Many risk factors are suggested to cause DS. The only well established risk factor is advanced maternal age and so age -specific rates have been documented(4). In our study, nearly 60% of mothers of non-disjunction cases were below 35 years. This is consistent with Astete et al., 1991 and Palka et al., 1990. In the latter study, the highest number of DS births was in the group of mothers between ages 25 and 29 years, this is in contrast to our finding in which the highest number of births was found in the group between 35 - 39 years. These findings are clearly related to the higher number of pregnancies in each age group(5,6).

One study was found out that women who had a reduced ovarian complement (congenital absence or removal of an ovary) were at increased risk of having an infant with Down syndrome. This may suggest that the increased risk of Down syndrome with increased maternal age may be related to the physiological status of the ovaries or the eggs at this age(7). Other potential explanations include delayed fertilization and changing hormone levels (8).