This is most common during the first trimester, but it can happen anytime during pregnancy. Dizziness and fainting are often caused by a drop in blood pressure. This is from the hormones released during pregnancy that relax the body's blood vessels. Too little blood is then pumped up to the brain. When this happens, you lose consciousness and faint or black out. Also Try to eat every few hours. Instead of eating 3 larger meals a day, try eating 6 smaller meals. Snack on healthy foods that contain some protein like nuts and peanut butter to avoid a drop in your blood sugar.
Make sure to talk to your OB doctor about your black outs.