"We have observed that a small but significant number of neurons in the mouse embryonic brain do not migrate to their proper positions in the cerebral cortex following prolonged and frequent exposure to ultrasound."
Writing in the Proceedings of the National Academy of Sciences, Rakic's team said the findings do not necessarily mean that ultrasound of human fetuses is dangerous but they said doctors and pregnant mothers should probably keep the scans to a minimum.
Ultrasound scans are one of the delights of pregnancy, giving the parents a peek at the unborn child and doctors a chance to see if there are any serious defects that might be corrected before or right at birth.
But several studies have suggested that ultrasound may affect the developing brain, not necessarily adversely.
For instance, a 1993 study published in the Lancet medical journal found that babies given ultrasounds before they were born were more likely to be left-handed. A separate study found a possible decrease in weight in newborns who were scanned, while a third found delayed speech.
But another study showed that children who had received ultrasound exams before birth actually did better on language tests when they were older, said Dr. Verne Caviness of Massachusetts General Hospital.
More study is clearly needed, Rakic said in a statement.
"We do not have any evidence ourselves that ultrasound waves cause behavioral effects in mice or have any effect on the developing human brain," he said.
"Therefore I want to emphasize that our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned. On the contrary: ultrasound has been shown to be very beneficial in the medical context," Rakic added.
He said the study suggests that pregnant women should not get multiple ultrasound scans for fun or out of curiosity. The American College of Radiology and the U.S. Food and Drug Administration currently recommend that women only get ultrasounds when medically needed.
For their study, Rakic and colleagues did scans of pregnant mice on the 16th day of gestation. This is the last week of gestation and a time when, in mice, the brain cells known as neurons move to a new position in the brain.
After prolonged, multiple scans, some of these cells went to the wrong place, they found.
"Does this study indicate that we should be concerned about human fetal ultrasound?" Caviness asked in a commentary published in the same journal.
He said the implications were not known and noted a human fetus has a much larger and denser brain, and that scans usually just pass over the brain for a few seconds.
"The corresponding neurons in the human brain would probably be formed in the 16th week and continue to migrate for at least 1-2 weeks," Caviness wrote.
No one knows how sound waves might disturb a developing fetus or embryo, both Caviness and Rakic said. |
