I'm 37 weeks pregnant. normal bp. heart rate 75-95 bpm. when laying down on either side i feel as tho i'm smothering. sitting upright helps some. heart pounds. i'm well hydrated & no other symptoms. just feel as tho i can't breathe. started yesterday. i s

Lack of blood return: Large uterus tend to compress the vena cava when lying down. Thus, reducing blood return and yo fill like your going to pass out. When lying down you should lay on your left so the weight of the uterus lies on the aorta. This major vessel has muscularis layer which protects it from collapsing.

You should not: Be having to ask that ? Here, call your OB a so you can be seen so he or she can make sure everything is all right, you may need a cardiac work-up or other testing, but you should call your OB asap, you can always have an amnio for lung maturity and get delivered early if necessary.