To evaluate the relationship between blastocyst quality and embryo scoring on day 3 and compaction on day 4. Analysis of an RCT in 749 patients undergoing COS with highly purified menotropin (HP-hMG, MENOPUR, Ferring Pharmaceuticals) or rFSH (FOLLISTIM, Merck) in a GnRH antagonist cycle [MEGASET; ClinicalTrials.gov NCT00884221]. All oocytes retrieved (N = 7,156) were cultured and MII oocytes were inseminated via ICSI. Oocyte fertilization was evaluated on day 1 and embryos with 2pn continued culture. Embryo development was assessed daily to day 5 after retrieval. Overall embryo score on day 3 was classified as top-quality, good-quality or minimum-quality. Embryo compaction was evaluated on day 4. Overall blastocyst score on day 5 was classified as excellent-quality (expansion and hatching status 4, 5 or 6, inner cell mass A and trophectoderm A or B) or good-quality (expansion and hatching status 3, 4, 5 or 6, inner cell mass A or B and trophectoderm A or B). Oocyte maturation and fertilization as well as embryo quality on day 3, 4 and 5 after retrieval were generally similar in the HP-hMG and rFSH groups, and data were therefore combined for this analysis. The embryo development status on days 3 and 4 versus day 5 is presented in the table.Tabled 1Blastocyst quality on day 5Excellent-qualityGood-qualityOther qualityNot blastocystEmbryo quality on day 3 Top (n = 1,316)22%24%27%27% Good (n = 1,424)10%14%28%47% Minimum (n = 444)<1%6%17%76%Compaction status on day 4 Full (n = 1,558)27%26%26%21% Partial (n = 870)8%17%35%40% None (n = 1,697)<1%3%15%82% Open table in a new tab Top-quality embryos on day 3 and fully compacted embryos on day 4 both have similar likelihood of ultimate blastocyst quality categorization on day 5. Embryos of lower quality on day 3 or lack of full compaction on day 4 are likely to develop into blastocysts of a lower quality or not develop further at all.