The fertilized egg is surrounded by a thin membrane, called the zona pellucida. Once fertilized, the egg undergoes its first divisions within the zona. When the embryo is ready to attach itself to the wall of the uterus the zona splits open. This "hatching" normally takes place during the blastocyst stage of development. We suspect that some IVF procedures may fail because the zona does not properly hatch.	Photo Slideshow: A Look Inside IVF
Embryologist are able to dissolve portions of the zona or drill openings in it with lasers. We utilize this assisted hatching when we suspect that the normal hatching might not occur. Likely situations for assisted hatching include when the women is 40 years or older, when the zona appears thickened, and when an embryo biopsy (PGD) is performed.

Why don't we hatch every embryo?
There may be a down side to assisted hatching. Many centers report an increase chance of identical twinning following assisted hatching. We believe the embryo may get caught in the artificial opening and split into two pieces which then proceed to develop into identical (monozygotous) twins. The chance of pregnancy complications is significantly increased when identical twins are present. Therefore, we utilize assisted hatching only in those cases which we feel truly require the procedure.