I found the following information about lying supine when pregnant and why it isn't recommended:
"After the fourth month of pregnancy, if a mom-to-be is in the supine position, on her back, there is a risk that the uterus may compress the vena cava, which could decrease cardiac output, reducing maternal blood pressure. Signs that this has occurred include: lightheadedness, dizziness, nausea and becoming pale or flushed. Circulation should return to normal if you roll onto your left side. Because of this risk, ACOG recommends no exercise in the supine position after the fourth month of pregnancy. However, other fitness organizations say that exercise while on the back may be done for four to five minutes at a time, so it is a bit controversial."
I guess it's up to each individual what they feel is best, but almost all current literature I've read (and I read a lot!) has recommended not laying in the supine position for very long when pregnant.
"After the fourth month of pregnancy, if a mom-to-be is in the supine position, on her back, there is a risk that the uterus may compress the vena cava, which could decrease cardiac output, reducing maternal blood pressure. Signs that this has occurred include: lightheadedness, dizziness, nausea and becoming pale or flushed. Circulation should return to normal if you roll onto your left side. Because of this risk, ACOG recommends no exercise in the supine position after the fourth month of pregnancy. However, other fitness organizations say that exercise while on the back may be done for four to five minutes at a time, so it is a bit controversial."
I guess it's up to each individual what they feel is best, but almost all current literature I've read (and I read a lot!) has recommended not laying in the supine position for very long when pregnant.