TUESDAY, May 31, 2011 (HealthDay News) -- Getting a flu shot during pregnancy appears to offer some protection from premature births and low birth weight babies, a new study finds.
U.S. researchers looked at data on nearly 4,200 births between June 2004 and September 2006 in the state of Georgia. About 15 percent of the women received a flu shot during pregnancy.
Pregnant women who received the vaccine and who gave birth during the assumed flu season (from October through May) were 40 percent less likely to have a baby born prematurely, that is, before 37 weeks' gestation, the study found.
"The effect is significant during the flu period, and it goes up along with the intensity of flu circulation," said lead study author Saad B. Omer, an assistant professor of global health, epidemiology and pediatrics at Emory University Schools of Public Health & Medicine.
Women who gave birth when there were some, but not widespread, reports of flu were 56 percent less likely to have a premature baby than unvaccinated women. During peak flu season, generally January and February, pregnant women who got the flu shot were 72 percent less likely to deliver prematurely.
The study, published May 31 in PLoS Medicine, also found a slight association between flu vaccine and protection from "small for gestational age" babies (a birth weight, head circumference or length in the bottom 9 percent) during peak flu season, but not at other times.
Babies born during peak flu season to mothers who were vaccinated against flu were 69 percent less likely to be small for their gestational age, the researchers found.
Outside flu season, researchers said the study found no association between premature births and flu vaccination. This was expected and lends support to their hypothesis that the effect they're seeing is actually due to the flu vaccine and not some other factor.
However, they emphasized that their findings merely show an "association" between the vaccine and reduced risk of prematurity, not a direct cause and effect.
Infections during pregnancy can affect fetal growth and development, according to background information in the article. Respiratory infections, particularly pneumonia, are associated with low birth weight and increased risk of pre-term birth.
Influenza is particularly dangerous for pregnant women, who have a greater risk of serious illness and death. "There is a lot of evidence that flu is much more severe in pregnant women than in women of a similar age who are not pregnant," Saad said.
Toward the end of pregnancy, women's lung capacity decreases, and the heart must work harder to pump blood to support the fetus, which taxes the body. Pregnancy may also make it more difficult for the immune system to mount a response to the flu.
Prior research conducted in women in Bangladesh found that getting the flu vaccine during pregnancy can even help protect the infant from flu after birth.
Taken together, the evidence is clear that women who are pregnant during flu season should get the flu vaccine, said Dr. Siobhan Dolan, a consultant to the March of Dimes and an associate professor of obstetrics and gynecology at Albert Einstein College of Medicine.
"This study adds to the body of evidence of how beneficial the flu vaccine is, both in decreasing the risk of pre-term birth and slightly decreasing the risk of low birth weight," Dolan said.
The American Academy of Pediatrics, the American College of Gynecologists and Obstetricians and the March of Dimes all recommend women get vaccinated against the flu during pregnancy.
Despite that recommendation, only about 15 percent of pregnant women get the seasonal flu vaccine annually, experts said, although that rose to nearly 50 percent during the 2009-2010 H1N1 crisis.
"It's good for you, and it's good for your baby," Dolan added. "It's the right thing to do, and the data is clear."
The March of Dimes has more on vaccines and pregnancy.