A human blastocyst (left side of picture) hatching from its shell (right side)
The human egg is surrounded by a protein shell called the "zona pellucida". This shell has several important functions. Most importantly, it allows only one sperm through to fertilize the egg. After fertilization, the egg divides into 2 cells and this surrounding shell prevents these 2 cells from becoming separated. Each of the 2 cells divide again, giving the embryo a total 4 cells approximately 48 hours after the egg retrieval. By 72 hours, the embryo can have 8 cells. As the embryo continues to through these rounds of cell division, the zona keeps the cells together. Embryos are usually transferred to the uterus when they have between 4 and 8 cells (2-3 days after retrieval).
After the embryo has been transferred, around Day 5, it has grown to the Blastocyst stage of development. At this stage the embryo must hatch out from its shell in order to implant in the uterus. If the embryo fails to hatch, it cannot implant.
Assisted hatching is a technique developed to encourage the embryo to hatch from its shell. It has been in use for about 15 years.
Assisted hatching is performed in the laboratory just prior to embryo transfer. It is a simple and precautionary procedure where the Embryologist makes a small hole in the outside shell (the zona) of each embryo to be transferred. Because the zona is not a living part of the embryo, making this hole does not harm the embryo. In fact, this facilitates the embryo in hatching from the zona once transferred into the uterus. A more advanced, less invasive process using laser technology makes the hole. Prior to laser technology this opening was made with an Acidified Solution, which slowly dissolved away part of the shell until a small opening was achieved. Now with the laser, a beam of light creates a much more precise opening in the shell. This procedure can be performed much more quickly; therefore reducing the amount of time an embryo is being handled. Traditionally, Assisted Hatching took about 5 minutes, with the laser this time is reduced to less than 1 minute.