In natural pregnancy, the oocyte (egg) is fertilized by sperm in the uterine tube and the formed embryo develops for 5 days and is transferred to the uterine cavity, reaching the endometrium in the blastocyst phase. Inside the uterus, the embryo hatches through the outer layer, called the pellucid zone. This hatching is essential for embryonic implantation. The pellucid zone is an important layer in natural pregnancy and classic IVF to prevent the penetration and fertilization of more than one sperm. It is the structure that surrounds the oocyte, also the pre-embryo during the first stages of its development and remains intact during the first cell divisions until the blastocyst stage. In this stage, the internal pressure exerted by the growing embryo, forces the thinning and consequent rupture of the pellucid zone for implantation of the embryo.
Assisted Hatching is a technique performed within embryology laboratories, which helps in the process of opening the pellucid zone (glycoprotein layer around the embryos) to facilitate the embryo’s hatching into the uterus on the day of transfer. It is usually indicated for those couples who are already trying to do IVF without successfully fixing the embryo in the endometrium. One of the reasons that may explain the implantation failure is the fact that many embryos are unable to leave the pellucid zone.
It can also be done when embryologists realize that the embryonic pellucid area is very thick. According to the American Society for Reproductive Medicine (ASRM), the technique increases the rate of implantation and pregnancy for women with:
- Pellucid zone with increased thickness, usually seen in women over 38 years old and high concentrations of FSH;
- Embryos of low morphological quality;
- Repeated implantation failures in other assisted reproduction attempts;
- Transfer of frozen and thawed pre-embryos.
There are some techniques that can be used, such as the laser (a form of energy fired extremely precisely over the pellucid zone, making an opening), currently more common and with better results, acidic (Tyrode) or mechanical solution. The procedure is not indicated for any case and should be discussed with the specialist doctor.