Researchers from Yale University School of Medicine report that a small, but significant, number of nerve cells in fetal mouse brains did not migrate to the correct location after being exposed to prolonged ultrasound waves.
This process of cell migration, known as neuronal migration, is essential for proper brain development.
The findings fall far short of proving that ultrasounds, which use sound waves to create an image, pose any threat to developing humans.
But they do raise concerns about the growing commercialization of ultrasounds marketed to parents-to-be for prenatal keepsake photos, experts tell WebMD.
"There is nothing to show that ultrasound is in any way harmful to the human fetus if the test is done when medically indicated by people who know what they are doing," Jacques S. Abramowicz, MD, tells WebMD. "But exposures should be kept to the lowest levels needed to achieve the medical aims."
Abramowicz directs the ultrasound unit at Chicago's Rush University Medical Center, and also chairs the bioeffects committee for the American Institute of Ultrasound Medicine (AIUM).
The AIUM has come out strongly against commercially marketed, nonmedically indicated fetal imaging, as has the FDA.
In a 2004 report, the FDA cautioned pregnant women to avoid what it called "entertainment ultrasounds" -- those typically found at shopping malls with names like "Womb with a View" and "Fetal Fotos."
The government agency noted that long-term effects of repeated ultrasound exposures on the fetus are not known. "In light of all that remains unknown, having a prenatal ultrasound for nonmedical reasons is not a good idea," the FDA said.
Findings Need Confirmation
The newly reported study, published online Aug. 7 in the Proceedings of the National Academy of Sciences, is one of the first to examine the possible effect of exposure to ultrasound waves on neuronal migration and brain development.
Yale researchers injected more than 335 fetal mice with special markers that tracked nerve cell development in the womb.
They found exposure to ultrasound waves for 30 minutes or more changed the path of a "small but statistically significant" number of nerve cells. The longer the ultrasound exposure, the greater the disturbance in neuronal migration.
The research did not show the change resulted in behavioral differences in the mice after birth, but the findings were of concern, study co-author Pasko Rakic, MD, PhD, tells WebMD.
"Clearly, we have to confirm these findings by looking to see if they occur in larger animals," he says.
The Yale researchers have begun just such a study in monkeys. Rakic says the results will not be known for several years.
"Our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned," he says, adding the benefits of medically indicated fetal ultrasounds clearly outweigh potential risks.
But he agrees pregnant women should avoid having ultrasounds that aren't medically indicated.
Entertainment ultrasounds could turn out to be this generation's equivalent of X-raying feet to determine shoe size, Rakic says. That practice was common in the late 1940s and early 1950s, and most shoe stores had their own X-ray machines.
"We now know that X-rays can be dangerous, and that exposures should be limited to only those that are medically necessary," he says.
Joshua A. Kopel, MD, a professor of obstetrics, gynecology, and reproductive sciences at Yale, says there is no indication the widespread use of fetal ultrasound over the last three decades has influenced brain development.
"There is no evidence that our children are dumber than we are. If anything, studies suggest that they are smarter," Kopel says. "But any time an ultrasound is used there ought to be a medical question being answered."