Using an ultrasound machine, millions of high-frequency ultrasonic sound waves are transmitted into the human body—and the reflections, or echoes, they send back are used to see what’s going on inside. Ultrasound is commonly used to look at how fetuses are doing inside their mother’s wombs. This is a picture of an ultrasound done when the fetus is 17 weeks old. (Darren Brode/ Shutterstock) Read More