Your blood pressure will drop during the first trimester. For some women, this involves dizziness and, in some cases, fainting. It usually helps to sit on the edge of the bed for a while before getting up. Avoid standing for extended periods of time. Your blood pressure will rise during the second trimester and any dizziness usually improves or disappears completely.
As the baby and your uterus grow and get bigger, you could feel dizzy if you lie flat on your back. This is called vena cava syndrome. It can occur when the baby in the uterus compresses (presses on) a large vein, located behind the uterus against the back. As a result, the oxygen flow decreases to your heart. Although not dangerous, it can feel unpleasant. Try to quickly turn onto your side and the dizziness will disappear quickly. You can try placing a pillow under your buttock/lower back so that your baby/uterus tips a bit to the side. This will prevent you from lying flat on your back and your uterus will not press on the vena cava. If this does not help, you’ll just have to accept the situation and start resting/sleeping on your side.
Source: - Lärobok för barnmorskor. Faxelid, E. Course literature 2001. - Myles Textbook for Midwives. Fraser, D. och Cooper, M. Churchill Livingstone 2003.