RENEE MONTAGNE, Host:
Ultrasound images allow doctors to diagnose a wide-range of problems while a fetus is still in the womb. But some parents ask for ultrasound just to add one more baby picture to the collection. Doctors have long discouraged the practice. Now a study of mice offers a new reason to be cautious about unnecessary ultrasound exams.
Here's NPR's Jon Hamilton.
JON HAMILTON: Most previous studies of the safety of ultrasound have looked for signs that a human fetus is affected by typical testing. These studies haven't found any obvious problems, and ultrasound has been around for more than 40 years. But researchers at Yale University wanted to know whether very long exposures to ultrasound might cause subtle changes in one of the most delicate processes in a developing brain. It's called neuronal migration.
Brain cells travel from the center of the brain - where they're formed - to distant locations like the cerebral cortex. It all happens according to a complex and highly precise choreography. Drugs and viruses can disrupt this migration, so what about the high frequency sound waves used to create an image of a developing fetus? To find out, Dr. Pasko Rakic led a team that studied several hundred mice. Half were exposed to ultrasound. Rakic says high levels produced a small change in the way the brain cells migrated.
PASKO RAKIC: We have difference in dispersion of cells in their position.
HAMILTON: The brain cells were a bit less likely to get to the right place. Rakic says it's not clear whether that had any affect on brain function. And he says the change was so subtle, it was difficult to detect.
RAKIC: It's not that you can see that in individual animals. You have to look at 300, and then you statistically analyze and you see one group is a little different from the other.
HAMILTON: The difference showed up in animals exposed to more than 30 minutes of ultrasound at levels comparable to those used to look at a human fetus. But Rakic says there's no direct comparison, because a mouse brain develops in days, not years, and has lots of other differences.
RAKIC: We cannot compensate for the fact that mouse is 1,000 times smaller than human.
HAMILTON: So Rakic has begun a study of monkeys. He says results of that study are several years off.
RAKIC: However, we publish this (unintelligible) just to warn people not use ultrasound unnecessarily. If there is no medical benefit, it shouldn't be used. But if it's a medical benefit, of course, it should continue to be used.
HAMILTON: The study appears in the current edition of the Proceedings of the National Academy of Sciences.
Jon Hamilton, NPR News.
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