Arms over the head is definitely an old wive's tale ...

One of the women at our antenatal classes brought this up as she had been yelled at by her MIL and the midwife said that it was a load of cr*p and there was nothing wrong with it as long as you're not overdoing it of course (like anything). She was pleased to be able to and tell her MIL later ...

Lying on the back ... you'll definitely know when you can't do it anymore ... I can't stand it but can't wait to be able to again (that and lying on my stomach again).

Last couple of checkups they've had me on my back for like 5 minutes checking position and stuff (little bugger won't sit in the one spot for 5 seconds) and I get dizzy real easy and start feeling a bit sick.