Hatching is the natural process where the blastocyst (embryo on the fifth or sixth day of development) frees itself from the zona pellucida, initiating implantation that leads to pregnancy.
Embryo implantation is one of the most important steps in assisted reproduction treatments. In many cases, when all possible factors have been ruled out and the embryo has been studied but implantation is not achieved, assisted hatching is recommended. This technique helps remove the presence of a thickened outer membrane (called the zona pellucida), which can hinder the embryo from breaking out and prevent its cellular adhesion to the uterus.
The zona pellucida is the outer layer of the embryo, released between days 5 and 7 of development (in the blastocyst stage) to implant in the endometrium.
This membrane acts as a protective barrier that allows only one sperm to enter for fertilisation, then it changes to prevent other sperm from penetrating the egg. It also helps control the embryo's initial cell divisions until it reaches the blastocyst stage.
The embryo must apply pressure to tear the zona pellucida. It forms a cavity called the blastocoel, which gives the blastocyst its name. This cavity fills with fluid, expands, and puts pressure on the zona pellucida until it rips, allowing the embryo to emerge and implant in the uterus.
However, for some women, especially those over 35, this layer can be too thick, preventing the embryo from hatching on its own and resulting in a failure to achieve pregnancy.
Assisted hatching is a lab technique that complements in vitro fertilisation (IVF). It involves creating a small opening in the zona pellucida of the embryo before embryo transfer, making implantation easier.
This procedure is also used in ICSI (intracytoplasmic sperm injection) to inject sperm into the egg, and for reimplantation genetic testing (PGT), where a small opening in the zona pellucida allows us to extract a cell and analyse its genetic content.
Depending on the day of development, the state of the zona pellucida, and the appearance of the embryo, we can distinguish between three types of blastocysts:
The embryo is large, and the zona pellucida is thinner.
The zona pellucida is breaking, and the embryo is starting to emerge.
The embryo has completely emerged from the zona pellucida.
Assisted hatching is commonly recommended in the following situations:
Assisted hatching is not routinely performed on all patients, only in specific cases. Thanks to assisted hatching, we can overcome one of the most important barriers in cases of repeated implantation failure or when working with frozen embryos: the embryo's inability to hatch naturally.
Typically, assisted hatching is done on the third day after fertilisation, when the embryo has about eight cells and has not increased in size. There are three possible techniques:

Since it is a complex technique, all three methods require the use of an inverted microscope with micromanipulation equipment to ensure precision and safety.