The ultrasound source produce vibrations at periodic intervals, causing the surrounding tissue particles to be disturbed, and also vibrates in a direction parallel to the direction of the sound waves.
As the tissue particles nearest to the ultrasound source vibrates due to the disturbance from the sound source, they will collide against neighbouring particles, transferring the vibrations from one particle to the next, producing alternate regions of compressions and rarefactions, transmitting the sound waves through the tissues, until the waves eventually reach the fetus, and reflect off the fetus body back to the receiver.
