The mosquito-borne Zika virus may be even more dangerous than previously thought, scientists in Brazil say.

They told the BBC that Zika could be behind more damaging neurological conditions, affecting the babies of up to a fifth of infected pregnant women.

Rates of increase in Zika infection in some parts of Brazil have slowed, thanks to better information about preventing the disease.

But the search for a vaccine is still in the early stages.

And Zika continues to spread across the region.

Most doctors and medical researchers now agree that there is a link between the Zika virus and microcephaly, where babies are born with abnormally small heads because of restricted brain development.

While it is estimated that 1% of women who have had Zika during pregnancy will have a child with microcephaly, leading doctors in Brazil have told the BBC that as many as 20% of Zika-affected pregnancies will result in a range of other forms of brain damage to the baby in the womb.

A separate study, reported in the New England Journal of Medicine, said that "29% of scans showed abnormalities in babies in the womb, including growth restrictions, in women infected with Zika".

"Our findings are worrisome because 29% of ultrasonograms showed abnormalities, including intrauterine growth restrictions and foetal death, in women with positive Zika infection," the study said.

Zika: What are the symptoms?

Deaths are rare and only one in five people infected is thought to develop symptoms.

These include:

• mild fever
• conjunctivitis (red, sore eyes)
• headache
• joint pain
• a rash

A rare nervous system disorder, Guillain-Barre syndrome, that can cause temporary paralysis has been linked to the infection.

There is no vaccine or drug treatment so patients are advised to rest and drink plenty of fluids.

But the biggest concern is the impact it could have on babies developing in the womb.

Zika virus: Special report

Many of the conditions that Brazilian doctors are noticing in babies born to mothers who had Zika are not as obvious to the untrained eye as microcephaly.

They may not have the same impact on the child's development either but the frequency with which they are occurring is alarming.

Doctor Renato Sa is a senior obstetrician who works in both public and private hospitals in Rio de Janeiro.

"There are cerebral calcifications, an increase in the number of dilation of cerebral ventricles and the destruction or malformation of the posterior part of the brain," he says.

In my notebook he makes a list of the conditions that they are now witnessing with increasing regularity; ventriculomegaly, damage of the posterior fossa, craniocynostosis and cerebral calcification.

He says that an added concern is that often there is no obvious sign or symptom of the neurological damage until later observations of the child's development, "perhaps convulsions or other tell-tale signs".

'Shocked'

There is an obvious urgency to understand more about Zika and develop a vaccine.

At the Instituto D'Or laboratories in Rio de Janeiro they are using stem-cell technology to create new cells that develop like human brains.

They call them "mini brains" and infect them with Zika.

"What we observed is that the Zika virus is able to kill cells, is able to affect the growth of cells," says Dr Stevens Rehen, the unit's senior neuroscientist.

Dr Rehen has noted, as have others, that there is something particularly virulent about the strain of Zika that arrived in Brazil and is presently spreading across the continent.

"There is something in the Zika virus that makes it more prone to kill neurocells during development. Now we need to search and understand what makes that virus more aggressive to the brain under development."

The team was shocked by what it discovered - a huge reduction in growth of the cerebral cortex, the critical outer layer of the brain.

"The effect of the Zika virus was very impressive," says Dr Patricia Garcez.