More than one in 10 babies whose mothers were infected with the Zika virus in the first three months of pregnancy will go on to have birth defects, researchers have found.
The latest study is a reminder of the toll of the virus, which swept through the Americas and parts of the Caribbean in 2015 and 2016.
It should also act as a warning to women planning to travel to Zika affected countries.
The virus, which is spread to humans by infected mosquitoes, causes a mild fever and rash but is particularly harmful for pregnant women as it has been linked to birth defects.
During the height of the epidemic in 2016 more than 3,000 babies, the majority in Brazil, were born with neurological defects such as microcephaly, where the head circumference is smaller than average, damage to the eyes and restricted movements.
This latest study of more than 500 babies and foetuses in Guadeloupe, French Guiana and Martinique, where there was an outbreak in early 2016, found that the risk to the unborn child is greatest if the mother catches Zika during the first three months of pregnancy: 12.7 per cent of babies were born with birth defects.
As the pregnancy progresses the risk of complications goes down, the researchers found, with 3.6 per cent of those whose mothers caught the virus in the second three months being born with defects.
However, the rate increased in the last three months of pregnancy with 5.3 per cent born with defects. The overall rate was 7 per cent, the researchers said.
The findings are similar to those reported by US researchers last year, who found that around 6 per cent of babies born to mothers infected with the virus had birth defects, rising to 11 per cent among those infected during the first trimester.
However, a smaller study of babies born in Brazil found a much higher rate of defects: 42 per cent. Scientists are still unsure as to why the rate of defects was so much higher in Brazil.
Arnaud Fontanet, co-author of the study and director of the Centre for Global Health Research and Education at the Institut Pasteur in Paris, said even though the rates of complication are low compared to other viral infections in pregnant women, such as rubella, they are worrying because of the large numbers of people infected.
“We are no longer in the epidemic phase of Zika but we know that when an epidemic goes through a region about half of the population becomes infected,” he said.
Zika virus | Quick facts
- The Zika virus was first identified in Uganda in 1947
- It is transmitted through the bite of an infected mosquito
- The link between the virus and birth defects was only identified when there was an outbreak of the disease in South America in 2015 and 2016
- More than 3,000 babies have been born with birth defects linked to the virus
- Scientists have yet to work out exactly why the virus causes birth defects
Some 80 per cent of people infected with Zika do not show any symptoms and the researchers said it was important that countries which had experienced Zika outbreaks monitor rates of birth defects in the whole population, not just in those whose mothers were infected with the virus.
The researchers said that the effects of exposure to Zika during pregnancy on children as they get older are unknown. It is too early to say whether healthy babies will develop problems later, said Professor Fontanet, whose next study will be to follow children born to Zika-infected mothers and compare them with those whose mothers did not become infected during pregnancy.
“It is important that we continue to follow up the babies because even if they looked healthy at birth they may develop some difficulties in learning how to walk, talk or to read. We need to have a full account of the consequences of Zika infection during pregnancy,” he said.