It was suggested that non-specific aging of ova predispose to meiotic defect. It was proposed that oocytes with greater number of chiasmata and the least predisposition to non-disjunction are formed earlier in embryogenesis and ovulated in early adult life. This causes increased rate of spontaneous abortion of aneuploid embryos and fetuses with advancing maternal age. Another hypothesis is the oocyte selection model, which states that a small number of aneuploid cells preexist in the germline that have arisen by mitotic non-disjunction during ovarian embryogenesis. Selection against the utilization of these aneuploid oocytes is exerted during each successive round of oocyte activation. Therefore, with time, older mothers gradually acquire a pool enriched with aneuploid oocytes(9). More recently it was proposed that age-dependent susceptibility to chromosome 21 non-disjunction result from age-dependent loss of spindle forming ability(2).

It was evident from our study that the mothers of translocation cases were younger than those of the non-disjunction cases. This is in agreement with the original finding from NDSCR which reported reduced maternal age in robertsonian translocation compared with the UK birth population (10).