On the fifth day of development in the laboratory, the human embryo, or the blastocyst as it is called at that stage, is a sphere of about 150 cells surrounded by a protective layer called the zona pellucida. In order to implant in the uterus, the blastocyst must emerge from the zona pellucida during a process called hatching. The precise mechanism of human embryo hatching is not known, but it appears to occur following multiple cycles of blastocyst contraction and expansion. It is also possible that hatching is helped along by enzymes released from the embryo or the uterus.
Scientists who now work at the ART Institute of Washington concluded more than 20 years ago that some embryos developing in vitro lack the ability to hatch spontaneously. They suggested to artificially open the zona pellucida in order to facilitate hatching and improve the chances of implantation. This procedure is called Assisted Hatching. Assisted Hatching has been the subject of study in over 400 scientific publications and it is becoming increasingly clear that it is effective in some embryos and for some patients. In most patients only a proportion of the embryos will be manipulated while others are not, hence the procedure is commonly referred to as Selective Assisted Hatching. Embryos that may benefit from assisted hatching include those that have been cryopreserved (and thawed), those that develop poorly in culture, embryos from older patients, and embryos from patients with multiple failed IVF attempts. Assisted hatching has been proven to be safe in a number of studies. However, it should be noted that it is not always entirely clear whether or not an embryo will benefit from this treatment. In our laboratory the procedure is performed under a microscope using a laser. The laser opens the zona pellucida over an area that is roughly a thousandth of an inch and creates a hole through which the blastocyst can hatch. The procedure is usually applied on Day 3 of development when the embryo is transferred on that day. Assisted hatching is also performed on blastocysts that have been vitrified (frozen) and warmed (thawed). Patients whose embryos undergo micromanipulation for assisted hatching may be prescribed a course of antibiotics and corticosteroids depending on the circumstances.