Developmental ability of chromosomally abnormal human embryos to develop to the blastocyst stage

BACKGROUND: A correlation between morphology, developmental competence and chromosome abnormalities is established. However, since absolute correlatio ns are rare, embryo selection remains one of the most arduous tasks in assi sted reproduction. This study was undertaken in order to determine which ch romosomal abnormalities are compatible with development to the blastocyst s tage. METHODS: Embryos diagnosed by preimplantation genetic diagnosis (PGD) as chromosomally abnormal or unsuitable for transfer were cultured to day 5 or 6. Morphology and development were observed daily. After extended cult ure, embryos were fixed and analysed by two rounds of FISH with the same pr obes used for PGD. RESULTS: Some types of numerical chromosome abnormalitie s do not preclude full differentiation in vitro. For instance, extensive mo saicism was detected in blastocysts; and trisomic embryos reached the blast ocyst stage with a frequency of 37%. Interestingly, only those monosomies c ompatible with first trimester development (monosomy X and 21) were detecte d at blastocyst stage. CONCLUSION: Even though there is a strong selection against chromosomally abnormal embryos, extended culture to day 5 or 6 cann ot be used as a reliable tool to select against clinically relevant chromos ome abnormalities such as trisomies.