For women who did get the flu, the risk of fetal death increased dramatically, the researchers found. Among vaccinated women, the risk of fetal death was far less, they added. Fetal death was defined as any recorded miscarriage or stillbirth after the first trimester.
Moreover, the vaccine was safe, wasn't linked to fetal deaths, and may have reduced the risk of fetal death, Stoltenberg said.
Experts weren't surprised by the results.
"This study confirms what we already know, that pregnancy is a dangerous time for the flu, and H1N1 was particularly problematic for pregnant women," said Dr. Marc Siegel, an associate professor of medicine at NYU Langone Medical Center in New York City.
The flu is especially risky for pregnant women because the virus can pass through the placental barrier and infect the fetus, Siegel explained.
This can result in fetal death or developmental problems, including mental development, he added.
"It's crucial for pregnant women to get a flu shot," Siegel said. "It's important to educate women, and this study helps," he added, noting some women may need convincing because they've been told to avoid certain medications during pregnancy.
Another expert, Dr. Loralei Thornburg, assistant professor of obstetrics and gynecology at the University of Rochester in Rochester, N.Y., said the body's response to infection changes during pregnancy.
"It's kind of an immunosuppressant," she said. "So when you get a bad virus in pregnancy, your body doesn't have the same ability to respond. Preventing infection in pregnancy is really the key."
The bottom line: "Every woman should get the flu vaccine," Thornburg said.
For more information on flu and pregnancy, visit Flu.gov.
SOURCES: Camilla Stoltenberg, M.D., director
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