Exposure to ultrasound might affect an unborn baby's brain development, suggest preliminary findings in research with mice.

Scientists injected about 335 fetal mice with markers to track their brain development while in utero. The mice that were carrying those young were subsequently exposed to ultrasound, and a number of their offspring exhibited brain abnormalities. Some of the animals' nerve cells did not reach the correct parts of their developing brains, the researchers observed.

In mammals, brain cells multiply and neurons migrate to their proper destination in the brain, where they are then assigned a function. Environmental or genetic factors -- such as drugs or alcohol -- can disrupt that process, resulting in neurons going to the wrong part of the brain and impairing brain function.

The research findings, which are published in the Proceedings of the National Academy of Sciences, indicate a need for further research with nonhuman primates as well as comprehensive epidemiological studies with humans, the scientists said. They intend to conduct further research using monkeys.

In the meantime, lead researcher Dr. Pasko Rakic cautioned that pregnant women should not avoid getting ultrasounds when recommended by their physicians. In the medical context, they are highly beneficial, he stressed. However, based on the study data, pregnant women should not have ultrasounds for nonmedical reasons.

Even so, it is not conclusive that effects seen in mice would be replicated in people. For one thing, the developmental period during which cells migrate to specific parts of the brain is much shorter for mice than for humans. Thirty minutes of ultrasound may have a far greater impact on a mouse's developing brain than on a developing human's.

It is also worth noting that most ultrasounds on pregnant women take fewer than 20 minutes, and settings are kept as low as possible to achieve a diagnosis.