It was documented that there is a significant excess of males among both the newborn children with DS and fetuses with DS aborted after prenatal diagnosis. Sex ratio in our study was as reported in other studies in all types of DS. In trisomy 21, about 54% are male fetuses or babies against the normal birth sex ratio of 51% male (3). However, in mosaics DS, the male to female ratio is 0.8 to 1 (10). This finding has subsequently been confirmed elsewhere , but remains unexplained (23).

In our study, family history of previously affected pregnancy or in close relative was present in 6% of cases; all of them were in non-disjunction cases, which is a much higher figure than that obtained in NDSCR (1%). A number of recognized reasons were suggested as a cause of recurrence (3):

One parent may be mosaic trisomy 21. While this is rare it is a known cause for recurrence in young patients.

There may be some other rearrangements (not involving chromosome 21 translocations) present, which lead to imbalance during gametogenesis which is unproven yet.

There may be other genetic predisposing conditions leading to non-disjunction. This might be in the germ cells or in somatic cells. The occurrence of regular trisomy in a wider pedigree could support this theory.