Second-trimester ultrasound a viable alternative to amniocentesis, expert says. (Testing for Down Syndrome).

SACRAMENTO -- A second-trimester ultrasound exam using three parameters is a viable alternative to amniocentesis for evaluating the possibility of Down syndrome, Dr. John C. Hobbins said at an ultrasound update sponsored by the University of California, Davis.

Those data come from an eight-center study that examined 176 fetuses with Down syndrome. The goal of the study was to consider how well each of about 12 different possible features seen on ultrasound predicted that a fetus had Down syndrome.

The three markers found to have the most predictive value were short femur length, the presence of structural anomalies such as a heart defect, and nuchal fold thickening. When those three features were used, they positively identified 58% of the fetuses with Down syndrome, said Dr. Hobbins, chief of obstetrics at the University of Colorado Health Sciences Center, Denver.

Viewed another way, the absence of any of those findings would mean the risk that the fetus had Down syndrome would be 50% lower, he said in an interview. Therefore, when combined with a triple screen, the ultrasound evaluation can be a fairly certain alternative to amniocentesis.

The risk of losing a fetus following amniocentesis is 0.5%-1%, Dr. Hobbins estimated.

But in a woman over age 35 years, the risk that her fetus has Down syndrome may become as low as 1 in 500 if she has a negative triple screen and none of the three findings are seen on ultrasound. "If it was my wife I would opt for that," Dr. Hobbins said in the interview. "I'm a guy who has done amnios for 35 years. I have a healthy respect for it."

Ultrasound evaluation for Down syndrome, generally using about eight or nine markers together, was becoming an increasingly common practice until recently A metaanalysis published last year has put a damper on the enthusiasm, Dr. Hobbins said.