Babies with brains damaged by the Zika virus might still appear normal, a large study of Brazilian babies shows, BBC reports.
Babies born with tiny heads - or microcephaly - is the main concern in the Zika outbreak.
But the findings, published in 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".