Birth order showed a higher number of first and second borns. This is consistent with the study of jyothy et al., 2000 and Hay and Barbano, 1972. The latter suggested that the first-born infant is at higher risk independent on the maternal age(24,25). On the other hand, Stoll et al., 1990, reported firstborns to be at lower risk of DS (26).

Lejeune et al., 1959 was supported with the discovery of the chromosomal basis of DS (27) . Shortly thereafter, Polani et al., 1960, discovered translocation DS in the daughter of a 21 - year-old mother who was selected for study because the investigators had reasoned that some individuals may be affected through a separate, maternal-independent chromosomal mechanism (28). The following year, Clark et al., 1961 reported mosaic DS, in a 2 year-old girl with physical stigmata of DS but near -normal intelligence (29). It is well known that non-disjunction contributes to most cases of DS, followed by robertsonian translocation and mosaicism. Table 9 and 10 present a comparison of the cytogenetic data between our study and different published large international studies.

Non- classical (%) ... Mosa-ic

(%) ... Translocation (%) ... Non-

disjunction (%) ... No. of cases studied

1 ... 4 ... 95 ... 5737 ... Mutton et al., 1996 (10)

3.6 ... 5.7 ... 90.7 ... 1778 ... Kovaleva et al., 1999 (30)

4.39 ... 7.69 ... 87.92 ... 1001 ... Jyothy et al., 2000 (25)

0.5 ... 2.2 ... 4.3 ... 92.9 ... 17,738 ... World Wide

0.68 ... 1.84 ... 3.5 ... 93.98 ... 1030 ... Our study

Table (9): Cytogenetic data published in several large studies of DS patients: