Sometime between days 5 and 8 of early development the human embryo (called "Blastocyst") hatches out of the "zona pellucida" (shell-like structure surrounding the embryo) in order to implant onto the uterine wall and establish pregnancy. Hatching of the blastocyst is an essential process in natural embryo implantation. The "zona pellucida" is made of proteins forming a layer that surrounds the egg and the early embryo sort of like an "eggshell". In order for the embryo to implant into the uterine lining it must "hatch" out of the zona pellucida.

Failure to hatch from the zona might be one reason for failed embryo implantation. A significant increase in the implantation rate of embryos after mechanically weakening the zona just before transfer was first reported in 1989. Selective Assisted Hatching (SAH) is performed by making a small incision in the zona pellucida surrounding the embryo just prior to it's replacement in the womb. Our laboratories use a fast laser technique to make the zona incision. With the laser, exposure of the embryo to sub-optimal conditions outside of their culture environment is minimized. In our laboratories, embryos scheduled for cryopreservation (freezing) also undergo assisted hatching. This is due to the likelihood that the freeze-thaw process may artificially harden the zona pellucida.

There has always been some controversy about which patients might benefit from Assisted Hatching. Some studies have suggested that assisted hatching improves implantation rate in women over 37 years old, women with poor ovarian reserve, women whose embryos exhibit an unusually thick zona and patients whose embryos exhibit significant cell fragmentation. Other studies show no benefit but they have involved in a general patient population. Interestingly, most of the studies showing improved implantation involve specific patient populations such as multiple IVF failures, embryos with sub-optimal morphology, or embryos with thick zonae.

The use of assisted hatching can allow the removal of excess fragments from the embryo or improve the chance of hatching for embryos with thickened zonae. Some data suggests that removal of fragments (embryo defragmentation) may improve the implantation potential. Embryo defragmentation is a delicate procedure which requires a high level of expertise. There is some experimental evidence that in some cases, embryo quality can be restored or improved with defragmentation. Fragments are membrane-bound, non-nucleated pieces of cytoplasm that are lost during early embryo cell division. Fragmentation significantly reduces the developmental potential of human embryos. When roughly 15% to 35% of the total volume of the embryo has been lost to fragmentation and a number of normally appearing blastomeres (embryo cells) are still viable, defragmentation might help restore development potential.

Defragmentation is performed after selective assisted hatching; embryo fragments are removed by extremely gentle aspiration with a specially made micro instrument 10 to 12 µm in diameter. The procedure is performed under high magnification and using a state-of-the-art inverted microscope; the embryo is continually repositioned and observed constantly during the procedure.

Despite many years of experience with Assisted Hatching at multiple fertility centers, the benefit of assisted hatching remains controversial. Nobody can guarantee improved pregnancy rates or that pregnancy will result from using assisted hatching. Furthermore, we cannot assure you that the resulting pregnancies will be normal in course or outcome. Additionally, there is a potential risk for damage to the embryo during the assisted hatching process. Although this situation is very rare, in expert hands, this may occur in less than 1% of cases. Current data indicate that assisted hatching does not appear to affect the overall developmental potential of the embryo. However, the exact likelihood of success for a given embryo or patient cannot be predicted. It is well documented that the implantation rate per embryo rises. This rise in implantation rate may also increase the risk for multiple pregnancy when more than one embryo is transferred. There may be an increase in the occurrence of identical twinning after assisted hatching. Identical twins carry all of the risks of any multiple pregnancy as well as some exclusive to identical twins; these include premature labor and delivery, poor growth of one or both babies, tangling of the umbilical cords leading to intrauterine fetal death, "twin-twin" transfusion leading to discordant growth of the babies.