Effectiveness of routine ultrasonography in detecting fetal structural abnormalities in a low risk population.

OBJECTIVE To review the efficacy of routine prenatal ultrasonography for detecting fetal structural abnormalities. DESIGN Retrospective study of the ultrasonographic findings and outcome of all pregnancies in women scanned in 1988-9. SETTING Maternity ultrasonography department of a district general hospital. SUBJECTS 8785 fetuses. MAIN OUTCOME MEASURES Correlation of prenatal ultrasonographic findings with outcome in the neonate. RESULTS 8733 babies were born during 1988-9, and 52 pregnancies were terminated after a fetal malformation was identified. 8432 (95%) of the fetuses were examined by ultrasonography in the second trimester. 130 fetuses (1.5%) were found to have an abnormality at birth or after termination of pregnancy, 125 of which had been examined in the second trimester. In 93 cases the abnormality was detected before 24 weeks (sensitivity 74.4%, 95% confidence interval to 66.7% to 82.1%. Two false positive diagnoses occurred, in both cases the pregnancies were not terminated and apparently normal infants were born. This gives a specificity of 99.98% (99.9% to 99.99%). The positive predictive value of ultrasonography in the second trimester was 97.9% (92.6% to 99.7%). Of the 125 abnormalities, 87 were lethal or severely disabling; 72 of the 87 were detected by the routine screening programme (sensitivity 82.8%, 73.2% to 90.0%). CONCLUSION Routine fetal examination by ultrasonography in a low risk population detects many fetal structural abnormalities but can present several dilemmas in counselling.