A protein coat called the zona pellucida surrounding eggs and embryos helps to regulate fertilization by preventing more than one sperm from penetrating the egg. It also acts as protection during the early stages of development.
However, to implant into the endometrium (the lining of the womb) and so establish a pregnancy, the embryo must first shed this coat, which it does usually on day six to seven of development naturally – this process is called hatching. At this point, the embryo is a hollow ball of cells called a blastocyst.
Assisted conception treatments most commonly fail after embryo transfer and this may be because the transferred embryos fail to hatch, perhaps because the zona is affected by cryopreservation (freezing) or in vitro lab culture.
Benefits of assisted hatching
To counteract this, assisted hatching aims to improve the chances of successful hatching by creating a hole in the zona pellucida. Views remain mixed as to whether the technique actually improves the chances of pregnancy, but most agree that it may be helpful in selected groups of patients.
The most commonly used method of assisted hatching is where fine glass pipette is used to blow weak acidic medium on to the zona. This thins an area before a hole is punched through the centre of the thinned section. The procedure is carried out on day three of embryo development, when the embryos have started their third cell cycle. By this time sufficient cell-to-cell bonding has taken place to minimize the loss of cells through the hole that’s been created.
Assisted hatching may be suitable for:
- Women who are over 39 years old and are using their own eggs
- Women who have had two or more embryo transfers without a pregnancy
- Women who have elevated follicle stimulating hormone levels (FSH)
- Women whose embryos appear to have unusually thickened zona