WHAT: Typically, an IVF laboratory grows embryos for about three days. However, with blastocyst transfer, a laboratory would grow embryos for five days.

WHY: For most patients, the best embryos are identifiable at day 3. However, if the best ones are not identifiable at this stage or if a patient has a large number of embryos, clinicians may decide to wait 2 more days and transfer at the blastocyst stage. Waiting until day 5 could provide better information on how well the embryos are developing and help make a better choice for transfer.

HOW: After five days of growth, the cells of the embryo should have divided many times over, and have begun to differentiate by function. The embryos (called blastocysts) that survive to this stage of development are usually strong and healthy.