The mosquito-borne Zika virus causes microcephaly and other birth defects, U.S. health officials confirmed in the New England Journal of Medicine on Wednesday. The Centers for Disease Control and Prevention said mounting evidence showed the virus was the cause of the rare fetal defect, which results in an underdeveloped brain. However, more research is needed to conclusively say it is causal, the CDC said in its study review. Health officials said they need to establish the range of defects caused by a prenatal Zika virus infection and also explore other factors that could affect birth-defect risk. The CDC said it will continue to recommend that pregnant women in the U.S. avoid traveling to areas at risk of Zika virus transmission and that men with pregnant partners either abstain from sex or use a condom after traveling to an affected region.