Introduction
With the development of assisted reproductive technologies, particularly in embryo culture and cryopreservation, the clinical pregnancy rate (CPR) is increasing along with the multiple pregnancy rate (MPR). Multiple pregnancy is considered the most significant adverse event associated with ART and is linked to an increased risk of maternal and neonatal morbidity. Reducing the number of transferred embryos is a key factor in reducing multiple pregnancies.1 However, this method has not been widely used in clinical practice due to concerns pertaining to the lower pregnancy rate following the reduction of the number of transferred embryos. Compared to cleavage-stage embryo, extending embryo culture to blastocyst allows for better evaluation of the implantation potential of embryo, and the implantation rate (IR) of blastocyst is higher.2-4Therefore, selective single top-quality blastocyst transfer should serve as a practical strategy in reducing the number of transferred embryos while maintaining CPR. Previous findings have suggested that comparable CPR could be achieved in a clinical setting when utilizing elective single blastocyst transfer in regard to patients with a good prognosis. Although live birth rate (LBR) equivalence was not demonstrated, it was thought the additional complications associated with multiple gestations outweighed the potentially higher LBR.5 Moreover, frozen single blastocyst transfer resulted in a higher CPR compared to that of fresh single blastocyst transfer in ovulatory women with a good prognosis.6 In advanced female age (≥ 40 years), single blastocyst transfer and double blastocyst transfer achieved similar CPR and LBR, while MPR was lower in the single blastocyst group.7 Although research on blastocyst transfer has been on the rise recently, these studies did not comprehensively analyze the effects of number and quality of transferred blastocysts as well as female age on pregnancy and clinical outcomes. In the present study, the effects of number and quality of transferred blastocysts and female age on clinical outcomes are compared in order to determine a more appropriate blastocyst transfer policy to decrease MPR while maintaining LBR.