Recently, a novel swine-origin influenza A/H1N1 virus was identified as a significant cause of febrile respiratory illnesses in Mexico and the United States. It rapidly spread to many countries around the world, prompting the World Health Organization to declare a pandemic on June 11, 2009. Pregnant women are at an increased risk for serious consequences of influenza infection. A 15 microgram (mcg) dose of unadjuvanted inactivated H1N1 vaccine is recommended for healthy adults and recent preliminary data indicates this dose is likely to be protective for pregnant women. However, a higher dose of an unadjuvanted, inactivated influenza H1N1 vaccine may be necessary to confer optimal protection to this at risk population. This protocol will explore the antibody response following vaccination of pregnant women at 2 different dose levels (15 mcg and 30 mcg) and a non-pregnant control group receiving a 15 mcg dose. Antibody responses will be assessed at 21 days and 180 days post vaccination. For pregnant women, maternal and cord blood will be collected to determine the level of H1N1 antibodies transferred to the baby. An optimal immune response in pregnant women would impact transplacental transport of protective antibodies which is important since vaccines are not available for infants younger than six months, another at risk population for severe H1N1 disease. Sustained immunity for at least 6 months post-vaccination would impart benefit not only to the woman herself but also decrease a primary exposure risk (infected mother) for the newborn infant. Furthermore, a systematic evaluation of the kinetics of maternally transferred antibodies to 2009 H1N1 influenza virus will help improve and develop strategies to protect infants from influenza. This is an open label, Phase II study in pregnant and non-pregnant women, aged 18-39 years old designed to investigate the safety, reactogenicity, and immunogenicity of an inactivated influenza H1N1 virus vaccine. Pregnant women will be randomized into 2 groups (100 per dose group) to receive intramuscular (IM) inactivated influenza H1N1 vaccine at 15 mcg (Group 1) or 30 mcg (Group 2). A non-pregnant control group of 100 subjects will receive IM inactivated influenza H1N1 vaccine at 15 mcg (Group 3). Following immunization, safety will be measured by assessment of adverse events (AEs) through 21 days post vaccination (serious AEs and new-onset chronic medical conditions monthly through 6 months post vaccination (Day 180). Reactogenicity to the vaccine will be assessed for 8 days following vaccination (Day 0-7). Immunogenicity testing will include hemagglutination inhibition assay (HAI) and neutralizing antibody testing on serum obtained on Days 0, 21, and 180. For the pregnant groups, HAI antibody testing will also be performed on serum from the participant at delivery, on serum from cord blood collected at delivery and, when possible, on serum samples collected from the participant at day 180 and from the infant at 6 weeks, four months, or six months after delivery. The primary safety objective is to assess the safety of unadjuvanted, inactivated H1N1 influenza vaccine in pregnant women when administered at the 15 mcg or 30 mcg dose. The primary immunogenicity objective is to assess the antibody response to unadjuvanted, inactivated H1N1 influenza vaccine in pregnant women when administered at the 15 mcg or 30 mcg dose. The secondary objective for pregnant women is to assess the efficiency of placental transport of maternal influenza antigen specific antibodies to the neonate.