Women who are pregnant when the next flu pandemic strikes will find themselves with special needs, concerns and risks — and very little science to help decide things like whether it's safe to take flu drugs or necessary to wear medical masks in public.

Recognizing this group's vulnerability, the U.S. Centers for Disease Control is gathering experts with backgrounds varying from drug metabolism in pregnancy to baby delivery to come up with special pandemic guidelines for pregnant women.

The nearly 70 experts convene in Atlanta on Thursday and Friday to go over what little data exists. In the process, they will try to figure out what to recommend on issues such as antiviral drug use in pregnant woman and how to try to ensure continuity of obstetrical care for mothers-to-be during what is expected to be a medical emergency.

An expert from the University of British Columbia who will be attending the meeting acknowledges at least some of the recommendations will not be typical evidence-based guidance, but rather advice "couched in the language of ignorance."

"I think we will explicitly state that … we're dealing with very, very limited data," says Dr. Jan Friedman, a medical geneticist and clinical teratologist. Teratology is the study of abnormal fetal development and congenital malformations.

He expects the group will come up with some advice, for instance weighing the evidence on how flu drugs like Tamiflu work in non-pregnant people against the fact pregnant women are at higher risk of complications and death from pandemic and even seasonal influenza.

"But we will say at the same time that the quality of data is very poor and these are the information gaps that we have. And we should try to fill as many of these gaps as possible before there is a pandemic situation so that we can make better informed recommendations," Friedman explains.

Previous pandemics offer few clues

Flu pandemics are rare and not well-documented events. The most recent was 40 years ago. So the picture of what it's like for pregnant women when a new flu strain spreads across the globe is murky. But a recent review of available data, published in the journal Emerging Infectious Diseases, cited figures that are cause for concern.

In the 1918 Spanish flu pandemic, one study reported on 1,350 pregnant women who became infected; 27 per cent died from the flu. In the milder Asian flu pandemic in 1957, half the women of reproductive age who died from flu in Minnesota were pregnant.

"From the limited information that we have from previous pandemics, it looks like pregnant women are expected to be a vulnerable population for future pandemics," says Dr. Sonja Rasmussen of the CDC's National Center on Birth Defects and Developmental Disabilities, the lead author of the review article and one of the organizers of this meeting.

Doctors know there is something about pregnancy — the carrying of a foreign body — that renders women more susceptible. The normal protections of the mother's immune system have to be altered to allow the fetus to grow.

"And that alteration can make her at an increased risk of infection or increased morbidity (illness) or mortality with different infections," Rasmussen explains.

Little is known about the impact on a developing fetus of a bout of influenza in the mother. But one of the hallmarks of flu is fever. Fever in pregnancy has been linked to an increased rate of neural tube defects such as spina bifida.

Friedman says there is good evidence to urge that caregivers try to bring down a fever in a pregnant woman with influenza. "And there are safe, relatively well-studied medicines to do that."

Likewise there is evidence that regular flu vaccine is safe for pregnant women.

But vaccines against novel strains of influenza such as H5N1 avian flu haven't been tested in this population. Nor have the immune boosting additives, called adjuvants, that will almost certainly be used to stretch limited global supplies of vaccine when the next pandemic hits.

As for antiviral drugs, "we just don't have data on them," says Dr. Tina Chambers, a teratologist at the University of California at San Diego who will also be attending the meeting.

"There's the animal data and … limited or no information on human pregnancy. So you're just, like with so many other things, you're kind of going into it blind."

One of the big questions is how to safely provide care to pregnant women at a time when hospitals will be both taxed to the limit and epicentres of contagion.

"That's one of the things which is worrying us," Dr. Matthews Mathai, of the World Health Organization's Department of Making Pregnancy Safer, says of the question of who will care for pregnant women. Mathai is representing the WHO at the meeting.