Crew are generally advised not to fly in the first trimester due to the amount of radiation received. Radiation in the first 3 months of the foetus' development can cause all sorts of abnormalities. Flying as a passenger should be fine, its just that crew get a lot of radiation due to the number of flights (and are officially monitored by their employers as radiation workers).
After the first trimester it is OK to fly as crew, in the US it is quite common and many airlines now have maternity uniforms for pilots.