Ultrasound, a commonly used imaging technique that scans the fetus, may be able to detect Down syndrome before birth, Boston researchers reported last week.

Fetuses suffering from the chromosomal defect that results in mental retardation tend to have shorter than normal bones in the arm and thigh, according to doctors at Brigham and Women's Hospital in Boston.

The finding was based on a study of 24 fetuses later found to have Down syndrome who were compared to 400 normal fetuses. The bones that provided an indication of abnormality were the femur -- or thigh bone -- and the humerus, the long bone in the arm.

The difference was more pronounced in the arm bone, the researchers said.

In addition, the scientists told a meeting of the Radiological Society of North America, ultrasound may be able to detect an abnormal skin fold at the back of the neck that is present in about 40 percent of babies with Down syndrome.

More than half of all pregnant women in the United States -- mostly women over 35, the age at which the risk of birth defects begins to increase -- are believed to undergo ultrasound tests in which sound waves are used to project an image of the developing fetus.

"If during these examinations attention is paid to measurements such as the length of the humerus, it may help more accurately assess an individual's risk for having a Down syndrome baby," said Beryl Benacerraf, a member of the research team.

Benacerraf said the bone information is not definitive but should help a woman decide whether to proceed with other tests, including amniocentesis, in which a sample of the fluid surrounding the fetus is tested for abnormalities.

Scientists at the University of Colorado issued a separate report at the meeting, in which they disagreed with the Boston group's finding as it pertains to thigh bones. They said a shorter than normal femur is a sign of short parents, and nothing else.