Assisted hatching can be performed using two different methods, namely chemical hatching or laser assisted hatching.
A covering layer or ‘shell’ surrounds embryos or blastocysts called the Zona Pellucida (ZP). The zona has an important role in fertilisation as it allows only one sperm to penetrate the zona and enter the egg to achieve fertilisation. It also acts to prevent premature implantation in the Fallopian tube and may help prevent the early embryo from being attacked by cells of the immune system.
The embryos have to hatch or break out of the zona so that they can embed into the endometrium lining the uterine cavity. This occurs about 4 to 5 days after embryo transfer when the embryo is at the blastocyst stage. Naturally this takes place by expanding/contracting of the zona until it distorts, allowing the blastocyst to hatch.
If the zona is not functional, this hatching may not occur. It has been reported that up to 75% of normal embryos never hatch through the protective layering of the zona. Laboratory techniques involved in IVF may result in hardening of the zona. In natural fertilsation there are enzymes present within the fluid in the Fallopian tube, which may soften the zona. This does not happen in IVF as the tube is bypassed. The zona may also be thicker following IVF, especially in older women. Frozen embryos may also have a hardened zona.
Chemical hatching involves chemicals (acid tyrodes) used to produce a weakened area of the zona. The chemical is applied through a microtool which is manipulated by an embryologist. It is not entirely accurate though, because when working with such small amounts of chemicals, it is hard to quantify the dose with pinpoint accuracy. The acid then has to be washed off in order to prevent further damage to the embryo.
Laser Assisted Hatching
The zona is drilled by the microlaser system called Fertilase. It creates a clean-cut precise incision in the zona. This method is more accurate for several reasons. Laser assisted hatching is fast in comparison with the chemical method, thus the embryo does not have to be held still by a suction pipette. Another reason why it is more accurate is due to the fact that we use a vibration free table to avoid any accidental movement of the embryo. The accuracy of the laser is predetermined by its programming, and is therefore not dependent on the skill of the operator. There is a high degree of reproducible accuracy with laser assisted hatching.