The aim of this study is to evaluate the impact of the biopsy of one or two cells in a preimplantatation genetic diagnosis program (PGD) for the screening of aneuploidies. This prospective and randomized study will be performed at the Instituto Valenciano de Infertilidad in Valencia (Spain) in those patientes included in our clinical program. Our purpose is to optimize the methodology employed in our laboratory in order to guarantee optimal implantation and pregnancy rates without a detrimental effect on the accuracy and efficiency of the cytogenetic analysis. Patients will undergo and IVF cycle, embryo biopsy will be performed on day 3 embryos in two fashions: in some patients only one cell per embryo will be retrieved whereas in others, 2 cells will be retrieved in good morphology embryos and only 1 in the slowl ones. In all cases aneuploidy screening will be performed by FISH for chromosomes 13, 16, 18, 21, 22, X and Y. Embryo developmente will be checked every 24 hours and chormosomally normal embryos will be replaced into the uterus on day 5 of development.