Detection of Fetal Anomalies with Routine Ultrasound
Abstract & Commentary
To determine how effectively routine ultrasound and other prenatal diagnostic techniques identify fetal malformations, Boyd and associates reviewed the experience at the Oxford Women’s Centre in Great Britain between 1991 and 1996. Data for the study were derived from a registry of congenital malformations including chromosomal abnormalities diagnosed clinically or pathologically in all livebirths, neonatal deaths, stillbirths, and induced and spontaneous abortions delivered at the center. Of 33,376 babies, 725 (2%) were judged to be abnormal at birth. An anomaly scan at 18-22 weeks gestation was available to all the women, and approximately 90% had this study. While alpha-fetoprotein screening for neural tube defects was performed routinely, neither triple marker screening nor nuchal translucency screening has been done at Oxford.
More than half of the malformed fetuses and infants (n = 396, 55%) were correctly identified before delivery. The rate of detection improved over the six years of the study—from 42% in the first three years to 68% for the second three years. In 174 fetuses, an abnormality was suspected on ultrasound but the fetus was subsequently found to be normal. In more than 90% of these cases (n = 160, 92%), the false-positive result was due to the observation of a "soft" ultrasound marker, nuchal thickening, choroid plexus cysts, and echogenic bowel. These soft markers were responsible for a 4% increase in the detection rate of anomalies. These markers were also responsible for a 12-fold increase in false-positives, from 1/2332 to 1/188.
Boyd et al conclude that more than half of all malformed fetuses can be prenatally detected in routine practice, most after an abnormal ultrasound at 18-22 weeks gestation. The use of soft markers significantly increases the rate of false-positives and could result in greater parental anxiety. (Boyd PA, et al. Lancet 1998;352:1577-1581.)
COMMENT BY STEVEN G. GABBE, MD
The investigation describes the role of routine ultrasound in the prenatal diagnostic screening from a single center in Great Britain. While not a prospective, randomized trial, the size of the database (more than 33,000 cases) provides important information. This study, as have others, demonstrates that more than 50% of major fetal malformations can be detected by a routine ultrasound evaluation of fetal anatomy at 18-22 weeks gestation. It confirms that the detection rates for some malformations are significantly greater than for others. For isolated neural tube defects, the detection rate was highest—98%. For cardiac malformations, only 38% were recognized. Ultrasound proves to be far more important than other prenatal strategies, such as chromosome analysis for advanced maternal age or biochemical screening tests, in identifying fetal malformations. The information on the value of "soft" ultrasound markers is especially important. Of these variants of normal, nuchal thickening, and choroid plexus cyst, only one has a confirmed anomaly at delivery. Isolated nuchal thickening was seen in 78 pregnancies and seven of these infants had trisomy 21. Fortunately, only two pregnancies were terminated for soft findings. In both cases, this action had not been advised by the prenatal diagnosis team and, in both cases, the fetus was normal at autopsy.
The investigation confirms the value of routine ultrasound in the identification of serious anomalies, but it raises questions about the expanding use of soft ultrasound markers in such screening programs.
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