They say pregnant woman are not suppose to lay or sleep on there back is this true ? it slows blood flow to the fetus ?

I vote "It's a myth": Normal pregnant women, or any woman, may sleep in whichever position she finds comfortable. Getting a good night's sleep is important, regardless of whether one is pregnant or not. All over the internet, people post "advice" (but without "proof") about pregnancy sleep positions, but one can read about some actual research results at: http://www.jogc.com/abstracts/full/200710_Commentary_1.pdf