In our study only 7.1% of fathers were above 49 years. In spite of the fact that advanced paternal age (>49 years) has been associated with increased risk of DS births in a few studies (17), the risk has diminished with the appropriate adjustment for maternal age. Other studies documented that paternal age is not considered as a risk factor for conception of a child with DS (18).

Parental consanguinity was present in 12 % of cases which is much lower than the reported rate in general Egyptian population (32 - 35%) (19). It could be suggested that consanguinity has a little association with the incidence of DS and that the process of non-disjunction during oogenesis and spermatogenesis is rather a consequence of other causes particularly increased maternal age (20).

On the other hand, Alfi et al., 1980 observed an increased frequency of consanguineous parents among their DS patients. They postulated the existence of a gene that could influence mitotic non-disjunction in the zygot, followed by the loss of monosomic cells and the formation of a complete trisomic or mosaic embryo (21). Authors suggested that the results of Alfi et al., based only on 20 cases could be influenced by some bias in selection, and were not confirmed by others who done their studies on much larger samples. Moreover, it is known that most of the free trisomies originate during the first meiotic division of the gamete, and this implies that the proportion of trisomic 21 produced by the action of such a gene , if it exists, would be very low (22).