why are you not allowed to lay on your right hand side when pregnant?

[Alan]

You can sleep on any side that you find comfortable. Sleeping flat on your back when you have a big belly is not recommended as there is a small chance that the weight of your baby could compress some major blood vessels in your back and thus reducing the blood flow to your placenta.

[JennaJayen]

Just had to add my little bit while preg with my DS I couldn't sleep on my left side after hearing my hip go "POP" after sitting down one weekend around 7wks preg. I still can't lie on my left side, even though he's over a year old now. I was told by my physio that I should just sleep in whatever position was most comfortable for me, and if it got too uncomfortable on my back or on my right side to prop myself with pillows under my left hip and shoulder.

I have never heard of not sleeping on the right side though.

[UntoldAngel]

Well I hope you can lay however you want because I have the following issues because of 2 car accidents:
1. sleep on back = severe pain
2. sleep on left side = loss of arm function for half a day
3. sleep on right side = painful hips / pelvis
Sleeping on my tummy is the only comfy position... which I know will become an issue once I get bigger...