It all depends on what you mean by "frequent". It is considered safe to occasionally fly on holidays, or trips. If a pregnant woman is a part of the flight crew, i.e. a pilot or a flight attendant, or for work or other reasons she often uses air transport, it is advisable to consult her doctor about the safe frequency of flying. Here, too, the individuality of each case applies, so the general question about the safe frequency of flying cannot be answered with certainty.