The finding comes from a randomized trial whose primary finding, Steinhoff said, was that the flu vaccine reduced cases of influenza in babies by 63 percent.
"It shows again that when you prevent flu in a pregnant woman, you benefit the mother, you benefit the infant, and, we think, this also shows that you benefit the fetus," Steinhoff said.
Omer and colleagues reported last month, in the American Journal of Obstetrics & Gynecology, that a review of existing literature shows that pregnant women are at elevated risk of flu- related complications.
On the other hand, they said, the risk of vaccine side effects is small.
"There is substantial evidence that vaccination is not only safe for pregnant women," Omer said in a statement, "but that it is critical for protecting women and their infants against serious complications from the flu."