You may faint, or almost faint, during pregnancy, especially if you get up quickly. Feeling dizzy is normal and common.
Fainting and dizziness can occur in pregnancy at any time. When standing, more blood goes to the uterus and legs and away from your head.
Anemia or low blood pressure
If you also feel tired, have an irregular heartbeat and paleskin or feel out of breath, ask your doctor if you may suffer from anemia. Low blood pressure can also cause dizziness.
From angelababy In Q&A