Assisted Hatching: When is it necessary?

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Assisted hatching is often offered as part of an IVF process. It’s a procedure and as such has its risks. Let’s explore why you may want to do this.

The process of assisted hatching refers to procedures done to the zona pellucida. The zona pellucida is the shell that surrounds the egg. Its clearest purpose in nature is to become suddenly hardened once one sperm has entered the egg. This prevents other sperm from entering the egg.

But this hardening of the egg shell can happen at other times too. For some women with egg quality concerns, perhaps women over thirty-seven years, the zona pellucida is hard from the beginning and this otherwise fertile egg will not be able to be fertilized naturally.

Under these circumstances, ICSI (intracytoplasmic sperm injection, the placement of sperm inside the egg) should help. But it still leaves a very hardened zona pellucida behind, and this can cause problems later in development.

By day five of embryo development, there are enough cells within the embryo that it is generally at the “blastocyst” stage, and the embryo attempts to hatch out of the zona pellucida. This generally goes without incident, but if the zona pellucida is too hard and too thick, the hatching may not occur and implantation will be compromised.

Assisted hatching is the process by which, generally on day three of embryo development, the integrity of the zona pellucida is compromised to weaken it to allow for the embryo to hatch more easily later on.

Historically, this was done chemically, but the problem with this approach was that the chemicals could sometimes leach further than the zona pellucida, damaging the embryo itself.

Modern technique is to use a laser, which can be so finely calibrated, that we routinely see no compromise at all. For that reason, assisted hatching can be offered with relative safety, for the downside risks appear to be small. There have been some worries that, even with a laser, there could be some residual damage. What if, for example, the sharpened edges associated with the laser would somehow damage an embryo that is hatching, perhaps severing the embryo in two? Were that to be the case, one would anticipate increased risk of monozygotic twinning (identical twins) associated with assisted hatching. People have looked to this in great detail with no definitive answers.

In our practice, we do see a heightened chance of monozygotic twinning compared to the background rate that might be expected in the general population, but have been unable to ascribe this to assisted hatching (or, in fact, to any particular event that occurs in the lab. It is possible that a subset of fertility patients is simply more at risk of monozygotic twinning than the background population).

It is not just women with egg-quality concerns that it may be beneficial for. For all embryos that are being cryopreserved, the zona pellucida is expected to come out harder than on a fresh embryo. Any couple with these circumstances may benefit from assisted hatching.

Furthermore, assisted hatching is helpful when doing pre-implantation genetic screening; it allows the biopsy catheter to access the cells to be assessed.

Does all of this mean that assisted hatching is right for you? That is a conversation for you and your physician, but assisted hatching is something that we are willing to consider for the majority of our patients at our clinic who are looking to IVF.

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