RESUMEN
Cromosomal abnormalities are the most frequent cause of first trimester spontaneous abortions (SA). During the period Sep. 1989 through May 1996 we have cytogenetically studied 640 embrionic tissue samples obtained from cases of SA. Of these, 609 samples (95,1 percent) were successfully karyotyped. An abnormal karyotype was observed in 388 cytogenetically normal and abnormal embryos. The most frequent abnormalities detected were autosomal trisomies (239/388 [61.6 percent]) followed by triploidy (62 cases, 16 percent), monosomies (41 cases, 10.6 percent) and teraploidy (17 cases, 4.4 percent). The single most common anomaly observed was trisomy 16 (73 cases, 18.8 percent). The frequency of trisomies, with the exception trisomy 16, was related with advanced maternal age
