Assisted hatching is a laboratory procedure that facilitates an embryo's ability to release itself from the confines of its "egg shell." After fertilization, the egg begins to divide, doubling in cell number to two cells, then four cells, then eight cells and so forth.  By the fifth to sixth day after fertilization, the embryo is 50-100 cells and is ready to "hatch" from its "egg shell." The "egg shell" (or zona pellucida) is a noncellular secretion of carbohydrate and protein that is secreted around the egg during its development in the ovarian follicle and persists after the egg is fertilized until the fifth or sixth day.

The embryo has expanded such that it strains at the confines of the zona pellucida until it "hatches" through a weakened area in the zona pellucida. This escape from the zona pellucida is required in order for the embryo to implant in the uterus and establish a pregnancy.

In some cases, hatching may be more difficult or impossible for the embryo due to various factors. The assisted hatching technique is used to weaken the zona by opening a hole in the zona pellucida. This opening is typically created in the laboratory by carefully expelling minute quantities of an acidic solution on the surface of the zona pellucida, creating a tiny hole. By preparing the zona for hatching, pregnancy rates are enhanced in patients with unusually thick zona pellucidas or other risk factors for failed hatching.

Who may benefit from assisted hatching?

• The older patient

• The patient whose embryos have unusually thick zona pellucidas

Sources:

Handbook of In Vitro Fertilization , 2nd. edition. Copyright 2000 by CRC Press. Editors: A.O. Trounson and David K. Gardner

In Vitro Fertilization , 2nd edition. Copyright 2000 by Cambridge University Press. Editors: Kay Elder and Brian Dale