Blood tests and ultrasound have led to the more selective use of invasive procedures like amniocentesis and chorionic villus sampling (CVS) to detect abnormalities in the fetus, a study released today suggests.

“Ideally,” the authors say, prenatal screening will become accurate enough so that invasive testing would only be needed to confirm, or precisely define, a chromosomal abnormality detected noninvasively.

“Our results show that there has been substantial progress toward this objective,” Dr. Peter A. Benn from the University of Connecticut Health Center in Farmington and colleagues write in the medical journal Obstetrics and Gynecology.

Pregnant women over 35, who are more likely to have a child with Down’s syndrome or other genetic abnormalities, are likely to benefit most. Amnio or CVS can detect these problems at an early stage, but the procedures entail some risk.

The investigators reviewed all amnio and CVS samples processed at a single laboratory over a 12-year period (1991-2002).

Despite an increase in the number of women becoming pregnant after age 35, there was a decline of more than 50 percent in the number of women undergoing amniocentesis or CVS - from 1,988 in 1991 to 933 in 2002.

There also was a significant 68 percent drop in the number of women referred for invasive prenatal testing solely on the basis of their age (from 1,314 in 1991 to 423 in 2002).

Despite the decline in the number of amniocentesis and CVS tests performed, the number of fetuses detected with Down’s syndrome increased from 20 cases in 1991 to 31 in 2002. Even so, this number was only about half of the fetuses actually affected, the authors note.

They say the proportion detected is “less than might have been expected, given the improvements in screening over time.”

The researchers thus conclude that it will be important to determine whether this is a result of patients’ attitudes toward prenatal testing, a desire not to know whether the fetus is affected, or whether there are “substantial access barriers” to screening.

SOURCE: Obstetrics and Gynecology, June 2004.