Before implanting in the uterus, the embryo must emerge from its membrane (covering) in a process called "hatching." In some women, the membrane seems to harden, particularly as women age. This can interfere with the hatching process. In these cases, the embryo membrane can be thinned with a dilute acidic solution or laser prior to embryo transfer. This may help the hatching process, and increase the chances of pregnancy in older women or those who have not achieved pregnancy after several IVF cycles. Assisted hatching may also be done in some cases following cryopreservation and embryo thawing.