The “best” embryo for transfer is typically selected based on morphology (grading) and/or preimplantation genetic screening (PGS).  Morphologic grading of cleavage-stage embryos (Day 2 and 3 embryos) involves the cell count and “fragmentation” rate of cells. Morphologic grading of blastocyst-stage embryos (Day 5 and 6 embryos) involves assessment of a group of cells that line the periphery of the embryo (called the “trophectoderm”) and a group of cells that are concentrated in a corner of the embryo (called the “inner cell mass”). Based on morphologic grading, the most advanced, well-graded embryo(s) are selected for transfer to the uterus.  If PGS is opted for, an embryo biopsy procedure is performed by removing cells from the trophectoderm of blastocyst-stage embryos, freezing (via vitrification, which is a fast-freeze method of cryopreservation) the biopsied blastocysts, and sending the biopsied cells to a reference lab for chromosome analysis which can aid in selection of the “best” embryo, based on chromosome analysis.  The combination of the embryo with the highest graded morphology, along with a normal chromosome complement, is the ideal embryo for transfer to the uterus.  Time-lapse monitoring (TLM) is a relatively new technology that is currently being studied and may prove to have value in the future.