Chromosomal abnormalities detected after an abnormal ultrasound in pregnancy

Improvements in ultrasound technology have resulted in an increasing number of requests for prenatal chromosome testing because of fetal abnormalities detected in utero. Between January 1990--January 1991, 388 tissue samples were referred to our laboratory for cytogenetic analysis, of which 202 were amniotic fluids samples, 157 chorionic villus biopsies and 29 fetal blood specimens. Of these 54 were referred for fetal abnormalities detected prenatally on high resolution ultrasound. Chromosomal analysis was successful in 50 cases, and included 6 (12%) chromosomally abnormal fetuses: 2 trisomy 21, 2 trisomy 18, one 45,X and one unbalanced translocation. The maternal age for three of the four cases of autosomal trisomy were below 35 years (the cut-off for amniocentesis for advanced maternal age). In contrast, 273 prenatal chromosome studies performed for advanced maternal age (AMA) produced only 4 (1.5%) chromosomally abnormal fetuses. These abnormalities detected on ultrasound indicate a significant population of fetal chromosomal aberrations which would otherwise not be detected prenatally.