Last Tuesday, Tom Cruise revealed that he had purchased an ultrasound machine to glimpse inside the womb of his pregnant fiancee, Katie Holmes. Doctors voiced concern at the news that Cruise and Holmes would be performing sonograms at home; a representative from the American College of Radiology said that "if it is not medically necessary, the use of ultrasound raises unnecessary physical risk to the fetus." Are sonograms really dangerous?
They can be. An ultrasound machine works by sending vibrations into the body and then waiting for them to bounce back. The machine can use information from the echoes to produce a moving image of a fetus. But not all of the energy that goes into the body comes back out—some gets absorbed in the tissues. This can cause cells to heat up, or it can make trapped gas bubble up. Studies of ultrasound in lab animals have shown that heat and bubble formation (or "cavitation") can damage internal organs.
Few studies of ultrasound have been conducted on the human fetus (for ethical and logistical reasons), and there's no smoking gun to suggest that the machines are causing harm. We've known for a long time that ultrasound heats up human tissue—that's the rationale for its application in physical therapy. Several experiments conducted overseas have shown an increase in left-handedness (or at least a reduction in right-handedness) among those exposed to prenatal ultrasound, which suggests that the test could have neurological effects.
Even if ultrasound is relatively safe for developing babies, doctors try to limit the number, duration, and intensity of imaging sessions. Doctors sometimes use ultrasound in the first trimester to confirm the baby's heartbeat, screen for genetic defects, find the source of pain or bleeding, or for other reasons. Most pregnant women get one at about the 20-week mark, to make sure that the fetus is healthy and growing at a normal rate. More sonograms might be taken later with a specific medical rationale. *
To help mitigate the risk of heat and cavitation, doctors check for two numbers displayed on the screen of a modern ultrasound device—the "thermal index" and the "mechanical index." These indices give a sense of how much the vibrations might affect the body, so doctors try to keep them below a certain threshold whenever possible.
Is Tom Cruise putting his baby—or his fiancee—at risk? It depends on what kind of machine he's using, and whether he's got a trained sonographer to help him out. He may have a machine that doesn't have the more dangerous high-power settings. Doctors tend to use those settings when a regular ultrasound won't penetrate an especially thick layer of fat. (It's hard to imagine that you'd need to turn up the machine for Katie Holmes.) Cruise isn't necessarily putting his baby at greater risk by looking at it multiple times—unlike X-rays, ultrasound seems not to cause cumulative damage across sessions. But without medical supervision, each individual sonogram could be harmful.
In view of the possible risks posed by ultrasound, the FDA regulates its use. If Cruise is operating the machine himself, he may be breaking the law. Companies that offer parents a nonmedical, "keepsake" sonogram may also be breaking the law. So far, the government hasn't spent too much time enforcing its ban on the use of ultrasound "for entertainment purposes." FDA officials have bigger fish to fry.
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Explainer thanks Dr. Joshua Copel of Yale University and Dr. Carol Rumack of the American College of Radiology.
Correction, Dec. 6, 2005: This piece originally stated that pregnant women rarely get sonograms during the first trimester. The practice is not uncommon. The piece also implied that most women receive only one sonogram during their pregnancies. In many cases, a fetus will be imaged multiple times. (Return to corrected paragraph.)