There is no easy answer to this; it has many ramifications and is something that needs to be discussed with your doctor to understand all the implications.
The appearance of the blastocyst has nothing to do with the chromosomal make-up. In fact, aneuploid embryos sometime make the most beautiful blastocysts.
What is possible is that the embryo is a mosaic and this is not the predominant cell line. That's one of the problems with a Day 3 biopsy; you are testing just one cell of an eight cell embryo.
You will have to rely on your doctor's advice on how to proceed here.
Arthur L. Wisot, M. D.
Reproductive Partners Medical Group
Redondo Beach, California