10w5d pregnant-do electric blankets cause harm to the fetus or is it just a myth?

No one knows: There is no evidence that electric blankets are harmful. No extensive testing has been done on pregnant women other than despite extensive use no harm has been documented.

You're fine: The stuff about the electromagnetic fields being deadly has taken a long time to vanish after being discredited at least in the minds of most of the scientists whose work i've followed. It seems to me to be recall bias and attempts to find somebody to blame. The beating heart produces an electromagnetic field. Set the temperature low enough to be safe.