Subzonal injection is a method of in vitro fertilization. In this procedure, a drop of fluid containing sperm is placed under a microscope. Then five or ten of the healthiest looking sperm are scooped up into a threadlike, hollow glass needle. The needle is used to penetrate the shell of the ovum (or egg), creating an entry point for the sperm. The egg itself, however, is not touched or penetrated.
Subzonal injection was developed in the 1990s and has been a fairly successful procedure. It does require several healthy sperm and at least one of the sperm must penetrate the egg on its own. To be successful, only one sperm can fertilize an egg. If more than a single sperm fertilizes the egg, the egg will not develop into a normal embryo.