The research found no evidence that the vaccine increases the risk of losing a fetus, and may prevent some deaths. Getting the flu while pregnant makes fetal death more likely, the Norwegian research showed

The flu vaccine has long been considered safe for pregnant women and their fetus. U.S. health officials began recommending flu shots for them more than five decades ago, following a higher death rate in pregnant women during a flu pandemic in the late 1950s.

But the study is perhaps the largest look at the safety and value of flu vaccination during pregnancy, experts say.

This is the kind of information we need to provide our patients when discussing that flu vaccine is important for everyone, particularly for pregnant women,” said Dr. Geeta Swamy, a researcher who studies vaccines and pregnant women at Duke University Medical Center.

The study was released by the New England Journal of Medicine on Wednesday as the United States and Europe suffer through an early and intense flu season. A U.S. obstetricians group this week reminded members that it's not too late for their pregnant patients to get vaccinated.

The new study was led by the Norwegian Institute of Public Health. It tracked pregnancies in Norway in 2009 and 2010 during an international epidemic of a new swine flu strain.

Before 2009, pregnant women in Norway were not routinely advised to get flu shots. But during the pandemic, vaccinations against the new strain were recommended for those in their second or third trimester.

The study focused on more than 113,000 pregnancies. Of those, 492 ended in the death of the fetus. The researchers calculated that the risk of fetal death was nearly twice as high for women who weren't vaccinated as it was in vaccinated mothers.

U.S. flu vaccination rates for pregnant women grew in the wake of the 2009 swine flu pandemic, from less than 15 percent to about 50 percent. But health officials say those rates need to be higher to protect newborns as well. Infants can't be vaccinated until 6 months, but studies have shown they pick up some protection if their mothers got the annual shot, experts say.

Because some drugs and vaccines can be harmful to a fetus, there is a long-standing concern about giving any medicine to a pregnant woman, experts acknowledged. But this study should ease any worries about the flu shot, said Dr. Denise Jamieson of the Centers for Disease Control and Prevention.

"The vaccine is safe,” she said.