Pregnant women with the pandemic avian influenza had a greater risk for death and hospitalization and nearly 65 percent of them deliver prematurely, prompting the Centers for Disease Control and Prevention (CDC) to urge vaccinations against this virus.

Among 347 pregnant women severely ill with the H1N1 virus between 2009 and 2010, 75 died and 272 were admitted to an intensive care unit and survived, the CDC reports on its Morbidity and Mortality Weekly Report Friday.

Among those who survived and delivered while hospitalized for influenza, 63.6% delivered preterm or very preterm and 43.8% delivered low birth weight infants compared with U.S. averages, according to the research.

Also, 25% of babies born after their mother's influenza hospitalization were small for their gestational age, compared with 10.0% of the general population, the CDC reported.

"The severe impact of 2009 H1N1 influenza among pregnant women and their infants emphasizes the importance of prevention in this group," the CDC stated.

"The cornerstone of influenza prevention among pregnant women remains promotion of influenza vaccination...regardless of trimester."

In this study the CDC registered the effect on babies' outcomes for the first time.