Most people who have had significant contact with the US medical establishment have had an ultrasound. The machines are cheap and easy to use, and provide a rough picture of the state of our internal organs without an invasive procedure or exposure to ionizing radiation. These two safety factors probably contribute heavily to its use for pre-natal checkups that ascertain the health of the developing fetus, the results of which many of you have also seen. But a just-released study suggests that ultrasound may have some subtle effects on embryonic brain development.

The study looked at the developing cortex of the brain. This portion of the brain develops through a stereotypical pattern. The precursors of neurons exist in a single layer of proliferating cells at the base of the cortex. These cells mature into neurons in waves, each of which migrates to the top of the structure. Thus, as more waves are produced over time, the cortex begins to resemble a layer cake, with each layer performing a specialized function. One consequence of this process is that the later waves of cells have to migrate ever further before settling down.

The researchers gave pregnant mice injections with a chemical that marked dividing cells, allowing them to track a single wave of differentiating neurons. One half of the mice were given ultrasound, while controls were put through the same procedure without the ultrasound device being activated. In all cases, the migration of neurons in the embryonic cortex was examined after birth. In both control and exposed samples, a small number of neurons either never made the migration, or got lost along the way, taking up residence in the wrong layer. The difference was in the numbers; once the ultrasound exposure went on for a half-hour or more, the number of neurons getting lost was consistently larger in the exposed group.

It's important to note that these results, although statistically significant, were variable, and the sample size was relatively small. The net result was also simply an enhancement of a situation that's apparently perfectly normal. The authors also discuss the significant differences between the mouse and human brains, not the least of which is that the human cortex develops over a time period that's three times longer than the entire mouse pregnancy. At the same time, the distance human neurons travel is over an order of magnitude further, and the complex brains of primates may have a lower tolerance for error. In short, these results are suggestive; a cause for further study, but not a cause for alarm. The discussion of the paper wraps up by noting that the FDA already recommends against unnecessary ultrasounds, and calls for similar studies to be performed in non-human primates. It's hard to argue with that conclusion.