The US Centers for Disease Control and Prevention (CDC) said that doctors should not delay giving antivirals to pregnant women with flu symptoms because the risks of premature labor, pneumonia and dehydration are too great and outweigh the risk of the drug affecting the baby.

Dr. Anne Schuchat, Interim Deputy Director for Science and Public Health at the CDC said in a press briefing about the H1N1 situation on Tuesday:

"We are ... seeing some severe complications among pregnant women in this year's novel H1N1 virus problem."

The CDC is currently investigating about 20 cases of pregnant women infected with the virus, some of them with severe complications.

One of the three H1N1 deaths in the US was a pregnant woman in Texas who did not take anti-flu drugs.

Flu usually affects pregnant women more severely than other people because their immune system is weaker. Shuchat said pregnant woment are more vulnearable to severe complications include pneumonia and dehydration as well as complications for the newborn, like premature labor".

She said the CDC urge doctors caring for pregnant women who they think may have flu to treat them promptly with antivirals.

"Sometimes, physicians are reluctant to treat pregnant women with medicines, and sometimes pregnant women are reluctant to take medicines because, of course, they are sometimes risky during pregnancy," said Shuchat.

But she said that experts who have looked at the situation in detail are adamant that the benefits to pregnant women with flu of taking antivirals are much greater than the "theoretical concerns about the drugs".

Without actually naming them, Shuchat said that either of the two medicines that the novel H1N1 is susceptible to should be used, but she was probably referring to Tamiflu and Relenza.

Relenza (zanamivir) and Tamiflu (oseltamivir) belong to the same class of drug known as neuraminidase inhibitors used to treat Influenzavirus A and Influenzavirus B.

In a separate briefing reported by the New York Times, CDC medical officer Dr Denise Jamieson, also advised doctors not to delay. If there is flu in the community, and you would otherwise not hesitate to give antivirals to a non-pregnant patient with flu symptoms, then:

"Don't delay because she's pregnant ... the benefit of giving Tamiflu outweighs the risk," said Jamieson.

She added that "Tamiflu and Relenza are fairly safe in pregnancy".

Shuchat said that while they didn't know a lot about this new H1N1 and pregnancy, it was important to look back on what is known about seasonal influenza and pregnancy, and the recommendation is that pregnant women are strongly urged to take the seasonal flu vaccine to protect themselves from complications during pregnancy.

She said the authorities are keen to stress the importance of prompt antiviral treatment in pregnancy and the CDC will be shortly be issuing a Morbidity and Mortality Weekly Report (MMWR) with some clinical data about the H1N1 pregnancy cases they have been investigating.

Schuhat also said that the way the US was using antivirals in response to the new H1N1 outbreaks was different to European countries. She said the circumstances in the US were very different to that of many countries in Europe.

In the US the virus is pretty much in every state, and she suspects it is also in the states that haven't reported confirmed cases yet.

"We don't have a situation where we can contain the virus's geographic distribution, and our focus is on reducing illness and death and mitigating the impact that this virus has as well as focusing our efforts on areas where they can have the most impact," said Schuhat.

So the priority is to use antivirals where the treatment will make a difference, she explained.

"And that's for people with severe illness presentation or for people who have underlying medical conditions or pregnancy, where the complications of an influenza infection might be worse than in other people," said Schuhat.

While other countries are still using the drug for preventive reasons, in the US the focus is on treatment. Prevention is not " likely to have a benefit here in the United States based on the transmission patterns we're seeing and the stage of the outbreak that was present by the time we recognized this virus", said Schuhat.