Embryos are typically cultured three to five days after fertilization at which time they are ready for transfer to the uterus. A blastocyst is an embryo that has developed into two cell types: 1) the trophectoderm which will eventually form the placenta and 2) an inner cell mass that will become the fetus.
Only the most viable embryos will develop to the blastocyst stage. For this reason, fewer blastocysts can be transferred thus lowering the incidence of multiple births while maintaining good pregnancy rates.
Blastocyst transfer is especially beneficial when many fertilized eggs are available in an IVF cycle. As embryos develop, some will fail to progress. If there are only 2-3 embryos available on day three, it is likely that a transfer on that day will include the best possible embryos. If there are a higher number of healthy embryos on day 3, blastocyst culture will allow the best embryos to “select” themselves by forming good blastocysts by day 5.
The developing embryo must “hatch” from its surrounding coating, the zona pellucida. Assisted hatching is a procedure associated with IVF that enhances the ability of the embryo to hatch and embed in the endometrium.
Assisted hatching involves making a small whole in the zona pellucida using a chemical known as Acid Tyrodes or using a laser. It is believed that the opening in the zona pellucida will assist the embryo to more easily hatch and improve the likelihood of implanting into the uterus.
Assisted hatching is typically performed on developing embryos just prior to transfer. Assisted hatching is also routinely done on embryos after they are thawed in a frozen embryo cycle.