WASHINGTON (NYTIMES) - Pregnant women looking for guidance on Covid-19 vaccines are facing the kind of confusion that has dogged the pandemic from the start: The world's leading public health organisations - the US Centres for Disease Control and Prevention (CDC) and the World Health Organisation - are offering contradictory advice.

Neither organisation explicitly forbids or encourages immunising pregnant women. But weighing the same limited studies, they provide different recommendations.

The CDC's advisory committee urged pregnant women to consult their doctors before rolling up their sleeves - a decision applauded by several women's health organisations because it kept decision-making in the hands of the expectant mothers.

The WHO recommended that pregnant women not receive the vaccine unless they were at high risk for Covid-19 because of work exposures or chronic conditions.

It issued guidance on the Moderna vaccine on Tuesday, stirring uncertainty among women and doctors on social media. (Earlier this month, it published similar guidance on the Pfizer-BioNTech vaccine.)

Several experts expressed dismay at the WHO's stance, saying the risks to pregnant women from Covid-19 were far greater than any theoretical harm from the vaccines.

"There are no documented risks to the foetus, there's no theoretical risks, there's no risk in animal studies" from the vaccines, said Dr Anne Lyerly, a bioethicist at the University of North Carolina, Chapel Hill. "The more that I think about it, the more disappointed and sad I feel about it."

The difference of opinion between the CDC and the WHO is not rooted in scientific evidence, but the lack of it: Pregnant women have been barred from participating in clinical trials of the vaccines, a decision in line with a long tradition of excluding pregnant women from biomedical research, but one that is now being challenged.

While the rationale is ostensibly to protect women and their unborn children, barring pregnant women from studies pushes the risk out of the carefully controlled environment of a clinical trial and into the real world. The practice has forced patients and providers to weigh sensitive, worrisome issues with little hard data about safety or effectiveness.

Vaccines are generally considered to be safe, and pregnant women have been urged to be immunised for influenza and other diseases since the 1960s, even in the absence of rigorous clinical trials to test them.

"As obstetricians, we are often faced with difficult decisions about using interventions in pregnancy that have not been properly tested in pregnancy," said Dr Denise Jamieson, an obstetrician at Emory University in Atlanta and a member of the Covid-19 expert group at the American College of Obstetrics and Gynecologists.

The college strongly advocated including pregnant and breastfeeding women in the vaccine trials.

"What many people miss is that there are risks to doing nothing," Dr Jamieson said. "Not offering pregnant women the opportunity to be vaccinated and protect themselves, where there are known and severe risks of Covid amplified by pregnancy, is not a wise strategy."

The uncertainty isn't limited to Covid-19 vaccines: Many, if not most, medications, including widely used drugs, have never been tested in pregnant women. It can take years or decades for adverse side effects to come to light in the absence of a study with a control group for comparison.

"This isn't a story about the WHO or other people advising against vaccination in pregnancy," said Dr Carleigh Krubiner, a policy fellow at the Centre for Global Development and a principal investigator for the Pregnancy Research Ethics for Vaccines, Epidemics and New Technologies project (PREVENT).

"It's a story about the failure to timely and appropriately include pregnant women in vaccination studies."

Saying she understood the commitment of the WHO and other advisory bodies to rely on scientific studies, Dr Krubiner added: "The reality is that we don't yet have the data on these vaccinations in pregnancy, and it's very difficult without that data to come out and give a full-throated recommendation in support."

In a statement, the CDC said on Thursday that based on how the Pfizer-BioNTech and Moderna vaccines work, "they are unlikely to pose a specific risk for pregnant women".

The CDC's recommendation may make sense for the United States, where women may easily be able to consult their healthcare providers, said Mr Joachim Hombach, a health adviser to the WHO on immunisations. But the WHO provides guidance to many low and middle-income countries where women do not have access to doctors or nurses, he said.

The WHO's recommendation was also made "in the context of limited supply" of the vaccines, Mr Hombach said. "I don't think the language is discouraging, but the language is stating the facts."

Pfizer did not include pregnant women in its initial clinical trials because it followed the policies outlined by the Food and Drug Administration to first conduct developmental and reproductive toxicity studies, said Ms Jerica Pitts, a spokesman for the company.

Pfizer and Moderna both provided results from toxicity studies in pregnant rats to the FDA last December.

Pfizer plans to begin a clinical study in pregnant women in the first half of 2021, Ms Pitts said. Moderna is establishing a registry to record outcomes in pregnant women who receive its vaccine, according to Colleen Hussey, a spokesman for the company.

Critics of the companies' decisions to exclude pregnant women from trials say the reproductive toxicity studies could have been carried out much earlier - as soon as promising vaccine candidates were identified. The companies should have added a protocol to enroll pregnant women once it was clear the vaccines' benefits outweighed potential harm, Dr Krubiner said.

"It's hard to understand why that delay is happening and why it wasn't initiated sooner," she said. "The bigger issue is, we're going to have lost months by the time they start them."

Dr Akiko Iwasaki, an immunologist at Yale University who has advocated immunisations for pregnant women, questioned the underlying issue that prompted the WHO's decision.

"Whatever it is, I wish the WHO would be more transparent in their reasons behind this recommendation," she said. "Women's lives depend on it."

The toxicity data released by Pfizer and Moderna last December found no harmful effects from the vaccines to pregnant rats - evidence cited by the WHO in its guidance.

Covid-19 has proved to be dangerous to pregnant women. A large CDC study published last November found that pregnant women with Covid-19 who were symptomatic were significantly more likely to be hospitalised or to die when compared with non-pregnant women who also had Covid-19 symptoms.

The evidence prompted agency officials to add pregnancy to the list of conditions that heighten the risk of severe disease and death from Covid-19.

Britain initially starkly recommended against Covid-19 vaccines for pregnant women but has since revised its guidance to authorise inoculating pregnant women who are front-line workers or otherwise at high risk. "I'm hoping the WHO will reconsider as well," Dr Jamieson said.

Some experts said the recommendations are not as divergent as they may appear at first glance.

"The CDC is more inclined to say that pregnant women should have access to the vaccine but should discuss their circumstances with their providers," said Dr Ana Langer, a reproductive health expert who leads the Women and Health Initiative at Harvard's T.H. Chan School of Public Health.

"The WHO's interim recommendation says that women who are at particularly high risk of exposure or getting Covid-19 should get the vaccine. So where's the big difference here?"