In pregnant women, flu may cause complications like pneumonia (infection of the lungs) or hospitalization. In the United States (US) it is recommend that all women get flu vaccine if they are going to be pregnant or deliver during the flu season but only a few studies have measured a pregnant woman's immune response (the body's defense against the flu) after getting the flu vaccine. About 200, 18-39 year old, inclusive, pregnant women in their second or third trimester (from 14 weeks of gestation to term, inclusive) will be enrolled in this US based study. Participation will be about 8 months in duration. Women will be randomized (assigned by chance) to receive either Fluzone® or Fluarix®. Blood collection will occur on Day 0 and 28 days post vaccination.