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. (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. 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. 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. Last month, the CDC said there was enough evidence to declare that Zika causes microcephaly and other brain defects. 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. That study estimated risk of microcephaly at 1 percent for pregnancies in which the mother was infected in the first trimester. The findings are being published online today in Ophthalmology, journal of the American Academy of Ophthalmology.
– 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.)