The influenza vaccine is recommended by the American College of Obstetricians and Gynecologists " ...in the second and third trimester during the flu season, and  women at high risk for pulmonary complications regardless of the trimester."[1].

Studies of influenza immunization of more than 2,000 pregnant women have demonstrated no adverse fetal effects associated with influenza vaccine. Pregnancy and lactation are not contraindications [2,3].

1. ACOG Committee Opinion. Immunization during pregnancy.Obstet Gynecol. 2003 ;101:207-12.PUBMED

2. Centers for Disease Control & Prevention. Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 47 (No. RR-6): 6, 1998.