This paper from the American College of Obstetricians and Gynecologists reviews the guidelines in detail. Here's a summary:
~The magnitude of in-flight exposure depends on altitude and on the 11 year solar sunspot cycle. There is less exposure on a short-haul low altitude flight (SF - Seattle) vs a long-haul high altitude flight. Travel bet New York (NY) and Tokyo on transpolar flights approximates at 150 microsievert = 15 mrem whereas between NY and Seattle is 60 microsieverts = 6 mrem.
~Most radiation experts agree that there's no fetal harm such as mental retardation, growth restrictions, or miscarriages at a dose below 20 millisieverts (mSv) = 2 rem. The long term risk, however, in terms of future risks such as leukemias has not been established - neither confirmed nor ruled out.
~Maximum exposure for a woman through her 40 week pregnancy is advised to be 1 mSv = 100 mrem. So the 2 trips described above would give 15% and 6% of the maximum permitted exposure. You would need at least 7 trips of the transcontinental or 17 trips of the intercontinental duration to reach the maximum dose allowed. The actual amount during each flight can be calculated here if you know the altitudes.
~ The FAA gets an advisory about the solar activity and when to avoid routes when the activity is peaking. Real time proton intensity info is available online from Space Environment Center of the National Oceanic and Atmospheric Administration.
In terms of non-radiation risks with air travel, it should be noted that the safest time to travel is considered to be during the second trimester between 18-24 weeks when the risks for miscarriages and preterm delivery are lowest. In an uncomplicated pregnancy, it is advised not to travel after 36 weeks. In a complicated pregnancy the recommendations would be different.