Like most of your symptoms, this one's due, in part, to hormone and blood pressure changes. As baby continues to grow, the pressure your uterus places on blood vessels can add to the dizziness. Take care of yourself in basic ways — eat regularly and choose healthy snacks, drink lots of water, wear loose and comfortable clothing, get up slowly from sitting or lying down, try not to stand for long periods, don't lie on your back during the second half of pregnancy, and avoid getting overheated — to keep dizziness at a minimum. If your dizziness is accompanied by vaginal bleeding or severe abdominal pain, call your doctor immediately — this might be a sign of ectopic pregnancy. It's also time to talk with your doc if things get so bad that you faint.
When you start to feel lightheaded, sit or lie down right away, and (if your belly allows it) put your head between your knees. Always lie on your left side — this increases blood flow to the heart and brain. Since dehydration can make you dizzy, drink a glass of water. You’ve probably been told countless times since getting pregnant to just take it easy, and that’s exactly what you should do when you start to feel lightheaded.