Babies with brains damaged by the Zika virus might still appear normal, a large study of Brazilian babies shows.
Babies born with tiny heads – or microcephaly – is the main concern in the Zika outbreak.
But the findings, published in the Lancet, show a fifth of babies that would be classed as normal actually had brain abnormalities.
And the Brazilian researchers warned Zika infection in newborns could also lead to brain damage.
Zika infection is largely mild, with most people having no symptoms.
But the World Health Organization has declared the Zika virus a global public health emergency because of the risk to newborn children.
In severe cases children can die and those that survive face intellectual disability and development delays.
The researchers’ analysis of every reported case – 1,501 – in Brazil up to February shows the risk might be greater than thought.
The head has done most of its growth by 30 weeks’ gestation. It is thought Zika infection after this point still affects the brain, but no longer shows up as microcephaly.
Pregnant women who had a rash – the main sign of Zika infection – late in the pregnancy were more likely to have a child with a normal-sized head.
Prof Cesar Victora, a researcher from the Universidade Federal de Pelotas in Brazil, said: “One in five definite or probable Zika cases had head circumference values in the normal range – therefore the current focus on microcephaly screening alone is too narrow.
“Our findings suggest that among pregnancies affected by Zika virus, some foetuses will have brain abnormalities and microcephaly, others will have abnormalities with normal head sizes, and others will not be affected.
“A surveillance system aimed at detecting all affected newborns should not just focus on microcephaly and rash during pregnancy and should be revised, and examination of all newborns during epidemic waves should be considered.”
The fact the brain can still be affected in the later stages of pregnancy has led the researchers to speculate that “Zika virus infection in newborn babies might lead to neurological damage”.
The researchers also found that a rash was not present in a third of infected mums-to-be.
The report said the microcephaly cases peaked at the end of 2015, about six to nine months after the Zika virus epidemic peaked in north-east Brazil.
Prof Victora added: “Because a new wave of Zika virus infection took place in south-eastern Brazil in early 2016, there could be a second wave of microcephaly at the end of the year.”
Prof Jimmy Whitworth, from the London School of Hygiene and Tropical Medicine, told the BBC News website: “The message this sends out is that there is broader concern than originally thought.
“It is about more than microcephaly, which is the tip of the iceberg, and the risk might extend longer in pregnancy and even to birth.
“We’re hampered by the lack of a reliable diagnostic test for Zika. It’s hard to know if a woman has been infected.”