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Is the Flu Vaccine Safe During the First Trimester of Pregnancy?


Updated September 04, 2009

Question: Is the Flu Vaccine Safe During the First Trimester of Pregnancy?

My doctor is recommending I get a flu vaccine, but I'm only 6 weeks pregnant. Is there any risk that flu vaccines in the first trimester can cause miscarriage, especially if they contain thimerosal?


Safety of flu vaccines in the first trimester is a tricky question. At the moment, there aren't any studies out there on influenza vaccination specifically in the first trimester, so no one can really say for sure that it is safe. There are studies showing that vaccination against influenza appears to be safe during the second and third trimesters.

However, many clinicians and the CDC recommend vaccination during the first trimester anyway, with the belief that the benefits are likely to outweigh any theoretical risks. There are known risks to having the flu during pregnancy. Fevers, a common side effect of the flu, can cause harm to the baby (including posing an increased risk of neural tube defects), and there is some evidence of higher risk of neurological problems in babies whose mothers had the flu during pregnancy. Pregnant women also appear to face a higher risk of developing dangerous complications after the flu, such as pneumonia.

As for concerns over thimerosal, the CDC does not believe there is any risk to getting a thimerosal-containing vaccine during pregnancy, but the safety evidence is scarce on exposure to thimerosal during the first trimester. Most flu shots do contain thimerosal, but you should be able to ask for a thimerosal-free shot if you are worried. In any case, discuss your concerns with your doctor and see if you can agree on a plan that works for you.


Ayoub, David M. and F. Edward Yazback. "Influenza Vaccination During Pregnancy." Journal of American Physicians and Surgeons 11(2): 41-47.

Coincidental miscarriage an issue; CDC advisory panel: flu vaccine in first trimester is OK. OB/GYN News. Accessed: Sept. 1, 2009. http://findarticles.com/p/articles/mi_m0CYD/is_7_39/ai_n5996296/

Mak, Tippi K, Punam Mangtani, Jane Leese, John M Watson and Dina Pfeifer. "Influenza vaccination in pregnancy: current evidence and selected national policies." Lancet Infect Dis. 2008 Jan;8(1):44-52.

Munoz FM, Greisinger AJ, Wehmanen OA, Mouzoon ME, Hoyle JC, Smith FA, Glezen WP. "Safety of influenza vaccination during pregnancy." Am J Obstet Gynecol. 2005 Apr;192(4):1098-106.

Naleway, Allison L., Wendy J. Smith and John P. Mullooly. "Delivering Influenza Vaccine to Pregnant Women." Epidemiologic Reviews 2006 28(1):47-53; doi:10.1093/epirev/mxj002

Thimerosal in Seasonal Influenza Vaccine. CDC. Accessed: Sept. 1, 2009. http://www.cdc.gov/FLU/ABOUT/QA/thimerosal.htm

Skowronski, Danuta M. and Gaston De Serres. "Is routine influenza immunization warranted in early pregnancy?" Vaccine. 2009 Jul 30;27(35):4754-70. Epub 2009 Apr 16.

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