Assisted hatching (AH)

Assisted hatching is one of the treatment options that may be suggested to you if you are having a cycle of in vitro fertilisation (IVF). It may increase the chances of pregnancy in some circumstances, but research is still going on to find out more about its effects.

What is assisted hatching (AH)?

To understand assisted hatching, you need to know how the process of natural conception occurs.

As soon as an egg is fertilised by a single sperm, no other sperm must be allowed to enter. If this were to happen, the embryo would have too much DNA, and would probably not develop. Nature has evolved a clever method to make sure everything goes to plan: a few moments after fertilisation, the outer layer of the fertilised egg begins to thicken and harden, becoming a stiff gel. The thickened zona pellucida is then too tough to allow any further sperm in.

At this point, the fertilised egg is usually on its way down the fallopian tube towards the uterus. During the next few days, the egg ‘hatches’ by breaking down the thickened zona pellucida so that it can implant in the wall of the uterus, on day 6–7 after fertilisation. If this happens, you get pregnant. If it doesn’t, the embryo is lost and your period arrives on time.

Assisted hatching is a method of trying to imitate nature by thinning the zona pellucida of an embryo produced by IVF, before it is introduced into your uterus. The theory is that doing this will increase the rates of implantation and therefore enhance your chances of getting pregnant.

Methods of assisted hatching

Various methods are used. It is possible to thin the zona pellucida using a weak solution of acid or a laser, or to introduce a weak point with a tiny needle called a microtool.

These techniques need to be done very carefully by an experienced embryologist and is carried out on the third day of embryo development in vitro. A small hole is made in the embryo’s outer layer without disturbing the embryo’s cells.

The embryo is then introduced into the uterus and you will probably be given a course of antibiotics for a few days before and after as a precaution against infection in the embryo. Assisted hatching is not thought to harm the embryo in any way, and does not seem to cause any additional risk of abnormalities in the embryo.

Assisted hatching – what is the evidence?

Clinical studies have been in progress for several years to look at the effects of assisted hatching on pregnancy rates following IVF. Data has also been collected on how assisted hatching affects the rate of live births, multiple births, miscarriage and still births.

The findings of two of the most recent major reviews are worth noting.

Martins et al. 2011: published in Human Reproduction Update

This analysis combined results from 28 different studies and showed that assisted hatching generally made little difference to pregnancy rates if the embryos being introduced in the IVF cycle were fresh rather than thawed and frozen. However:

  • Women who had previously had at least two IVF failures were definitely helped by assisted hatching, even if fresh embryos were being implanted.
  • Women thought to have a low chance of pregnancy and who were having frozen embryo transfer also had higher rates of pregnancy after assisted hatching.
  • All women who had assisted hatching had a significantly increased risk of having a multiple birth.

Das et al. 2009: a Cochrane database review

This also included 28 studies (not necessarily the same ones) and concluded that there was a significant increase in pregnancy rates in women who had assisted hatching. They worked out that a clinic with a 25 % success rate could improve that to 29–49 %. Das et al. also concluded that:

  • Miscarriage rates were similar whether assisted hatching was used or not.
  • Women who had assisted hatching had a significantly higher chance of having a multiple birth.

What do the authorities say about assisted hatching?

The National Institute of Clinical Excellence (UK)’s current position (Dec 2011):

  • Assisted hatching is not recommended because the evidence does not demonstrate that it improves pregnancy rates. NICE recommends further research to look at how assisted hatching impacts on live birth rates and also to investigate the possible impact on children later in life.

The Practice Committee of the American Society for Assisted Reproductive Technology (June 2008):

  • The scientific evidence does show that assisted hatching is necessary in all IVF cycles.
  • Assisted hatching is recommended in women who have had two or more IVF cycles that failed, women judged to have a lower chance of success in IVF for whatever reason, and in women aged 38 and over.

Assisted hatching is not for every woman having IVF

If you are embarking on a cycle of IVF your specialist may suggest assisted hatching. This treatment is available in infertility clinics around the world but it will only be offered if your doctors think it will be of benefit to you.

Although the evidence that assisted hatching improves the success rates of IVF is not clear cut, some groups of women are more likely to get pregnant if their IVF cycle includes assisted hatching:

  • Women over 39 who are using their own eggs, particularly if they are frozen and then thawed. The zonae of embryos produced by older women are known to be tougher and thicker than embryos that develop from eggs from younger women. The freezing process may also thicken the outer layer of the embryo.
  • Women whose embryos have a thicker outer layer than is usual.
  • Women who have had two or more IVF cycles in which viable embryos have failed to implant.
  • Women who have higher than normal levels of follicle stimulating hormone (FSH), as they are known to produce eggs with thicker zonae.


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