Objective (s): one of the best tools to achieve high pregnancy rates is a successful embryo selection. The objective of this study is to assess the embryo grades on day five/six, and the potential of slow developing embryos. Rational: morphological assessment of embryos is a routine practice in IVF lab. Embryo quality is mainly examined by microscope at certain times using morphological scoring systems, and it is still the best strategy of selection of embryos. Design: prospective analysis of single embryo transfer (SET). Setting: study was conducted in Queens Fertility Center, Cairo, Egypt. Subject: women who had fresh single Embryo Transfer (ET) on day five/six, Age 20 – 35 years old were included. Embryo scoring was done using Gardner grading system, besides, 5 grades were added. Only non-compacting cells embryos were excluded. Intervention (s): chromosomal analysis of each grade may add more prediction. Main Outcome Measure (s): clinical pregnancy. Result (s): analysis of single embryo transfer (n=326) revealed positive clinical pregnancy 138 (42.3%). Analysis of difference resulted single embryo with expansion or hatching (EH) stages or 1, 5, or 6 were significantly associated with higher clinical pregnancy rates. Conclusion: This study concluded that, no a specific grade should be excluded except for arrested embryos, as no golden grade, however, prediction of pregnancy might be calculated according to our findings.