A former UCSF medical student, Carolyn Rees, now a doc in rural Idaho, wrote me a very informative letter — and raised several interesting questions — that are definitely worth a little discussion here.

Dr. Rees asked: Is there any evidence that ultrasound examination can affect brain development?

In fact, that evidence is mixed. Over the past 15-25 years, a number of smaller studies conducted principally in North America recorded cognitive and language impairments in children that were attributable to ultrasound examination — while results in several other subsequent large studies conducted principally in the public health systems in Europe were negative.

On the other hand:

  1. Elegant studies conducted in monkeys by an eminent brain scientist at Yale (Dr. Pasko Rakic) have shown that ultrasound exams result in an alteration of the normal, detailed organization of the cerebral cortex that specifically applies for neurons that are migrating into the cortex at the time of the exposure. In other words, across roughly the 2nd trimester of pregnancy when cortical layers are being formed, you can actually determine the time of administration of the ultrasound exam post hoc, by looking at the location of abnormally oriented neurons in the layers of the cerebral cortex.  Does this have a functional consequence for the brain?! No one really knows.
  2. Seven or eight years ago, Sandy Blakeslee, a science reporter for the New York Times (and a long-time friend), sent me the reference to a study from a Mayo research team in Phoenix in which scientists had measured the levels of audible sound stimulation that bombards the fetus during an ultrasound exam. It turns out that in one part of an ultrasound examination the very high (“ultra”) frequency sound is “modulated” at low frequencies to generate the sharpest images. That modulation creates an audible sound that is very intense (greater than 100 decibels). It is not surprising that the third-trimester fetus — whose hearing is intact across this period — writhes in the womb when the beam moves onto the head! For the ultrasound machines investigated by the Mayo scientists, the highest sound energies transmitted to the fetus were centered in the range of frequencies that are most crucial for resolving the sounds of aural speech. You might note that any untoward consequence of an audible sound-induced exam would be limited to the third trimester, because the baby has no effective hearing until roughly the beginning of the 7th month of gestation.

I talked a doctoral student on a rotation project in my laboratory into studying the neurological impacts of simulating 1) a single exam in the third trimester; or 2) five exams — in both cases using the rat infant as our model. Simulation was relatively simple in the rat because, relative to the human baby, rats are born at a young age; their hearing is not intact until they are 11-12 days old. Sound stimuli designed to mimic sounds received by human fetuses in ultrasound exams were created with the help and advice of the Mayo Research Institute scientists. We played them to our rat babies shortly after they acquired hearing, exposing them for the measured times that would apply in a real exam(s).

Even these brief exposures to these loud sounds degraded the representation of sound frequencies in both rat groups. That degradation was especially striking in the multiply-exposed rats. Strong negative consequences of this exposure endured into adulthood. We were surprised by the magnitudes of these recorded effects. A single exposure was limited to 2 minutes (simulating the time the beam might be directed toward the human fetus’ head); multiple exposures involved only 10 minutes of total, intense-sound exposure, delivered in 5 time-separated epochs.

Five points to emphasize:

  1. Given this outcome, ultrasound exposure may plausibly add to the risk of onset of a more devastating condition (e.g., autism) in an already-genetically-vulnerable fetus. It should be put on that short list of possible (unfortunately, STILL UNPROVEN) contributors to the increased rates of incidence of autism. As with the exposure to chemical poisons (non-coplanar PCBs; PBDEs), the use of ultrasound has increased dramatically over the past two decades, and third-trimester exams have become routine.
  2. Ultrasound examinations have been shown to have little or no medical value in the third trimester. I was surprised to learn from the medical literature that they do not make any key contribution to medical decisions or significantly change medical outcomes over this period. It can be argued that they have considerable sociological value strengthening doctor-patient and parent-fetus relationships — which are undeniably important. But beyond that, excepting a tiny percentages of cases, they are an unnecessary aspect of prenatal care — unless you want a picture of fetal-Sissie or fetal-Junior hanging above the mantlepiece!
  3. Boutique photography shops with ultrasound machines that can provide you with a crystal-clear picture of little Sally-fetus or Jerry, Jr-fetus would seem to this scientist to be more than a little bit over the top.
  4. Guess who is in line for MULTIPLE ultrasonic exams? Those kids already at greatest risk for cognitive problems are high on this list. Alas.
  5. Different ultrasound manufacturers use different strategies for modulating the ultrasound stimulation to generate the most-resolved images, and some generate more intensely audible sounds than others. I’ve had trouble running down these specs before writing this entry. I’ll contact the scientists in Phoenix and provide a table in a future entry.