The babies of women infected with Zika virus while they are pregnant can suffer the effects at any stage of gestation, researchers reported Friday in a troubling look at how Zika affects unborn children.
Two babies died just before they should have been born after their mothers became infected, the international team found. They also found the virus causes a range of birth defects beyond microcephaly.
They suggest a name for these effects: Zika virus congenital syndrome. Babies have been born with a range of brain and eye defects and some have also been abnormally small.
The findings, together with other studies, support what most experts already believe: that Zika is causing an epidemic of birth defects.
Earlier Friday, a different team reported that tests in lab dishes show that Zika goes straight into developing brain cells and turns them into virus factories before killing them.
And the findings strongly support the idea that Zika, a once-obscure virus believed to cause little more than a headache and a rash, can be a killer.
"Despite mild clinical symptoms, Zika virus infection during pregnancy appears to be associated with grave outcomes, including fetal death, placental insufficiency, fetal growth restriction and central nervous system injury," they wrote in their report, published in the New England Journal of Medicine.
"We saw problems with the fetus or the pregnancy at eight weeks, 22 weeks, 25 weeks, and we saw problems at 35 weeks," said Dr. Karin Nielsen, professor of clinical pediatrics at the David Geffen School of Medicine at the University of California Los Angeles, who helped organize the study.
"Even if the fetus isn't affected the virus appears to damage the placenta, which can lead to fetal death," Nielsen said. Two babies died just days before they were due.
Related: Zika Can Cause Eye Defects
"Any woman with Zika virus should be handled as a high-risk pregnancy," Nielsen told NBC News.
"There's more than microcephaly. There is a spectrum of disease. There are parts of the brain that are not formed. There are calcifications in the brain. There is in-utero growth restriction."
Many of the symptoms are similar to what's been seen when women get rubella while pregnant, including microcephaly, marked by a smaller than normal brain and head. Children who survive birth with microcephaly can grow up with often profound physical and mental disabilities.
Calcifications are hardened remnants of dead tissue. Other studies have shown that Zika in the brains of some babies miscarried or aborted after a mother was infected.
Nielsen was working with Dr. Patricia Brasil and colleagues at the Oswaldo Cruz Foundation in Rio de Janeiro. They were already running a study of dengue virus, asking pregnant women to come in and be tested if they showed symptoms of dengue, such as a rash.
Dengue is related to Zika and it's spread by the same mosquitoes, the Aedes aegypti mosquitoes that spread Zika. And like Zika, dengue is spreading fast across Brazil and other Latin American countries.
So Nielsen's team was up and running when Zika started causing alarm. They tested blood and urine from 88 women between September and February. More than 80 percent of the women with symptoms such as fever and rash tested positive for Zika in the blood, urine or both.
The women were at various stages of pregnancy. Nielsen's team was able to follow them in real time to see what happened to their pregnancies. They found 29 percent of the fetuses had some sort of problem, from brain damage to damage to the placenta.
Nielsen's study is the first to show what happens from the time a woman becomes infected. That can answer one big question: when the most dangerous time of pregnancy is.
"We know when the infection is taking place," she said. "We can associate that with weeks of gestation, so we can know if there are any malformations, what time they happened — we can associate them with a first trimester, second trimester and third trimester infection."
The answers are not reassuring. The experts have predicted that the first trimester would be the most dangerous time for a fetus if a woman became infected. But Nielsen's team found babies were hurt even late in pregnancy.
"One important finding was that there were problems with pregnancy in the third trimester as well, which was surprising to us," she said.
Some had looked normal on ultrasound, even, before they died in the womb. "It is not normal to find two fetal deaths that late in pregnancy in that small a group of women," she said.
The team is studying the babies to see if they can find out why they died. And they're watching the rest of the pregnant women to see what happens.
World Health Organization and Brazilian health officials are worried. They've seen more than 4,000 cases of microcephaly in recent months, and more than a million people have been infected. Colombia, the second Latin American country to get hit with Zika, has started to report Zika-linked cases of microcephaly.
"This virus hit in a perfect storm," Nielsen said. "You are talking about a very large population in Brazil. When you have a lot of people infected, you are going to see problems."