Have a look
here
excerpt:
Damage to the fetus when you are already pregnant
Once pregnant, the degree and nature of the damage to the fetus depends on the stage of development and the degree of exposure. According to a United Nations Scientific Committee and the National Radiological Protection Board, the risks can include severe mental retardation and childhood cancer.
For example, the risk of severe mental retardation is said to increase as much as 1 in 10,000 with every 1mSv of exposure during the fourth month. If you flew Athens-New York for 100 hours in that month under normal conditions, you would absorb almost 1mSv and there would be an increased risk of 1 in 10,000 that your baby would be born severely retarded. This risk would be higher if you flew during periods of high solar activity and lower if you flew less at lower altitudes and latitudes.
Given that there are about 96,000 female flight attendants in the US, if we were to assume that a generous 10% (or about 10,000) are pregnant, then one of those fetuses – assuming that their mothers are working full-time international flights with normal solar activity – will be born severely retarded. That statistic would probably horrify most mothers-to-be because the damage is preventable. But the devil's advocate might say that a small number of babies will be born with severe birth defects anyway and such a small addition doesn't justify the administrative and financial burden of giving pregnant flight attendants ground jobs or time off with pay. Other people are unsure of what to ask for. Would an enforced limit on work hours for pregnant flight attendants be popular? Clearly what is acceptable to some people is unacceptable to others.