There is an elastic outer membrane encasing each egg known as the zona pellucida. This membrane is involved in the interaction with the sperm and protects the embryo up until the time that it hatches, usually around day 6. It is believed that as women age, the zona loses elasticity, making hatching more difficult. Assisted hatching is recommended for women over 37 who have experienced unsuccessful cycles, or whose zona appear thick under the microscope. Performed using micromanipulators and very fine needles to create a small opening in the zona, this technique divides opinion. Some clinicians believe that the use of assisted hatching results in higher pregnancy rates and that the making of a 'weak point' in the zona may help implantation. Others feel that there is no convincing evidence that it helps to improve chances of pregnancy.