Ultrasound Detection of Down Syndrome

New Method Shown to be Effective for Detecting Down Syndrome at Mid-Trimester Scan

Looking at the nasal bones of fetuses at the mid-trimester scan could improve the detection of Down Syndrome during pregnancy, according to results to be published in the January 2003 issue of Ultrasound in Obstetrics & Gynecology, the official journal of the International Society of Ultrasound Obstetrics and Gynecology, which will be available online on 17 December 2002.

The most frequently occurring chromosomal abnormality, Down Syndrome affects every one in 500 to 600 live births. This genetic condition arises from an error in cell division that results in an extra copy of chromosome 21. More mature women have an increased chance of carrying a baby with chromosomal abnormalities, so they are offered amniocentesis, a procedure which involves taking a sample of amniotic fluid. This is currently the most effective way of testing for Down Syndrome, but there is a risk that miscarriage can be induced.

Professor Kypros Nicolaides of King’s Hospital Medical School has previously demonstrated using ultrasound that in the early stages of gestation the nasal bone is underdeveloped in about 70% of fetuses with a chromosomal anomaly, but in less than 1% of normal fetuses.

Now for the first time Professor Nicolaides and his team of researchers have studied the incidence of absent nasal bones in chromosomally normal and abnormal fetuses at the mid-trimester scan. This is the predominant scan that is routinely used to test for anomalies in most pregnancies.

The team found that the nasal bone was underdeveloped in 61.8% of the fetuses with Down Syndrome, but in only 1.2% of normal fetuses. According to Professor Nicolaides, “This shows that nasal bone underdevelopment at the mid-trimester scan is associated with a high risk for Down Syndrome and it is a highly sensitive and specific marker for this chromosomal abnormality.”

“We estimate that if examining the fetal profile for the absence of the nasal bone is incorporated with other commonly used methods in screening programmes the sensitivity would increase from the 60% that can be achieved with the mid-trimester blood test to more than 90%” said Professor Nicolaides. “This would also dramatically reduce the need for unnecessary amniocentesis.”

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