Ultrasound scan spots Down's syndrome

Nasal bone holds key to safer test for birth defects.
20 December 2002

KENDALL POWELL

Women over 35 have a 1-in-300 risk of having a Down's syndrome baby. © S. Cicero

An ultrasound scan for the fetal nose bone might cut the number of women whose fetuses need a test for Down's syndrome.

Nearly two-thirds of 15-22-week-old fetuses with Down's syndrome lack a nasal bone, fetal-medicine specialist Kypros Nicolaides, of King's College, London, and his colleagues found¹.

For normal fetuses, the figure is 1%. This makes it unlikely that the test would wrongly diagnose Down's syndrome.

The nasal scan is more accurate than previous ultrasound markers, such as the length of leg bones. Some UK and US doctors are already using the nose-bone scan in combination with other tests.

The detection of Down's syndrome requires a sample of fetal cells from the fluid surrounding the fetus. This test, called an amniocentesis, causes miscarriage in about 1% of women who have it.

The nose-bone scan will reduce these tests, says Nicolaides. Women over 35 have a 1-in-300 risk of having a Down's syndrome baby. Each year, in the United States, 375,000 such women are encouraged to have an amniocentesis.

The scan could save many mothers the agonizing decision over whether to have an amniocentesis and risk losing a normal baby. "If the nasal bone is present, then the risk of having a baby with Down's syndrome is automatically reduced to one-third of what it was before the scan," says Nicolaides.

The nasal bone can be a good marker for Down's syndrome, agrees Rosemary Reiss, who practises maternal fetal medicine at Brigham and Women's Hospital in Boston, Massachusetts.

But the proportion of false alarms jumps to 8% for patients of African descent, says Reiss. Differences among ethnic groups will have to be understood for screening to succeed, she says.

Testing time

The nasal-bone test gives similar results in fetal scans as early as 11-14 weeks². The 15-22-week point coincides with the traditional check-up for high-risk mothers. "This is a simple scan that women have anyway," says Nicolaides.

The new test will help give patients a better idea of the risk they face, says Richard O'Shaughnessy, director of the fetal treatment programme at Ohio State University. He suggests combining it with blood tests and another ultrasound marker that looks for fluid collecting behind the neck.

1. Cicero, S. et al. Nasal bone hypoplasia in trisomy 21 at 15-22 weeks' gestation. Ultrasound in Obstetrics and Gynecology, Published online, doi:10.1002/uog.19 , (2002). |Homepage|
2. Cicero, S., Curcio, P., Papageorghiou, K. H., Sonek, J.. & Nicolaides, A.B. Absence of nasal bone in fetuses with trisomy 21 at 11-14 weeks of gestation: an observational study. Lancet , 358, 1665 - 1667, (2001). |Homepage|