"When I use a stethoscope to listen to an [adult] patient's heart, the sound that I'm hearing is caused by the opening and closing of the cardiac valves," says Dr. Nisha Verma, an OB-GYN who specializes in abortion care and works at the American College of Obstetricians and Gynecologists.
The sound generated by an ultrasound in very early pregnancy is quite different, she says.
"At six weeks of gestation, those valves don't exist," she explains. "The flickering that we're seeing on the ultrasound that early in the development of the pregnancy is actually electrical activity, and the sound that you 'hear' is actually manufactured by the ultrasound machine."
That's why "the term 'fetal heartbeat' is pretty misleading," says Dr. Jennifer Kerns, an OB-GYN and associate professor at the University of California, San Francisco.