Ultrasound close ultrasoundSound with a frequency greater than 20,000 Hz (20 kHz). waves have a frequency higher than the upper limit for human hearing – above 20,000 Hertz (Hz). Different ...

A prenatal ultrasound is a non-invasive test that uses ... and whether you're carrying multiples. How far along the pregnancy is makes a big difference in the accuracy of so-called gender ultrasounds.