Ultrasound can pinpoint birth defects, study says

BOSTON — Ultrasound scans can be accurate enough to take the place of amniocentesis for checking fetuses for spinal abnormalities and some other severe birth defects, researchers say.

Amniocentesis, while generally safe, carries a small risk of triggering a miscarriage. Ultrasound poses no known risk to the growing fetus.

However, amniocentesis is often recommended when routine blood tests given to pregnant women raise the possibility that their unborn children have major birth defects.

The blood test checks for elevated levels of alpha-fetoprotein. While many things can cause this protein to be higher than normal, there is a 1 percent chance that it means the baby has a defect.

To make sure the fetus is healthy, doctors frequently withdraw a sample of amniotic fluid with a needle to see if it, too, contains elevated levels of alpha-fetoprotein, a sign of spina bifida, abdominal wall malformations and other serious birth defects.

The new study found that by using ultrasound to take pictures of the fetus, doctors can see these defects directly and accurately judge whether or not the fetus is healthy.

“We may have to modernize our way of thinking,” said Dr. Beryl R. Benacerraf, a co-author of the study. “I wouldn’t say that nobody has to have amniocentesis. But women can choose. They need to be involved in the decision and given the odds.”

The study, conducted at Brigham and Women’s Hospital in Boston, was published in Thursday’s issue of the New England Journal of Medicine.

The researchers conclude that if a woman has an elevated alpha-fetoprotein level and a normal ultrasound scan, there is less than one-tenth of 1 percent chance that her baby will have spina bifida or other related birth defect.

However, if she opts for amniocentesis, there is a half of 1 percent risk that the test itself will terminate the pregnancy.

Dr. Allan S. Nadel, the principal author of the study, noted that some women may still want to rule out the slight risk of a birth defect, even if that might also pose a small risk of losing their pregnancy.

In the study, doctors reviewed the ultrasound findings of 51 fetuses with spina bifida and related abnormalities that were delivered or aborted at the hospital. In every case, ultrasound had correctly identified the birth defects.

However, the doctors cautioned that ruling out these defects requires sophisticated ultrasound equipment in the hands of doctors who are well trained in its use. They said this expertise is available in most large medical centers.

Amniocentesis is also frequently recommended for pregnant women over age 35 to check for Down’s syndrome. Ultrasound is not routinely used to screen fetuses for this birth defect.