When pregnant women get vaccinated against flu, their babies are bigger, healthier and less likely to be premature, researchers reported on Thursday.

The studies show that influenza vaccines protect not only women, who are extremely vulnerable to flu when pregnant, but also their babies before and after birth, the researchers said.

They hope their findings, presented at a meeting in Philadelphia of the Infectious Diseases Society of America, will encourage women to get vaccinated and encourage their doctors to offer the shots.

"We are talking about one vaccine protecting two individuals," Dr. Marietta Vazquez of Yale University in Connecticut told a news conference. "Maybe if they are not getting vaccinated for themselves, they will do it for their babies."

Pregnant women are at special peril from flu in any year. Their immune systems are suppressed to keep the body from rejecting the fetus, and the growing baby presses on their lungs.

The U.S. Centers for Disease Control and Prevention has for years recommended that pregnant women be vaccinated against seasonal flu.

This year, with the H1N1 swine flu pandemic, pregnant women are at the front of the line. The CDC says more than 1,000 Americans have died of swine flu, and figures show that 6 percent of deaths have been among pregnant women.

But only 15 percent to 25 percent are ever vaccinated, and babies under the age of 6 months are too young to get a flu vaccine.

"Obstetricians do not offer influenza vaccine. They should know about this recommendation," Vazquez said.


She and colleagues studied some 350 pregnant women starting in 2000 — 157 who got flu and 195 who did not.

"Flu vaccine given to women during pregnancy is 85 percent effective in preventing hospitalization in their infants under 6 months of age," the team wrote in a statement.

Dr. Mark Steinhoff of Cincinnati Children's Hospital in Ohio and colleagues studied pregnant women in Bangladesh, who were randomly assigned to get influenza or pneumococcal vaccines.

The newborns of women who got flu vaccine were 63 percent less likely to be infected, Steinhoff told the news conference. And the babies born to vaccinated mothers weighed, on average, half a pound (215 gm) more, he said.

"When you prevent flu in a pregnant woman, you benefit the mother, you benefit the infant, and it is also shown that you benefit the fetus," Steinhoff said.

The team also found 25 percent of the infants were infected with flu during the first six months of life.

Dr. Saad Omer of Emory University in Atlanta and colleagues looked at 6,410 births between June of 2004 and September of 2006, checking to see how many babies were premature or small for gestational age.

When flu was the most widespread, vaccinated moms had an 80 percent lower risk than unvaccinated mothers of having a premature baby, Omer said. The risk of having a baby that was small for gestational age was 70 percent lower for vaccinated mothers.

There was little effect outside flu season, he said.

It is likely that many of the pregnant women had flu infections that they did not report or notice, but this would have affected the baby, Omer said.

"A mild flu infection probably reduces the amount of nutrition that goes through the placenta," Steinhoff said.