Before an embryo can attach to the wall of the womb, it has to break out or 'hatch' from its outer layer called the zona pellucida.
It has been suggested that making a hole in or thinning this outer layer may help embryos to 'hatch', increasing the chances of the woman becoming pregnant in some cases.
However, assisted hatching does not improve the quality of embryos.
The NHS guidelines on fertility, issued by NICE (the National Institute for Health and Clinical Excellence), say:
'Assisted hatching is not recommended because it has not been shown to improve pregnancy rates.' The guidelines also mention that further research is needed to find out whether assisted hatching can have an effect on live birth rates and to examine the consequences for children born as a result of this procedure