I dont know anything about safety issues, but I have to use mine loads for work (I am a translator and work from home) I bought this laptop table, and it has made a huge difference comfort wise. I am 36 weeks now and its invaluable as there is no real "lap" left to use a laptop on anymore!
I work with a lot (twenty plus) screens at work so researched this when I was pg as part of my risk assessment I could not find anything that said the radiation from the screens would harm an unborn baby.
forgot to mention that laptop table from Amazon (see link above) has a built in fan (gets power from your laptop via a USB), and because it has an inset that angles the laptop upwards the bottom part get lots of air around it, keeping it much cooler.
Thanks for all the replies.... I have been trying to stop putting it directly on my tummy.... I guess Ill probably just go with using a cushion for now, and if it gets tough as I get huge Ill consider a table of some sort.