by Photo: Siemens Medical Systems

Ultrasound waves are more than loud enough to rouse a sleeping fetus.

Nothing thrills expectant parents like an ultrasound vision of their developing fetus. But Mostafa Fatemi of Minnesota’s Mayo Clinic wondered why such fetuses always seem to move and stretch while their picture is taken. He found out by placing a tiny hydrophone inside a woman’s uterus during the procedure. The device registered up to nearly 100 decibels — as loud as a subway train. While we can’t hear it, a fetus can, says Fatemi, because “a fetus’s ears are filled with fluid, which is a better conductor of ultrasound waves than air.” There’s currently no evidence showing that such noise is harmful, but Fatemi says clinicians may want to aim their ultrasound probes more carefully, away from the baby’s ears, “so we don’t cause a fetus to move, roll, or stretch unnecessarily.”