Although fetal deaths were rare during the study, they were more common in pregnant women with the flu.
"Vaccination itself was not associated with increased fetal mortality and may have reduced the risk of influenza-related death during the pandemic" of 2009, said the team, led by Dr. Siri Haberg of the Norwegian Institute of Public Health in Oslo.
Dr. Jon Abramson of the Wake Forest Baptist Medical Center in North Carolina, who was not connected with the research, said it was already known that influenza can be unusually dangerous in pregnant women.
"The increased hospitalization rate is about fourfold higher among pregnant women compared to women of a similar age who are not pregnant," he told Reuters Health.
With some exceptions, the U.S. Centers for Disease Control and Prevention (CDC) recommends the flu shot for everyone over six months of age, including pregnant women.
Abramson called vaccination "a double bang for your buck."
The study, which appears online in the New England Journal of Medicine, encompassed the H1N1 pandemic in the fall and early winter of 2009 and took place after publicity about more than 30 cases of fetal death following vaccination.
Up until then, pregnant women in Norway were not routinely advised to get the flu vaccine. During the pandemic, as much as 30 percent of the Norwegian population fell ill with the flu.
The vaccine was offered beginning on October 19, 2009 and was usually given during the second and third trimester.
The vaccine wasn't foolproof, but it cut a woman's risk of developing influenza by 70 percent.
Among nearly 26,000 women vaccinated during pregnancy - usually during the second or third trimester - there were 78 fetal deaths, or three per 1,000 pregnancies. Among about 87,000 pregnant women who were not vaccinated, there were 414 fetal deaths, or close to five per 1,000 pregnancies.
When the researchers focused on the women who developed the flu, the risk of losing their child to miscarriage or stillbirth was almost double that of those who had not been vaccinated.
Among all women, vaccination during the study period reduced the likelihood of fetal death by 12 percent, but that difference could be due to chance, the researchers said.
The researchers also found no evidence that vaccination increased the risk of premature delivery or having a baby with a low birth weight.
"We found no evidence that influenza vaccination of pregnant women increased the risk of fetal death," the researchers said. "However, influenza infection itself posed a major risk; among pregnant women who received a clinical diagnosis of influenza, the risk of fetal death nearly doubled."
Smaller studies in Canada and Denmark have produced similar results.
"Because of all these mounting pieces of evidence about the effect of influenza on pregnant women, the World Health Organization in the last year actually recommended that the number one priority for getting the flu vaccine is pregnant women, pandemic or not," Abramson said.
SOURCE: http://bit.ly/13ExDIB New England Journal of Medicine, January 17, 2013.