A medical ultrasound technique, often used in obstetric ultrasonography (during pregnancy), 3D and 4D ultrasounds use sound waves to create three-dimensional images of your baby in your womb. Three-dimensional ultrasound isalso known as phased array ultrasonics. What’s different is that 3D ultrasounds create a three-dimensional image of your baby, while 4D ultrasounds create a live video effect, like a movie — you can watch your baby smile or yawn.