12 hr heatpacks - a warning!!

Hi All
Just a quick note, I have been having v bad back ache and my physio and ob have recommended heat (amongst other things). The physio always sticks one of those 12 hr heat packs on me at the end of a session and i use a wheat pack when i am at home..
I went to the chemist to buy some more heat packs for the office and was warned that many of them are UNSAFE during pregnancy. I doubted this as no one had mentioned but they gave me the number of an advisory svc at the royal womens hospital who said that yes some of them are not recommended due to arnica and something else they contain which can cause early contractions and some other things. there are a few brands that are OK - I got some by Elastoplast that they recommended. Anyway, just thought i would let you all know in case you are also a fan of them!
BTW the heat does wonders for the back pain so i do recommend just make sure you ask the pharmacist about them.