What Causes Fainting And Lightheadedness During Pregnancy?

My daughter is 19 weeks pregnant and just had a spell where she passed out. She said she felt weak really hot and light headed. She came to after about 30 to 45 seconds. Her blood pressure was 113/68 and pulse 71 what could have caused this and is there reason for concern? She was not doing any physical activity she has an office job and sits at a desk all day.