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We describe two cases of sonographic abnormalities associated with unusual chromosomal aberrations. Case 1 presented with a cystic hygroma at 12 weeks' gestation. Cytogenetic analysis revealed an unbalanced complex chromosome rearrangement implicating chromosomes 6, 13 and 21 (karyotype: 47,XX,t(6;21;14)(q14;q21;q21)mat,+21) and corresponding to a complete trisomy 21. This anomaly resulted from malsegregation of a maternal balanced three-way translocation. For case 2, an alobar holoprosencephaly was identified by ultrasonography at 23 weeks' gestation. Chromosomal analysis showed a recombinant rec (13), dup q chromosome, secondary to unequal crossing-over of a paternal pericentric inversion of chromosome 13, giving rise to partial trisomy 13q (karyotype: 46,XX,rec(13)dup(13q)inv(13)(p11q21)pat). These two cases illustrate the role of ultrasound in leading to detection not only of foetal chromosomal aberrations but also of rare balanced chromosomal rearrangements presented by one of the two parents., (Copyright 2004 John Wiley & Sons, Ltd.)