Assisted Hatching (AH)

Introduction

An embryo needs to break out or hatch from a gel-like shell surrounding it in order to attach to the wall of the womb (implantation). This shell is known as “zona pellucida”. It may be harder is a few embryos than others. Thickness of zona pellucida appears to get influenced by women’s age, profile of the FSH (Follicle stimulating hormone) during proliferative phase. This thickness inversely correlates with the rate of embryo implantation.

Failure of implantation after in vitro fertilization (famously known as IVF) may result from the inability of the embryo to hatch out of the zona pellucida. Artificial disruption of the zona pellucida to enhance the ability of the embryo to hatch has been proposed as a method for improving the success of IVF. Assisted hatching is a fertility procedure in which the doctor induces the embryo to hatch by causing the shell/ zona pellucida thinner. In certain instances, a small aperture is made in the embryonic shell to aid hatching. This technique reportedly increases rate of implantation significantly.

What is Assisted Hatching

A protein layer called the zona pellucida surrounding eggs and embryos helps to regulate fertilization by preventing more than one sperm from penetrating the egg. It also acts as protection during the early stages of development of the embryo.

However, to implant into the lining of the womb (the endometrium) and to establish a pregnancy, the embryo must first drop this shell, which happens in six to seven days of development. This whole process is called hatching. At this point, the embryo is a hollow ball of cells called a blastocyst. Cultured embryos also hatch and implant at lower rates than natural. Treatments of assisted conception most commonly fail after embryo transfer and this occurs because the transferred embryos fail to hatch, mainly because the zona pellucida is affected by cryopreservation (freezing) or in vitro lab culture.

Under these conditions, assisted hatching can be employed in following cases:

1. Embryos with thick zona pellucida

2. Recurrent failure of embryo implantation after embryo transfer

3. Advanced maternal age, generally over 37 years

4. Cryopreservation/ freezing cycles

5. Women with FSH levels

Benefits of Assisted Hatching

• Assisted hatching aims to improve the chances of successful hatching by creating a hole in the zona pellucida.

• Assisted hatching is offered at a number of centres around the world.

• Mixed views exist as to whether the technique truly improves the chances of pregnancy, but most agree that it may be helpful in selected groups of patients.

The Procedure

• A fine glass pipette is most commonly used in assisted, where it blows weak acidic medium on to the zona pellucida.

• This step causes thinning of the area, before a hole is punched through the centre of the thinned section.

• The procedure is carried out on third day of embryo development, when the embryos have started their third cell cycle.

• Sufficient cell-to-cell bonding has taken place by this time, to minimise the loss of cells through the hole that has been developed.

• Embryo implantation seems to occur earlier by allowing embryo-endometrium contact after assisted hatching.

• Partial drilling, thinning or total removal of zona pellucida are different techniques of performing assisted hatching.

• It becomes very critical to minimize the time for which the embryo is out of the incubator and to optimize the methods to reduce temperature and pH changes that can be detrimental for the embryo growth.

• Embryo transfer to the uterus has to be performed with no trauma, to avoid damage to cultured embryos.

1. Partial zona drilling- an artificial gap drilling can be made in the zona using micromanipulation technique. It is important that the size of the aperture made in the zona is large enough to avoid trapping of the embryo during hatching. This drilling can be done by mechanical, chemical, enzymatic, or thermolytic effect.

2. Zona pellucid thinning- zona pellucid is bilayered, with a thick, less dense, easily digestible outer layer and a more compact and sturdier inner layer. The aim of the zona thinning is to thin the outer layer of zona without complete lysis and perforation of the inner layer.

3. Zona pellucida removal- total removal of zona pellucida before the embryo transfer has been expected to bring about closer contact of the embryo with the endometrium, thus improving implantation.