The study included 340 pregnant women in Bangladesh who were divided into two groups -- 170 who received the flu vaccine and 170 who received a different vaccine that does not protect against the flu. All of the women were in their third trimester.
When the seasonal influenza virus was circulating in the population, the flu vaccine group had fewer babies who were small for their gestational age than the other group -- about 26 percent versus 45 percent.
The percentage of small-for-gestational-age births was similar in both groups when the influenza virus was dormant, according to the study published Feb. 21 in CMAJ (Canadian Medical Association Journal).
When the influenza virus was circulating, the mean birth weight was 7 pounds in the flu vaccine group and 6.6 pounds in the group that didn't get flu shots, the investigators found.
"Our data suggest that the prevention of infection with seasonal influenza in pregnant women by vaccination can influence fetal growth," Dr. Mark Steinhoff, of Cincinnati Children's Hospital Medical Center in Ohio, and colleagues wrote in their report.
Image: Pregnant woman, via Shutterstock.