“…Most authors assume that the main problem concerns meiotic segregation errors of a normal disomic oocyte, as reiterated by for example Hunt & Hassold (2008), Jones (2008), Mailhes (2008), Martin (2008), Oliver et al (2008), Vogt et al (2008), Allen et al (2009), Coppedè (2009, Driscoll & Gross (2009), Garcia-Cruz et al (2009) and Ghosh et al (2009). We have recently performed a study (Hultén et al 2008) relevant to the possibility that the maternal age effect is instead due to the accumulation of pre-existing T21 cells in ovaries of human females during their development from foetal life to adulthood, a hypothesis akin to the oocyte selection model by Vig (1984), Zheng & Byers (1992), Sensi & Ricci (1993), Zheng & Byers (1996a, 1996b, Zheng et al (2000) and Zheng (2004). We found that in fact all eight normal foetuses analysed in this respect were T21 ovarian mosaics (average 0.54%, range 0.20-0.88; S.D.…”