The most frequent examples are the recurrence risk of trisomy 21 (Down's syn­drome) and the risk for chromosomal aneuploidies at advanced maternal age (38 years and older).

The recurrence risk for trisomy 21 is 1% as calculated either from retrospective data (Pfeiffer et al., 1973) or from prospective series of prenatal diagnoses for this indication (Milunsky, 1973). The recurrence risk of other numerical aberra­tions is less well known. This is related to the question of the presence in man of a familial tendency to non-disjunction. Two types of evidence for this phenomenon are present:

a. A number of sibships have been described with aneuploidies for different chro­mosomes (Girardet et al.,1972; Bell & Cripps, 1974; Holmgren & AWhn, 1971; Hamerton, 1971).

b. Chromosomal studies on subsequent spontaneous abortions in the same family in­dicate that if the first abortion showed a chromosomal aberration, the following was likely to be caused by a chromosomal aberration, not necessarily being of the same type (Boue & Bou6, 1973).

These factors are to be considered when giving recurrence risks for a trisomy others than trisomy 21. Bell & Cripps (1974) advise prenatal monitoring in

every pregnancy in families where a child with an aneuploidy has been born. Chromosomal mosaicism in one of the parents might give a considerably higher risk for recurrence of the same aneuploidy than in the previous group and se­veral of these families were described (Beratis et al. 1972; Hsu et al., 1971). The recurrence risk of chromosomal mosaicism is probably in the order of 1% but cases of familial mosaicism were described, where higher risk figures are involved (Hsu et al. 1970; Shih et al., 1974).

Down's syndrome is not the only trisomy with an increased frequency at ad­vanced maternal age; other autosomal trisomies tend to be more common as well. However, these will be found relatively rarely since there is a high early abortion rate in these cases (Laurence et al., 1974 ). The risks for a chromosomal aberration !n the maternal age-groups of 35-39 years and over 40 years were found to be 1.5% and 3%, respectively in a collected series of prenatal diagnoses (Hsu & Hirschhorn, 1974).