New research from Australia confirms that the HIN1 flu hits pregnant women particularly hard-especially if they have asthma, obesity or diabetes.

“This finding underscores the importance of education regarding recommendations for vaccination in pregnancy and the need for rapid testing and earlier use of antivirals in suspected influenza,” Dr. Michelle L. Giles of Monash Medical Center in Clayton, Victoria, and her colleagues write.

While more than three-quarters of the women in the study had been treated with oseltamivir (Tamiflu), Giles and her team add, two-thirds had been sick for at least 48 hours before getting the drug. Flu medication should be started as soon as possible after symptoms begin to give patients the most benefit, they say.

Past studies have found that pregnant women with influenza may be at greater risk of complications like pneumonia, Giles and her colleagues note, although the effects of the flu on the fetus are less well understood.

To investigate the effects of the H1N1 influenza A strain in pregnancy, the researchers looked at 43 pregnant women with lab-confirmed H1N1 admitted to six different Victoria hospitals during the 2009 outbreak.

Two women were admitted during their first trimester, 13 during their second trimester and 28 in their third trimester. Twenty-five had been hospitalized due to flu-like illness; all but one of these women spent less than a week in the hospital. But among the 11 patients admitted for pneumonia, seven were hospitalized for at least a week.

Half of the women had at least one other health problem, such as asthma, obesity, or diabetes mellitus, but these women did not seem to be at increased risk of pneumonia or pregnancy complications compared to women without other health problems.

Fifteen of the women delivered their babies during hospitalization, six before 37 weeks’ gestation and 9 at 37 weeks or later. The researchers had outcome information on 24 babies at the end of July 2009. Twenty-one were alive, two had died in the womb (at 26 and 31 weeks’ gestation), and one died 26 days after delivery due to prematurity-related complications (this infant had been born at 26 weeks’ gestation). Seven of the babies, including the infant who died, were tested for H1N1, and none of them were infected.

Forty percent of the women went into preterm labor, Giles and her team note, while the normal rate of premature delivery for the hospitals included in the study was around 10 percent. All of the women who delivered their babies before 37 weeks and had chest X-rays were confirmed to have pneumonia.

“The mechanism by which pregnancy, particularly late pregnancy, increases disease severity is unknown,” Giles and her team write. Immune system changes could be a factor, they add. A woman’s expanding belly can reduce her lung capacity, they add; this as well as the increased demands pregnancy places on the heart and lungs could also be factors.

Flu shots are recommended for pregnant women, the researchers add, but many don’t get them. While some experts say this is because many women are reluctant to receive immunizations or take drugs during pregnancy, they note, there is evidence that doctors’ lack of awareness may be “a major contributor, with good acceptance by mothers when the risks and benefits are explained.”

SOURCE: Clinical Infectious Diseases, March 1, 2010.