Ultrasonographically adjusted midtrimester risk of trisomy 21 and significant chromosomal defects in advanced maternal age.

Objective: Our purpose was to determine whether normal midtrimester ultrasonography results reduces the risk of fetal Down syndrome and any clinically significant chromosomal defects in pregnancies with advanced maternal age and to develop risk tables on the basis of age and ultrasonographic results.

Study Design: Biometry data were obtained in 4079 women between 15 and 24 weeks undergoing amniocentesis for maternal age. Expected values of humerus, femur, both combined, and abdominal circumference based on biparietal diameter were generated from a normal group. Observed/expected values of each parameter and nuchal thickness > or = 6 mm was used to screen for Down syndrome and any clinically significant chromosome defects. Receiver-operator characteristic curves were used to determine threshold screening values for each parameter. By use of stepwise logistic regression the optimal measurements for detection of chromosome anomalies were established. An abnormal ultrasonography result was defined as either abnormal biometry choroid plexus cysts or a structural defect.

Results: The finding of shortened humerus (observed/expected < 0.90), abnormal nuchal thickness, or an anatomic defect had the highest sensitivity for Down syndrome detection, 70.6%, p < 0.0001, whereas a short humerus, small abdominal circumference, (observed/expected < 0.92), or an anatomic defect had a sensitivity of 46.7%, p < 0.0001, for any significant chromosome defect. With a normal ultrasonography the risk of Down syndrome in a 39-year-old woman falls from 1:100 to < 1:292.

Conclusion: A normal ultrasonography substantially reduces the risk of Down syndrome and any chromosome abnormality. This information is useful in counseling women who decline amniocentesis on the basis of maternal age.