Influenza is a significant cause of morbidity and mortality in the United States. Pregnant women and infants are at an increased risk for the complications of influenza. Pregnant women are considered a high risk population. In the United States, routine vaccination with inactivated trivalent influenza vaccine (TIV) for women who are pregnant or deliver during the influenza season is recommended. However, few studies exist on the safety and immunogenicity of administration of seasonal inactivated TIV. Although influenza vaccination has been recommended during pregnancy, the rates of immunization remain low, at about 13 percent, which partly reflects concern of safety among pregnant women and providers. It is important to gather prospective data on the safety and immunogenicity of inactivated TIV vaccine in pregnant women. This is a multi-site randomized, double-blinded clinical trial in 180 healthy 18-39 year old, pregnant women in their second or third trimester of pregnancy (from 14 weeks of gestation to 33 weeks/6 days gestation, inclusive) and 60 healthy 18-39 year old non-pregnant controls. Study subjects will be randomized 1:1:1 to receive one of the following three 2010-2011 seasonal inactivated trivalent influenza vaccines: Fluarix®, Agriflu®, or Fluzone® (60 pregnant women and 20 non-pregnant women per group). The study will begin enrollment when at least 2 of the 4 study products are available. Once enrolled, a blood sample will be collected and each subject will receive a single 0.5 mL dose of the assigned vaccine. Subjects will be observed for at least 15 minutes after immunization, and the subjects will maintain a memory aid to record oral temperature and systemic and local adverse events (AEs) for 8 days after immunization. Subjects will have a phone call on Day 2 and 8 for review of memory aid, concomitant medication assessment, and assessment of AEs. At approximately Day 28 post-vaccination, subjects will return to the clinic for blood sample collection, AE and concomitant medication assessment, and a targeted physical examination (if indicated). Pregnant subjects will also have scheduled phone calls from Day 28 until the time of delivery or Day 180, whichever is later, to assess for any new-onset chronic medical conditions, and pregnancy-related problems such as miscarriage or serious adverse events (SAEs) since the last visit. At approximately Day 180 post-vaccination, subjects will return to the clinic for blood sample collection and assessment of the receipt of any vaccines since the last visit. Subjects will be asked about any new-onset chronic medical conditions, SAEs since the last visit and pregnancy-related problems such as miscarriage (for the pregnant women cohort). The duration of the study will be approximately 6 months for the non-pregnant subjects and about 6-8 months for the pregnant subjects, depending on delivery date for the pregnant women. Pregnant subjects will have collection of maternal and cord blood at the time of delivery. Pregnant subjects will also have a phone call 1 month after delivery to assess for any new-onset chronic medical conditions or SAEs since the last visit. The primary objectives are to evaluate the safety of a single 0.5 mL intramuscular injection of the 2010-2011 inactivated TIV in pregnant women and to evaluate the immunogenicity of the 2010-2011 inactivated TIV in pregnant women by hemagglutination inhibition assay (HAI). The secondary objectives are to evaluate the persistence of antibodies to the 2010-2011 inactivated TIV in pregnant women at Day 180 and to assess the transfer of maternally derived antibody against viruses in the 2010-2011 inactivated TIV to infants bor