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You go into the doctors, they have you change your clothes, then you lay back and they will put this gel type thing onto your stomach and rub it so it covers your entire stomach area. Next they take something that is hooked to a machine and rub it on top of your stomach over top of the gel type thing, on the machine screen you can see your baby. I hope this helps, I tried to explain it the best I could. Congrats on your pregnancy!
The thing they rub on your belly is a transducer which both emits and receives pulses of ultrasonic sound which is reflected back to the transducer when the sound encounter any change is density of the material the sound encounters. A computer in the device which is connected to the transducer reconstructs an image of the surfaces that reflect the sound. In the case of obstetrics the images shows the outline of the fetus with such detail that you can often tell the sex of the fetus as well as the number of finger or toes.
For a detailed answer, read this article. http://www.mrcophth.com/commonultrasoundcases/principlesofultrasound.html#The