Pregnant women who were given H1N1 influenza vaccine in 2009 were less likely to face adverse fetal outcomes such as preterm birth. Also, they gave birth to heavier infants. These are the conclusions of a study published on Clinical Infectious Diseases, coordinated by Dr. Saad Omer, of the Rollins School of Public Health at Emory University, Atlanta. He and his colleagues conducted a retrospective cohort study of live births during the period of 2009 influenza A (H1N1) virus circulation, finding that infants of vaccinated mothers had 37% lower probability of being born preterm than infants of unvaccinated mothers. As for the birth weight difference, infants of vaccinated mothers weighed 45.1 grams more than those with unvaccinated mothers.

CID paper: http://cid.oxfordjournals.org/content/early/2013/01/31/cid.cit045.abstract?sid=616e87b3-e15f-4e62-8781-7a4d98b49990