Implantation of the embryo into the endometrium occurs several days after fertilization. Normally, the embryo erodes its own zona pellucida "from the inside", causing it to thin, so that the embryo hatches before it can adhere to the endometrium and implant. If the zona pellucida is thicker than normal, or if it has hardened due to prolonged in vitro culture, or freezing and cryopreservation, hatching is theoretically impaired. Therefore, the embryologist may facilitate embryo hatching after IVF, by opening a hole in the zona pellucida (either by chemical corrosion, or by using special laser attached to the microscope), before embryos are transferred into the uterine cavity.