Is the Zona pellucida thickness of metaphase-II oocytes in an IVF/ICSI program influenced by the patient's age?

Zona pellucida (Z.p.) thickness is believed to be considerably influenced by hormonal stimulation. Assessments of the influence of the patient's age on Z.p. thickness have been at variance. In our study we wanted to investigate, among several other parameters, the age of the patient as a possible factor influencing Z.p. thickness. Prior to intracytoplasmatic sperm injection (ICSI) we determined the mean thickness of the Z.p. of 843 metaphase-II oocytes from 100 patients, which had been obtained after ovarian stimulation in an in vitro fertilization (IVF)/ICSI program. The mean thickness of the Z.p. was significantly positively correlated with the age of the patient (p = 0.000, r = 0.129). When the intraindividual mean values of Z.p. thickness (n = 100) were used, a positive correlation with age (p = 0.174, r = 0.137) remained. However, this correlation was no longer significant. It is concluded that, beside other factors, a thicker Z.p. could be responsible for the possibly lower fertilization rate of older patients in a conventional IVF program.