Exposing a foetus to more than ten radians of radiation has been shown to increase the risk of learning disabilities and eye problems. But you don't need to be concerned. It is rare for X-rays to be stronger than five radians, and most chest x-rays are sixty millirads.
During pregnancy the baby will be exposed to about one hundred millirads of naturally occurring radiation from the sun and earth itself. To keep this in perspective, the amount of radiation the baby will be exposed to during the x-ray will be well below what they receive from natural sources. Just be sure to let the radiographer know that you are pregnant so they can shield you properly.