Why is it suggested for pregnant women to lay on their left side?

I have read this in a number of books and on pregnancy web sites but none have ever said why the left side is better than the right (I get the whole sleeping on your back or stomach thing). Just curious.
Update: Well, I was worried from the first two responses that I had not been very clear on what I was looking for...but the next three were a nice surprise...thanks ladies, that was what I wanted.

I can't understand why some of you were given thumbs downs...bitter about their own thumbs down maybe?
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