Friday, November 13, 1998 Published at 11:05 GMT
Abortion risk of prenatal scans
Ultrasound is getting more accurate
Ultrasound may be better at picking up abnormalities in foetuses than previously thought, but its results could also be misleading, causing parents to abort healthy babies.
Previous studies have shown a lower rate of detection.
The study, by doctors at Oxford's John Radcliffe Hospital, found that on average across the six years 55% of abnormalities were correctly identified by scans.
These included babies who were born with spina bifida and Down's syndrome.
In the last three years of the study, the success rate improved to 68%.
However, a small percentage of the ultrasound scans showed foetuses signs of abnormalities who later went on to be normal babies.
The researchers studied more than 30,000 babies, of whom 725 (2%) were deemed abnormal on delivery.
Another 174 foetuses had signs suggesting abnormality, but went on to be normal at birth. They are known as "false positives".
More than 90% of these signs were what is known as "soft markers", which include thickening of the neck - an indicator of Down's syndrome.
The researchers say doctors should ensure that parents are given the right information about "soft markers", the risk of their child developing abnormalities and the accuracy of tests.
They also call for more research into the impact and value of "soft markers", to "determine whether the benefits of reporting them exceeds the harm".
"Soft markers" led to a 4% increase in successful detection of birth defects, but a 12-fold increase in "false positives".
Robin Heise Steinhorn of the Department of Paediatrics, Gynaecology and Obstetrics at the State University of New York, said: "The Oxford study shows that ultrasonography is increasingly sensitive for the detection of many serious anomalies and therefore capable of providing good information to allow parents to make important decisions about their unborn child.
"Unfortunately, it also shows that ultrasonography may also provide information that is confusing or even misleading.
"Good decisions will be possible only if patients are given the right information. By themselves soft markers are not a good basis for these decisions."
Peter Moore, a British prenatal expert, said ultrasound was not usually a diagnostic tool, saying if a baby was sure to be born with an abnormality.
"Very often it just shows the chance that a child has some abnormality. It is how people cope with the concept of chance and probability that then gets confusing."
Midwife Carnel Lacey called for more pre-scan counselling of women.
She said many did not understand that scans did not give yes and no answers and that women were often unprepared for the consequences of a scan which showed abnormalities.
The Oxford study also suggests that ultrasound is most effective for picking up anomalies in the external structure of the foetus, such as defects of the abdominal walls.
It detected more than 90% of these cases.
However, it was less sensitive in cass of serious internal structural abnormalities like congenital heart disease, where the success rate was about 50%.
Mr Steinhorn said there were questions over whether the accuracy of scans could be improved, since it also showed that improving sensitivity to scans made it more difficult to pinpoint specific problems.
He added that scans did not change the fact that foetuses with abnormalities could be miscarried or stillborn, but they did offer parents the chance of preparing for potential problems.
In 43% of cases where abnormalities were detected, parents chose to abort their foetus. Their decision was not only based on the scan and often included further tests.