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 selected cases of implantation failure, assisted hatching may be considered as part of an assisted reproduction treatment. This laboratory technique involves creating a small opening in the embryo's outer layer, known as the zona pellucida, to facilitate the natural hatching process before implantation.
The zona pellucida is a protective outer layer that surrounds the oocyte and the early embryo. At the blastocyst stage, the embryo normally hatches from the zona pellucida before implantation 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 protects the early embryo during its initial stages of development until it reaches the blastocyst stage and begins to hatch.
As the blastocyst develops, a fluid-filled cavity called the blastocoel expands, causing the embryo to increase in size and the zona pellucida to become progressively thinner. The blastocyst can then begin to emerge through an opening in the zona pellucida, a process known as hatching.
In some cases, characteristics of the zona pellucida may make the hatching process more difficult. Assisted hatching may therefore be considered in selected patients, depending on their individual circumstances and treatment history.
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.
Micromanipulation of the zona pellucida is also used in other assisted reproduction procedures, such as ICSI (intracytoplasmic sperm injection) and preimplantation genetic testing (PGT). In ICSI, a micropipette passes through the zona pellucida to inject a single sperm directly into the oocyte, while in PGT, manipulation of the zona pellucida can facilitate embryo biopsy for genetic analysis. These procedures have different purposes from assisted hatching.
Depending on the stage of development, the appearance of the embryo and its relationship with the zona pellucida, different stages of blastocyst expansion and hatching can be observed:
The blastocyst has increased in size as the blastocoel expands, causing the zona pellucida to become progressively thinner. At this stage, the embryo is preparing for the hatching process.
An opening has formed in the zona pellucida and the blastocyst has begun to emerge through it. This is an intermediate stage in which part of the embryo remains inside the zona pellucida.
The blastocyst has completely emerged from the zona pellucida and is no longer surrounded by this protective layer. It can then come into direct contact with the endometrium, an essential step for implantation to begin.
Assisted hatching may be considered in selected cases, including:
Assisted hatching is not routinely performed on all patients, only in specific cases. In selected cases, assisted hatching may help facilitate the embryo's natural hatching process when characteristics of the zona pellucida could make this process more difficult.
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.