The study looked at how the quality and speed of development of embryos affect pregnancy outcomes in frozen embryo transfer cycles. They analyzed over 2,000 cycles and found that for poor-quality embryos on day 5, transferring a single embryo led to better pregnancy outcomes and fewer multiple pregnancies compared to transferring two embryos. However, for poor-quality embryos on day 6, transferring two embryos was better for pregnancy success. The type of inner cell mass or trophectoderm in the embryo did not affect pregnancy outcomes. So, for day 5 poor-quality embryos, transferring one is best, and for day 6 poor-quality embryos, transferring two is better.