The ovarian stimulation protocol and IVF-ET procedures were as previously described (Hu et al., 2020 (link)). Fertilization was performed by conventional IVF or by ICSI. Embryos were vitrified on Days 5–6 of embryo culture. The procedure of embryo vitrification and thaw in our centre was described in previous studies (Zheng et al., 2017 (link); Hu et al., 2020 (link)). Grading of blastocyst morphology was performed before vitrification on Day 5 or Day 6 of embryo culture according to the Gardner and Schoolcraft’s system which has been used in our centre since 2008 (Gardner et al., 2000 (link), 2004 (link)). The detailed scoring system is included in Supplementary Table SI. Blastocysts were graded according to three separate quality scores: the development stage status of the blastocyst (1–6), the grade of ICM (A, B, C), and the grade of TE (A, B, C). Blastocysts with stage status higher than 2, ICM grade higher than C and TE grade higher than C (≥3BB) were considered as good quality and blastocysts with grading lower than 3BB were considered as poor quality (Gardner et al., 2004 (link); Oron et al., 2014 (link)). In most cases, lower grade blastocysts were only used when no higher grades ones were available and women were fully informed of the risks associated with the transfer of a lower grade blastocyst and provided their consent before the embryo transfer. The blastocyst transfers were performed on Day 5 after ovulation in natural or ovarian stimulated cycles, or after 7 days of progesterone supplementation in artificial hormonal replacement cycles. The warmed blastocysts were cultured for 30–120 min to evaluate their quality and blastocysts with good survival signs (less than half of the blastomeres showing signs of damage) were transferred.