my friend is about 25 weeks pregnant and went for a ultrasound last week..during it she felt really faint and the lady doing the scan said to roll onto her side...to cut a long story short, the reason we get to told to lie on our sides or someother way, other than our back is because the baby pputs pressure on ur spine and depending where he/she is sitting/lying in ur belly, can be lying on nerves...which is why you can get sick/faint etc so there is a scientific thory; and for my friend it proved true- but personally i have been lying however i want...although i am only 11 and a half weeks now... i guess time will tell...dont stress too much girls...