The purpose of this study is to see how much antibody (proteins produced by the immune system that help fight infections) the body makes after getting a flu vaccine. Researchers will also look at how the body reacts to the flu vaccine and how it affects the babies of pregnant women. The study will enroll approximately 240 women ages 18-39 years, including 180 pregnant women in their second or third trimester of pregnancy (at least 14 weeks pregnant) and 60 non-pregnant women. Participants will be randomly (by chance) assigned to 1 of 3 vaccine groups. Each participant will receive one shot of a 2010-2011 flu season licensed vaccine. The vaccine will be given as an intramuscular injection (shot in the muscle) in the upper arm. Study procedures include pregnancy testing, blood draws, and memory aids. Patient participation may be up to 8 months. The information from this study will help guide researchers in developing flu vaccines for pregnant women.