Scientists say there's more evidence supporting a link between the Zika virus and a serious birth defect.
Researchers report that one in every 100 pregnant women infected with the virus during the first trimester will give birth to a baby with microcephaly (an abnormally small head) and the potential for neurological issues.
The new risk analysis did have one important caveat, however.
"The findings are from the 2013–14 outbreak [of Zika] in French Polynesia, and it remains to be seen whether our findings apply to other countries in the same way," study co-author Dr Simon Cauchemez said in a news release from The Lancet.
Read: Pregnant women confirmed with Zika infection in US
The analysis was based on data from an outbreak of Zika infections in French Polynesia, a group of islands in the South Pacific. Cauchemez and colleagues said over 31,000 cases of infection were reported during the 2013-2014 outbreak, and eight cases of microcephaly were confirmed.
"Data from French Polynesia are particularly important since the outbreak is already over," said study co-author Arnaud Fontanet, a colleague of Cauchemez at the Institut Pasteur in France.
"This provides us with a small – yet much more complete – dataset than data gathered from an ongoing outbreak," Fontanet added.
'A global public health dilemma'
The researchers believe that the findings strengthen the notion that maternal infection during the first trimester of pregnancy may be especially linked to microcephaly in babies.
Dr Richard Temes directs neurocritical care at North Shore University Hospital in Manhasset, New York. He called the emergence of the Zika-microcephaly link "a global public health dilemma".
Read: Surge of Brazilian babies born with small heads
"Although the risk of transmission is low in comparison to other viral infections, such as congenital rubella [German measles], the authors rightly conclude that the risk to the population is much greater given the higher incidence of Zika virus during outbreaks," Temes said.
In other related news, U.S. health officials on Friday gave tentative approval to a field test in the Florida Keys of mosquitoes genetically tweaked to help curb the spread of the Zika virus.
Officials at the U.S. Food and Drug Administration said they made the preliminary determination that the test of the genetically engineered insects poses little harm to people, animals or the environment, The New York Times reported.
But, final approval for the trial won't come until the FDA considers comments from the public, which is likely to take months, the newspaper said.
Linked to Guillain-Barre syndrome
U.S. health officials said they were learning much about the virus. However, the more they learn, the more they realise how much they don't know, Dr Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said during a media briefing.
"Unfortunately, the more we learn, the worse things seem to get," Fauci said.
Read: Zika virus: pregnant women shouldn't travel to Brazil
The Zika virus is suspected of causing an epidemic that started last spring in Brazil, where there have been more than 5,600 suspected or confirmed cases of microcephaly.
Zika has also been linked to Guillain-Barre syndrome, an immune system disorder that can occasionally lead to a fatal form of paralysis.
Dr Tom Frieden, director of the U.S. Centres for Disease Control and Prevention, said: "We are learning more about Zika every day. The link with microcephaly and other possibly serious birth defects is growing stronger every day. The link to Guillain-Barre syndrome is likely to be proven in the near future, and the documentation that sexual transmission is possible is now proven."
First discovered in Uganda in 1947, the Zika virus wasn't thought to pose major health risks until last year, when it became clear that it posed potentially devastating threats to pregnant women.
But, for most other people the virus offers little threat – approximately 80 percent of people who become infected never experience symptoms.
'An uphill battle'
Meanwhile, the virus continues to spread in Latin America and the Caribbean.
It is not expected to pose a significant threat to the United States mainland, federal health officials have said in the past.
In Puerto Rico, however, the situation is "of great concern", Frieden said.
Read: Pregnant women in SA warned about Zika virus
"Puerto Rico is on the frontline of the battle against Zika," said Frieden, who had just returned from the island. "And it's an uphill battle."
By next year, Frieden said, there could be hundreds of thousands of cases of Zika in the territory, and "thousands of infected pregnant women".
Last month, President Barack Obama asked Congress for $1.9 billion to fight the Zika virus. To date, Congress has not approved the funding and both Frieden and Fauci expressed concern that efforts to fight Zika are in jeopardy if the funds aren't forthcoming.
One goal is to create a vaccine that can be given to children before they reach puberty to prevent Zika infection, Fauci said. "We cannot do what needs to be done in a sustained way without those resources," he said.
The CDC currently has this advice for pregnant women:
- Consider postponing travel to any area where Zika virus transmission is ongoing.
- If you must travel to or live in one of these areas, talk to your health-care provider first and strictly follow steps to prevent mosquito bites.
- If you have a male partner who lives in or has traveled to an area where Zika transmission is ongoing, either use condoms the right way every time, or do not have sex during your pregnancy.
The Zika virus has now spread to over 33 countries and territories in Latin America and the Caribbean. The World Health Organisation estimates there could be up to 4 million cases of Zika in the Americas in the next year.
Zika virus: 14 things you need to know
Don't be fooled by 'mild' Zika virus
Brazil sees a growth in microcephaly cases