FILE - In this Wednesday, Feb. 24, 2016 file photo, 3-month-old Esther Kamilly has her head measured by Brazilian and U.S. health workers from the United States' Centers for Disease Control and Prevention... (Associated Press) 
 
 FILE - In this Wednesday, Feb. 24, 2016 file photo, 3-month-old Esther Kamilly has her head measured by Brazilian and U.S. health workers from the United States' Centers for Disease Control and Prevention (CDC) at her home in Joao Pessoa, Brazil, as part of a study on the Zika virus and the birth defect... (Associated Press) 
 
 FILE - In this Wednesday, Feb. 24, 2016 file photo, 3-month-old Esther Kamilly has her head measured by Brazilian and U.S. health workers from the United States' Centers for Disease Control and Prevention (CDC) at her home in Joao Pessoa, Brazil, as part of a study on the Zika virus and the birth defect... (Associated Press) FILE - In this Wednesday, Feb. 24, 2016 file photo, 3-month-old Esther Kamilly has her head measured by Brazilian and U.S. health workers from the United States' Centers for Disease Control and Prevention... (Associated Press) 
 
 NEW YORK (AP) — As the international epidemic of Zika virus disease has unfolded and led to devastating birth defects for at least 1,300 children in eight countries, an agonizing question has persisted: What is the chance that an infected pregnant woman will have a baby with these defects? 
 
 Researchers don't yet have a complete answer, but they are slowly homing in on one. 
 
 The largest study to ever look at the question says the risk of one especially severe type of birth defect is "substantial" — in the range of 1 percent to 14 percent. It also reinforces the understanding that women infected in the early stages of pregnancy face the greatest risk. 
 
 The range is so unusually wide because researchers are relying on imprecise and incomplete information as they to try to quickly estimate the level of risk in advance of what they say is likely transmission of Zika by mosquitoes in the U.S. later this year. 
 
 The study focused on what was seen in just one place, a state in northeast Brazil. And it looks only at microcephaly, a condition in which a baby's skull is much smaller than expected because the brain hasn't developed properly. But health officials say Zika can cause other birth defects, too. 
 
 "These numbers are probably only the tip of the iceberg," said Dr. Neil Silverman, a UCLA professor of obstetrics who has been advising the California Department of Public Health on Zika issues. 
 
 A study done in Rio de Janiero published earlier this year suggested there is a nearly 30 percent risk for all kinds of Zika-related birth defects and fetal death. The new research doesn't necessarily contradict that, said Silverman, who was not involved in the latest research. 
 
 The new study was done by government scientists at the Centers for Disease Control and Prevention. It was published online Wednesday by the New England Journal of Medicine. 
 
 The Zika virus causes only a mild and brief illness, at worst, in most people. But in the last year, infections in pregnant women have been strongly linked to fetal deaths and to potentially severe birth defects, mostly in Brazil. Last month, the CDC said there was enough evidence to declare that Zika causes microcephaly and other brain defects. 
 
 The virus is spread mainly through the bite of a tropical mosquito called Aedes aegypti. The bug can be found in the southern United States, but there's no evidence that they've been spreading the virus in the U.S. yet. 
 
 The new study is based on about 400 babies with microcephaly in the Brazilian state of Bahia, who were diagnosed between July and February. 
 
 The researchers made estimates based on what data they could get, but faced challenges. For example, they weren't certain all 400 microcephaly cases were correctly diagnosed. They didn't know for sure how many of the mothers of the affected children had a Zika infection, nor how many pregnant women in Bahia in total were infected with Zika virus during that time period. 
 
 But based on other data from Bahia and from two earlier Zika outbreaks, they could make calculations that accounted for different scenarios — including one which assumed most people were infected and another in which Zika infections were less common. 
 
 A large Zika epidemic would lead to more babies born with birth defects, but the chance that an individual baby would be affected shrinks as an outbreak progresses. That's because a large outbreak would expose more women to the virus and allow them to develop immunity by the time they got pregnant, Silverman said. 
 
 The study found the risk of a fetus developing microcephaly ranged from about 1 percent, when most people were infected, to nearly 14 percent, when only a small proportion were. 
 
 "This gave us an appreciation that the risk is substantial," said Michael Johansson, a CDC biologist who was the study's lead author. 
 
 Previously, the best estimate of the Zika-related risk of microcephaly came from a very small study based on eight cases that occurred during a Zika outbreak in French Polynesia in 2013 and 2014. That study estimated risk of microcephaly at 1 percent for pregnancies in which the mother was infected in the first trimester. 
 
 The range reported in the new study was also for women infected in the first trimester. For women infected later in pregnancy, the risks were well below 1 percent no matter how much Zika was spreading in the community, the authors found. The Rio study, though, found other harms in babies born to women infected during the second or third trimesters, experts noted. 
 
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 Journal: www.nejm.org ||||| Daniele Santos feeds her baby, Juan Pedro, 2-months-old, in their living room on February 3, 2016 in Recife, Brazil. (Photo: Mario Tama, Getty Images) 
 
 A fetus infected with the Zika virus during the first three months of pregnancy has about a 1% to 13% risk of developing microcephaly, an abnormally small head usually caused by incomplete brain development, according to new estimates from the Centers for Disease Control and Prevention. 
 
 The CDC created a mathematical estimate of the microcephaly risk, based on reports from French Polynesia, which experienced a Zika outbreak from 2013 to 2013, as well as in the Brazilian state of Bahia, where Zika cases have been diagnosed since last year. 
 
 The study is important, because it can help doctors counsel pregnant women about the risks to their babies, said Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston. Pediatricians and health systems across the Americas also will have to prepare to care for babies with extensive disabilities. 
 
 Even if only 1% of Zika-infected babies develop microcephaly, that could overwhelm the health systems of developing countries that aren't equipped to care for severely disabled children, said Amesh Adalja, a senior associate at the Center for Health Security at the University of Pittsburgh Medical Center. Dengue is common in Brazil and throughout much of Latin America and the Caribbean. 
 
 Although Zika was first identified in a monkey in 1947, doctors first linked it to microcephaly only in 2015, after a large outbreak in Brazil. 
 
 The new report suggests that it's possible the virus could have been capable of causing microcephaly in the past, but that earlier outbreaks were too small for relatively rare events such as microcephaly to be noticed. 
 
 During the Zika outbreak on the Micronesian island of Yap, for example, a microcephaly rate of 1% to 13% would have resulted in zero to four cases, according to the CDC report, published Wednesday in The New England Journal of Medicine. 
 
 The increase in microcephaly cases in French Polynesia, which had a Zika outbreak in 2013 to 2014, was so small that doctors didn't even notice it at the time. Researchers only detected the increase this year, after the Brazil epidemic prompted them to go back and take a closer look at women's medical records. Although 31,000 people sought medical attention for the Zika virus, doctors found just eight cases of microcephaly — three in babies and five in pregnancies that were aborted. 
 
 It's not clear why microcephaly seems so much more common in Brazil, Hotez said. Brazil's Ministry of Health has confirmed 1,434 cases of microcephaly, according to a statement released Tuesday. Brazilian officials have said there could be up to 1.5 million cases of Zika in that country. 
 
 Hotez said it's possible that Zika virus has mutated in a way that makes it more likely to cause microcephaly. Or, it's possible that something about the environment of Brazil or the health of women there make them more likely to deliver babies with birth defects. 
 
 Scientists are also trying to figure out if women who have had other mosquito-borne infections, such as dengue virus, might be more likely to give birth to babies with birth defects, Adalja said. 
 
 Authors of the CDC report note that microcephaly could be only one of a range of birth defects caused by Zika. Doctors caring for babies with microcephaly have diagnosed a variety of eye problems that could lead to blindness. 
 
 And in a Brazilian study of 88 women published in March, ultrasounds found major abnormalities in 29% of the fetuses from women who tested positive for Zika, but none of the women without Zika infections. Women were exposed to the Zika virus between the sixth and 35th week of pregnancy, according to the study, also published in The New England Journal of Medicine. 
 
 Although some of the babies in that study had microcephaly, others had restricted growth in the womb; poor development of brain structures; calcifications in the brain, which signal places where tissue has died; abnormal amniotic fluid levels; or abnormal blood flow in the fetal brain, umbilical cord or placenta, according to the study. Two fetuses in the March study were miscarried and two were stillborn. 
 
 Although Zika is the first mosquito-borne virus known to cause birth defects, doctors have known for decades that viruses such as rubella, or German measles, can cause microcephaly and other serious problems. About 21% of babies infected with rubella develop microcephaly, Adalja said. Congenital rubella syndrome, which can include blindness, deafness and intellectual disabilities, has been eliminated in the U.S. because of the measles-mumps-rubella vaccine. 
 
 Read or Share this story: http://usat.ly/1TERXUW ||||| 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. Credit: CDC/ Cynthia Goldsmith 
 
 A new estimate suggests that one in 100 women who become infected with the Zika virus during the first trimester of pregnancy will give birth to a child with microcephaly, a condition that causes an abnormally small brain and head. The finding comes from researchers' study of a single outbreak of Zika. 
 
 The new results support the idea that pregnant women who become infected with the Zika virus in the first trimester of pregnancy may have the highest risk of giving birth to babies with the condition, compared with women who become infected later in their pregnancies, researchers said. 
 
 "Our analysis strongly supports the hypothesis that Zika virus infection during the first trimester of pregnancy is associated with an increased risk of microcephaly," study co-author Dr. Simon Cauchemez, of the Institut Pasteur in Paris, France, said in a statement. 
 
 However, the findings are based only on data from one six-month-long Zika outbreak in French Polynesia, and so it is not yet clear whether this estimate will also apply in other countries affected by the ongoing outbreak, he noted. [Zika Virus News: Complete Coverage Of The Outbreak] 
 
 In the study, researchers looked at the prevalence of microcephaly among babies born during or around the time of a Zika virus outbreak in French Polynesia that began in October 2013 and ended in April 2014. The researchers identified eight babies born with microcephaly during the 23-month study period, between September 2013 and July 2015. 
 
 Among these cases, seven of the infants were born during the four-month period between March 2014 and July 2014. The researchers estimated, based on their calculations, that the women who became infected with the virus in the first trimester of pregnancy had a higher risk of giving birth to babies with microcephaly, compared with women who became infected later in their pregnancies, according to the findings, published today (March 15) in the journal The Lancet. 
 
 "The finding that the highest risk of microcephaly was associated with infection in the first trimester of pregnancy is biologically plausible," because the first trimester is an important period for the development of a fetus' brain, Dr. Laura Rodrigues, of the London School of Hygiene & Tropical Medicine, who was not involved in the new study, wrote in a related editorial published in the same journal. 
 
 But the finding that the risk of microcephaly is one in 100 (or 1 percent) among the babies born to women infected during the first trimester is lower than expected, the researchers said. That expectation is based on the risk of birth defects in babies born to women who had been infected with some other viruses, the investigators said. [Zika Prevention: Can Pregnant Women Safely Use Mosquito Repellants?] 
 
 For example, babies born to mothers who are infected during pregnancy with a common type of herpes virus called cytomegalovirus face a 13 percent risk of birth defects. And the risk of an infant having congenital rubella syndrome ranges from 38 to 100 percent if a mother is infected with rubella during her first trimester of pregnancy. 
 
 However, the percentage of people who become infected with these other viruses is normally much lower than the incidence of infections with the Zika virus during outbreaks, the researchers said. For example, only about 1 percent to 4 percent of pregnant women become infected with cytomegalovirus per year, the scientists said. 
 
 In comparison, during the Zika outbreak in French Polynesia, 66 percent of the population was infected, the researchers said. Therefore, the link between microcephaly and the Zika virus remains an important public health issue, they said. 
 
 Moreover, microcephaly is not the only birth defect that has been linked to the virus, said Dr. Gregory Kapinos, a neurointensivist at North Shore University Hospital in Manhasset, New York, who was not involved in the study. 
 
 "Severely small heads is only 'the tip of the iceberg,' as many other birth defects are now studied as potentially correlated with Zika's infection, too, increasing the burden of disability and devastation that these babies could be facing, if they survive," Kapinos told Live Science. 
 
 Follow Agata Blaszczak-Boxe on Twitter. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.
– A month after confirming Zika causes birth defects, CDC researchers say there is a "substantial" risk that a pregnant woman will have a baby with a birth defect if infected with Zika during her first trimester. More specifically, the risk for microencaphaly is 1% to 14% depending on the scenario, reports the AP. The range is wide given that research into the disease is relatively new. For the study, scientists analyzed data from 400 babies diagnosed with microcephaly—in which babies are born with abnormally small heads—in the Brazilian state of Bahia between July and February. Because the study focused exclusively on microcephaly in a single Brazilian state, "these numbers are probably only the tip of the iceberg," says a UCLA professor. One notable quirk: the bigger the outbreak, the better the odds are of having a healthy baby—because more women would be exposed before becoming pregnant and perhaps develop immunity. When the outbreak is small, the risk rises to the 14% level. An earlier study out of Brazil put the risk of Zika-related birth defects and fetal death at almost 30%, though one based on a Zika outbreak in French Polynesia put the risk of microcephaly at 1%. But even a 1% risk could be devastating in developing countries, where there are limited care options for disabled children, a health expert tells USA Today. A separate study published Wednesday notes three Brazilian infants with microcephaly showed eye problems not linked to Zika before, including retinal lesions, hemorrhaging, and abnormal blood vessel development, per a release. (The number of pregnant women with Zika in the US has nearly tripled.)