Researchers say un-vaccinated pregnant women are at an increased risk of complications from the flu.
Those complications include low birth weight and premature delivery.
Researchers looked at 4,000 cases.
They say infants born to vaccinated mothers were 40 percent less likely to be premature and 69 percent less likely to be low birth weight.
The study was a joint effort by Emory University, Johns Hopkins University and Cincinnati Children's Hospital Medical Center.