After the eggs were retrieved and fertilized with father's sperm in the lab, the embryos are grown up to day 5. On the fifth day, the biopsy for PGD is done on all appropriately developing embryos.
Biopsy involves removing a few cells from the trophectoderm, or the layer of cells that is 'hatching out' of the embryo at this stage of development. I'll come back to "hatching" soon to share some professional opinion. The embryos are stored while genetic material inside the removed cells is tested for abnormalities. Decisions regarding selection of embryos to transfer into the uterus are made with the advice of both the medical and genetics team.