If your microwave is old or damaged, however, some radiation might leak through.

http://www.babycenter.com/406_is-it-safe-to-use-my-microwave-oven-during-pregnancy_1245301.bc

Here is a way to test it: Open the door and place strip of paper in the door jam. Close the door, capturing the paper. Now pull the paper. If the paper slides through the door, the seal is poor. Replace the seal or don't use the microwave.