Assisted hatching
Following an embryo transfer, the embryos must continue to develop in the uterus for a pregnancy to occur. After approximately 5 or 6 days, as the embryo develops into a blastocyst, it must hatch out of its outer shell (known as the zona pellucida) so that the cells of the embryo can make direct contact with the cells lining the uterus. This is known as implantation. A blastocyst is a small ball of cells that expands to fill the zona and the pressure created should be sufficient to cause the zona to break and allow the blastocyst to hatch naturally.
