6-year experience of prenatal diagnosis in an unselected population in Oxford, UK

The benefits and harm associated with prenatal diagnosis are open to debate. We give a 6-year overview of the experience of one prenatal-diagnosis unit using a defined, unselected population.

All congenital malformations suspected prenatally and all congenital malformations, including chromosome anomalies, confirmed at birth were identified from the local Congenital Malformation Register. All fetuses or infants of women booked for delivery at the Oxford Women's Centre who had an OX postcode and date of delivery between 1991 and 1996 were eligible for the study.

725 (2%) of 33,376 babies, were judged abnormal at delivery. 396 (55%) malformed fetuses and infants had been correctly identified prenatally. 174 fetuses had a suspected abnormality identified on scan and subsequently proved to be normal. 160 (92%) of these false-positive results were attributable to the reporting of so called ultrasound soft markers. Accuracy of ultrasound diagnosis was good for structural malformations. Ultrasound soft markers were responsible for a 4% increase in detection of malformations (from 51% to 55%) and a 12-fold increase in false-positive rate (one in 2332 to one in 188). 171 pregnancies (43% of prenatally diagnosed malformed babies) were terminated because of suspected abnormality. Suspicion of abnormality in these cases was first aroused after ultrasound scan in 136 (79%); chromosome analysis because of advanced maternal age, family history, or higher risk in biochemical screening test in 25 (15%); and molecular analysis of single gene defect because of family history in ten (6%). There was a 20% reduction in prevalence of conditions compatible with survival beyond the neonatal period because of termination of such pregnancies.

More than half of all malformed fetuses can be identified prenatally in routine practice, mostly following initial suspicion from ultrasound examination. Ultrasound soft markers lead to a small increase in detection of malformations but a large increase in false positives. Further research on the impact, including psychological, and value of markers is required to determine whether the benefits of reporting them exceeds the harm. Because methods and techniques continually change, ongoing surveillance of prenatal diagnostic services is vital.