A new ultrasound procedure available at UCSF allows doctors to detect fetal chromosomal abnormalities, such as Down's syndrome, earlier in pregnancy and with a higher sensitivity than other screening methods, says a UCSF geneticist who is the only clinician in Northern California trained to perform this procedure.

The novel technique measures the fluid accumulation, or nuchal translucency, in the neck of fetuses that causes abnormal swelling or enlargement -- an indicator of a chromosomal defect.

"The advantage of fetal nuchal translucency screening is two-fold: it spots problems early in pregnancy during a woman's first trimester at 10-14 weeks, and detects 80 percent of Down's syndrome cases," says James Goldberg, director of the reproductive genetics unit in the UCSF department of obstetrics, gynecology and reproductive sciences. He adds that Down's syndrome affects one in 700 fetuses in the US annually.

Because of its timeliness and high detection rate, fetal nuchal translucency is a more effective screening method than a serum, or blood test -- the most commonly used detector for fetal abnormalities. A blood test is performed during a woman's second trimester at 15-20 weeks, and identifies only 60 percent of Down's syndrome cases.

Testing for chromosomal abnormalities in the first rather than second trimester has the advantage of earlier prenatal diagnosis and consequently a less traumatic termination of pregnancy if a couple chooses this option, Goldberg says.

A potential criticism of early compared to late prenatal diagnosis is that, in some cases, a woman may experience spontaneous miscarriage, removing the need, if she chooses, for an elective abortion, Goldberg notes. However, he explains that this is also true for mid-trimester screening and that each pregnancy differs, making it impossible to determine which pregnancies will miscarry.

In addition to detecting chromosomal abnormalities in single and multiple pregnancies, fetal nuchal translucency screening is also a potential marker for a wide variety of fetal problems and genetic syndromes.

"It appears that an increased nuchal translucency thickness, even if the chromosomes are normal, is associated with an increased risk of other types of birth defects, most commonly heart defects," Goldberg says. "We believe that problems with lymphatic or cardiac development are what causes abnormal swelling in the neck of fetuses."

In fact, data from as early as 1866, documented by Langdon Down, reported that the skin of affected individuals appears to be too large for their bodies. A normal nuchal translucency thickness ranges from one to two millimeters and varies with the gestational age of a fetus.

UCSF Medical Center, a part of UCSF Stanford Health Care, will offer fetal nuchal translucency screening and counseling to all pregnant women during their first trimester of pregnancy. Positive results will be confirmed by an invasive test, either an amniocentesis or chorionic villus sampling, in which a small amount of a woman's placenta is tested. These methods carry a slight risk of miscarriage, and ultrasound screening may cut down on unnecessary invasive testing, Goldberg says.

Along with a fetus' nuchal translucency thickness and gestational age and size, a woman's maternal age and history of other children with Down's syndrome are part of a formula that calculates her risk of having an affected pregnancy.

According to Goldberg, this screening process currently has a five percent false positive rate. "In the past, one of the problems with this approach has been an absence of uniformity in the way the nuchal translucency thickness is measured," he says. "The critical part is to measure consistently and apply the results to a formula that warrants accurate risk factors."

Goldberg recently received training to perform fetal nuchal translucency screening from the Harris Bright Research Center for Fetal Medicine at King's College in London, England, where a uniform method for measuring and interpreting nuchal translucency thicknesses was formulated. The center's method has been applied to over 100,000 pregnancies in Europe.

"We feel that this technique has been sufficiently established by the large number of cases performed in the UK," notes Goldberg. "We will maintain our quality assurance information to ensure that we are conducting this procedure appropriately."

by Abby Sinnott

1st appeared 3/26/98

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