As your uterus enlarges, it is possible that lying on your back (in a supine position) will cause you to feel faint or even pass out. This is caused by the weight of your uterus pressing on the main blood vessel that carries the blood back to your heart from your lower body. All that blood that isn’t getting back to your heart makes your blood pressure go too low, causing the faint feeling. This can be scary, but it is easily fixed by avoiding positions where you are laying on your back. If you start to feel faint in a back-lying position, you should turn to your side or sit up.