Zika virus was found in the brain of a fetus, the strongest evidence yet that the virus causes abnormally small heads and incomplete brain development, according to an article in The New England Journal of Medicine. This is the first documented case of virus transmission from mother to child, though it is not a definitive link between Zika and fetal abnormalities — a connection first suggested by the Brazilian minister of health. 
 
 This is the strongest evidence to date that Zika causes incomplete brain development The Zika virus appeared to particularly favor neurons, today's case report found. Damage from the virus may have halted brain development at 20 weeks, though it's not clear how the virus was infecting nerve cells. Some structures found at autopsy showed that the virus was reproducing in the fetus's brain. 
 
 In early 2015, a Zika outbreak was identified in northern Brazil, where other viruses spread by mosquitoes (like dengue) were circulating. By September, doctors began reporting an unusual number of babies born with microcephaly, a condition where infants are born with unusually small heads and underdeveloped brains. Zika has spread to many countries in the Americas, though Brazil bears the heaviest caseload, with estimates of 440,000 to 1.3 million infected as of December. The number of newborns with microcephaly has increased twentyfold in the northern part of Brazil, the health minister has said. That led to the suspected link — and five public health agencies in multiple countries have advised women not to get pregnant as a result. Until now, though, there was no evidence of virus transmission from a pregnant woman to her fetus. 
 
 "This is the critical point: you have a mother who's infected, a fetus that's abnormal, and in the fetus, you have the genetic signature of the virus," says Andrew Pekosz, director of the Center for Emerging Viruses and Infectious Diseases at Johns Hopkins Bloomberg School of Public Health. "This is clear data showing Zika can infect the fetus." Today's case report isn't proof that Zika is causing microcephaly, but it makes the link much likelier, Pekosz says. 
 
 Today's report isn't proof Zika causes microcephaly — but it makes the link likelier The woman was a 25-year-old European volunteer who had been working in Natal, Brazil, on the country's northeast tip. She got pregnant in February 2015; at the 13th week of pregnancy, she had a fever and rash consistent with Zika infection. Ultrasounds at 14 weeks and 20 weeks were normal. 
 
 Then, she returned to Europe at 28 weeks pregnant. An ultrasound in her 29th week showed the first signs of abnormalities. At 32 weeks, an ultrasound showed that the fetus was abnormally small — in the third percentile for fetal weight — and its head was below the second percentile for circumference. There were calcium buildups in the brain and placenta. The woman chose to get an abortion, and doctors performed an autopsy, where they found viral replication of Zika in the fetus's brain. The fetus was negative for 13 other viruses that may cause fetal abnormalities; the woman had no history of genetic abnormalities in her family. 
 
 The autopsy also showed severe structural abnormalities in the fetus's brain, said study author Tatjana Avšič Županc, a microbiologist at the University of Ljubljana in Slovenia. There was a total absence of brain folds; fluid had also built up in the brain. "Our evidence means a certain danger to pregnant women, especially those in the first trimester," Županc says. "Particularly for those who reside in or visit areas with highly endemic Zika virus." 
 
 the case provides support for the brazilian minister of health The case provides support to the Brazilian minister of health, who first suggested the link between Zika and microcephaly. But microcephaly can be caused by a number of viruses, including rubella and toxoplasmosis, as well as by malnutrition or maternal alcoholism. Some scientists have suggested the growth in cases is due to doctors keeping an eye out for microcephaly, as well as its relatively broad diagnosis criteria. That's why the Slovenian team tested for so many other viruses — to make sure Zika wasn't accompanied by another infection. 
 
 There are still a lot of questions to be answered about Zika's effects on fetuses. For instance, about 80 percent of infections are asymptomatic, says Nikos Vasilakis, a pathologist at the University of Texas Medical Branch. No one yet knows if asymptomatic infections can cause fetal abnormalities. Animal models could provide hints about these questions — but there are no animal models for Zika, says Johns Hopkins' Pekosz. Studies proving casuality and allowing doctors to begin assessing risks of birth defects will take years, Vasilakis says. And no one knows if infection in the first trimester — a crucial developmental period — is more dangerous than in later trimesters. "It's quite a challenge to figure out what happens," Vasilakis says. "It's going to take a lot of money and scientific discipline." 
 
 Correction: A previous version of the story incorrectly identified the woman as Slovenian. She is European, according to the NEJM report, which does not specify a home country. ||||| Daniele Santos, 29, holds her son Juan Pedro who is 2-months old and born with microcephaly, at their house in Recife, Brazil, February 9, 2016. REUTERS/Nacho Doce 
 
 CHICAGO (Reuters) - Researchers on Wednesday reported new evidence strengthening the association between Zika virus and a spike in birth defects, citing the presence of the virus in the brain of an aborted fetus of a European woman who became pregnant while living in Brazil. 
 
 An autopsy of the fetus showed microcephaly or small head size, as well as severe brain injury and high levels of the Zika virus in fetal brain tissues, exceeding levels of the virus typically found in blood samples, researchers in Slovenia from the University Medical Center in Ljubljana reported in the New England Journal of Medicine. 
 
 The findings help “strengthen the biologic association” between Zika virus infection and microcephaly, researchers from the Harvard School of Public Health and Massachusetts General Hospital in Boston, wrote in an editorial that accompanied the paper. 
 
 Researchers in Brazil are scrambling to determine whether the arrival of the Zika virus in that country has caused a major rise in microcephaly, with more than 4,000 suspected cases of the condition reported to date. Brazil has confirmed more than 400 of those cases as microcephaly, and identified the presence of Zika in 17 babies, but a link has yet to be proven. 
 
 The virus is transmitted by mosquitoes. There is no treatment or vaccine. 
 
 Professor Tatjana Avšič Županc, who led the researchers in Slovenia, said in an email her team’s findings “may present the most compelling evidence to date” that birth defects associated with Zika infection in pregnancy may be caused by replication of the virus in the brain. 
 
 For definitive proof, however, she said more studies are needed. 
 
 In the NEJM paper, the mother showed signs of Zika infection during her 13th week of pregnancy, but ultrasounds during her 14th and 20th week were normal. 
 
 It was not until the woman returned to Europe that researchers found ultrasound evidence of severe fetal abnormalities during an ultrasound in her 29th week. 
 
 That suggests that ultrasounds may pick up signs of severe fetal abnormalities “only very late in gestation - in many cases too late to terminate the pregnancy,” Drs. Eric Rubin, Michael Green and Lindsey Baden wrote in the editorial. 
 
 In the Slovenian case, the woman also noticed reduced fetal movement and was told the fetus had a poor prognosis. She requested an abortion, which was approved by state and hospital ethics boards and performed at 32 weeks gestation. A normal pregnancy lasts 40 weeks. 
 
 Aside from obvious microcephaly, the fetus showed no other deformities. The woman had no family history of genetic abnormalities that could have caused the microcephaly. 
 
 Researchers did an autopsy, and instead of normal grooves formed in the brain during growth, the surface of the brain was smooth and there were numerous calcifications, which are suggestive of inflammation. 
 
 Brain samples tested positive for the Zika virus, and no other viruses in the same class, such as dengue, yellow fever or West Nile virus. The doctors also ruled out many other potential infectious causes of microcephaly, including rubella, cytomegalovirus and toxoplasmosis. 
 
 From the brain samples, the team was able to identify the complete genetic sequence of the Zika viral genome, which most closely matched a Zika virus strain isolated from a patient from French Polynesia in 2013 and a Zika virus strain isolated in Sao Paulo, Brazil, in 2015. 
 
 Importantly, the team found no presence of the virus and no other defects in any fetal organs other than the brain, which suggests the virus selectively attacks nerve tissue, Avšič Županc and colleagues wrote. But exactly how the virus does this is not clear. 
 
 The discovery of viral particles and a high load of Zika virus RNA in the brain samples, coupled with the absence of other possible pathogens, and a complete viral genome present “strong evidence” of the Zika virus’ potential for causing birth defects, Avšič Županc said in an email. 
 
 In January, a team of researchers at the U.S. Centers for Disease Control and Prevention reported that they, too, found evidence of Zika virus in brain samples from Brazil taken from two miscarriages and two newborns who died shortly after birth. 
 
 CDC published full results of their findings earlier on Wednesday. ||||| This digitally-colorized image shows particles of Zika virus, which is a member of the family Flaviviridae. The virus particles are colored red in the picture. They are 40 nanometers (0.00004 millimeters) in diameter. 
 
 A woman in Brazil who became infected with the Zika virus gave birth to a stillborn baby, and large parts of the infant's brain were missing, according to a new report. 
 
 Moreover, the fetus had damage to tissues outside of the central nervous system, the researchers said. For example, the infant's body had an abnormal accumulation of fluids. 
 
 "These finding raise concerns that the virus may cause severe damage to fetuses leading to stillbirths, and may be associated with effects other than those seen in the central nervous system," study author Dr. Albert Ko, chair of the Department of Epidemiology of Microbial Diseases at the Yale School of Public Health, said in a statement. 
 
 However, this was an isolated case, and more research is needed to determine whether Zika virus can actually cause some of the health effects seen in the fetus, he said. 
 
 And because this was a single case, it is not possible to estimate the risk of stillbirth among women who are exposed to the Zika virus during pregnancy, the researchers said. 
 
 The 20-year-old woman described in the report had a normal pregnancy during the first three months. However, at around her 18th week of pregnancy, an ultrasound showed that the fetus weighed much less than normally developing fetuses weigh at that point. [Zika Virus Special Report: Complete Coverage of the Outbreak] 
 
 The woman did not have any of the common symptoms of Zika virus infection, such as rash, fever or body aches, either shortly before she got pregnant or while she was pregnant. 
 
 By the 30th week of the woman's pregnancy, the doctors knew the fetus would have several congenital conditions, the report said. For example, the fetus's head was abnormally small, and parts of the brain were missing. 
 
 An ultrasound in the 32nd week of pregnancy showed the fetus had died, and the doctors induced labor shortly after. Then, the researchers confirmed the presence of Zika virus in the fetus. It also turned out the fetus had joint deformities. 
 
 Some other mosquito-borne viruses can affect the brain of a person who is bitten. For example, some people with West Nile virus infections can develop a severe form of the disease called West Nile encephalitis. 
 
 But no other mosquito-borne virus has been linked to neurological effects in a fetus carried by a woman who was bitten, said Dr. Richard Temes, director at the Center for Neurocritical Care at North Shore University Hospital in Manhasset, New York, who was not involved in the report. 
 
 "This is really the first virus that is not causing neurological damage to the host, or the person the mosquito bites, but it is actually transmitted to the fetus that the host is carrying," Temes told Live Science. 
 
 Researchers don't know the potential mechanism behind the link between this neurological damage and the Zika virus, he said. However, it appears that the fetal brain may be particularly susceptible to damage from the virus in the first trimester of pregnancy, as this is when the most rapid brain development occurs, he added. 
 
 Doctors recommend that women in any stage of pregnancy avoid traveling to places affected by the Zika outbreak, such as Brazil, Temes said. 
 
 Most people who become infected with the Zika virus show no symptoms of the infection, according to Centers for Disease Control and Prevention. Researchers are concerned about the virus primarily over a possible link between Zika infections in pregnant women and a congenital condition called microcephaly in their infants. Babies with this condition are born with underdeveloped brains, and face severe, lifelong cognitive impairments. However, the link between the condition and the virus is not proven, and studies are underway to look at the link more closely. 
 
 The new report was published today (Feb. 25) in the journal PLOS Neglected Tropical Diseases. 
 
 Follow Agata Blaszczak-Boxe on Twitter. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.
– The Zika virus' apparent effect on fetuses is on shocking display in a new study in the New England Journal of Medicine that was rushed to publication given the virus' spread. Though it offers no definitive link between Zika and fetal abnormalities, the study does produce the first evidence of virus transmission from mother to child and "the strongest evidence yet that the virus causes abnormally small heads and incomplete brain development," reports the Verge. The study centers on a 25-year-old Slovenian woman who became pregnant while volunteering in Brazil in February 2015. She developed a high fever and rash during her 13th week of pregnancy, but ultrasounds at 14 and 20 weeks showed no issues. When she returned to Slovenia in October, however, doctors found extensive brain damage in the fetus and the pregnancy was aborted, per NBC News. An autopsy found the full genome of the Zika virus exclusively in the fetus' unusually smooth and scarred brain, suggesting the virus only attacked nerve tissue, per Reuters. The mother had no family history of genetic abnormalities and the fetus tested negative for 13 other viruses known to cause birth defects. "You have a mother who's infected, a fetus that's abnormal, and in the fetus, you have the genetic signature of the virus," says an infectious disease expert. "This is clear data showing Zika can infect the fetus." An epidemiologist adds "it sounds like a pretty clear case of extreme microcephaly." Scientists suspect brain development may have stopped around 20 weeks, leading one doctor to suggest that Zika, like other viruses linked to birth defects, may be most dangerous in the early stages of pregnancy. (The virus has been linked to other health problems.)