The cells that make up the early embryo are enclosed within a flexible membrane (shell) called the zona pellucida. During normal development, a portion of this membrane dissolves, allowing the embryonic cells to escape or “hatch” out of the shell. Only upon hatching can the embryonic cells implant within the wall of the uterus to form a pregnancy.
Assisted hatching is a laboratory technique in which an artificial opening or thinning is made in the shell of the embryo in an attempt to enhance its ability to hatch and implant after transfer. It has been suggested that this outer shell becomes thicker and hardened with culturing in the laboratory and aging of the oocyte. The hatching is usually performed on the day of transfer, prior to loading the embryo into the transfer catheter. A small opening can be made by a physical means as in vaporizing the shell with a high energy laser beam, or by a chemical means with a dilute acid solution.