To determine whether the age of the female patient, oocyte maturation stage, pronuclei, embryonic quality and/or cycle outcome are related to the intensity of the zona pellucida birefringence (ZPB) in oocytes during IVF procedures, a retrospective study was conducted on 214 IVF cycles using a polarisation imaging system. A negative correlation was found between ZPB intensity and the age of female patients (r = –0.44; P 0.05).