During pregnancy, more blood will flow towards the uterus but the return of blood to the upper part of your body is slowed down. This reduces the blood flow to the brain and therefore may cause dizziness or faint. Another cause of dizziness during second trimester is that your growing uterus put pressure the on blood vessel which then slow down the flow of blood to the brain.
Read further on how to prevent dizziness or faint during pregnancy and what to do when dizziness or faint occurs during pregnancy
http://www.babyment.com/pregnancy/dizziness.php
