Womens Health

How an Ultrasound Can Fail to Detect Fetal Anomalies

Frederick R. Jelovsek MD

This title is misleading because ultrasound is actually a very powerful diagnostic tool that has significantly changed diagnosis in pregnancy. On the other hand, it is not perfect and women will be misled and hurt if they think their baby can't have any birth defects or problems if the "ultrasound was normal". Well just how good can ultrasound be?

In a New Haven, Connecticut study, Magriples U, Copel JA: Accurate detection of anomalies by routine ultrasonography in an indigent clinic population. Am J Obstet Gynecol 1998; 179:978-81, 901 pregnant women underwent routine ultrasound scanning between 15-26 weeks of pregnancy. All of the mother's delivery records and the baby's nursery record were checked afterwards to determine whether there was or was not any birth anomaly of the baby.


The authors reported the following observations regarding the need for ultrasound scanning:

  • 21% of women needed an advanced scan of which half (10.8%) of those were because an abnormal result was suspected or found
  • 3.1% (a third of the suspected abnormal screening scans) showed a fetal anomaly
  • the average number of ultrasound scans per pregnancy was 1.8
  • 21 babies actually had anomalies and 9 babies with anomalies were missed (sensitivity about 70%)
  • 35 babies were suspected to have birth defects on screening ultrasound but turned out to be normal on targeted scan

Most of the anomlies that were found with ultrasound were major birth defects and the ones that were missed were lesser anomalies. The defects missed and the outcomes were:

  • heart septum (ventricular) defect - surgery at one month
  • imperforate anus - surgery at 3 days
  • abnormal ear, congenital facial nerve palsy - surgery at 3 months
  • Lung anomaly - baby died
  • malposition of heart vessels (transposition) - surgery times 3
  • clubfoot - nonsurgical therapy
  • an unknown defect syndrome with poor feeding - feeding tube, foster care
  • small (1 cm) spinal cord defect (meningomyelocoele) - surgery at 3 months

Well, what can we conclude from this study? If a second ultrasound is recommended after a screening scan, there's still a good chance that nothing is wrong, it just needs to be checked out. Most of the major birth defects are found but not all of them. Ultrasound can miss defects even in the best of hands. While no one can absolutely assure a pregnant mom that the baby will be normal, there's extremely good odds (97%) that the baby will be perfect.


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