This is the only info I can find on this issue. I would consult with your OB/GYN for an approval from them. The heat generated may indeed cause problems and if wireless could also. Other then the below this is all that I am finding. I would personally use them with caution.
http://forums.obgyn.net/pregnancy-birth/P-B.0102/2044.html
Let me know if you need further assistance.
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