavage-embryo transfer, impact of blastocysts transfer on the implications of the policy in that age group has been fully studied.
Aneuploid embryos, often age dependent, are less likely to continue development to the blastocyst stage,15 hence blastocyst culture theoretically would assist the selection of the most viable embryo. A number of studies have demonstrated significant differences in implantation and pregnancy rates in favor of extending the embryo culture to blastocyst-stage transfer.16-18
As extending embryo culture has been a routine practice in many clinics, it remains to be proven if it is appropriate to consider the transfer of one blastocyst-stage embryo for women at advanced age. We carried out the retrospective cohort study to compare the effectiveness of eSBT versus DBT on the cumulative live birth rates (cLBR) (fresh and frozen transfers accruing from a single oocyte retrieval) and perinatal outcomes. We have also assessed the applicability of eSBT by exploring whether its practice is associated with extending the time to live birth (TTLB).