“…There are some factors that impair hatching, including abnormality in ZP structure, increase in ZP thickness, and zona hardening. Advanced maternal age, ovarian stimulation protocol, sub-optimal in vitro culture and cryopreservation may induce zona hardening (Aston and Weimer 2010). Assisted hatching (AH), by creating an opening in the ZP, helps the embryo to escape from the ZP.…”