During the first days of its growth, an embryo is surrounded by a shell called zona pellucida from which the embryo has to escape in order to be implanted. If the zona pellucida is too hard or thick (as it may happen in women of advanced age) then the embryo may have difficulty in exiting the zona which would inhibit implantation. With the Assisted Hatching technique, a hole is opened in the zona pellucida just before the embryotransfer hoping that this will facilitate implantation.