New figures released Wednesday by Brazil's Health Ministry as part of a probe into the Zika virus have found fewer cases of a rare birth defect than first feared. Researchers have been looking at 4,180 suspected cases of microcephaly reported since October. On Wednesday, officials said they had done a more intense analysis of more than 700 of those cases, confirming 270 cases and ruling out 462 others. Brazilian officials still say they believe there's a sharp increase in cases of microcephaly and strongly suspect the Zika virus, which first appeared in the country last year, is to blame. But the World Health Organization and others have stressed that any link between Zika and the defect remains circumstantial and is not yet proven scientifically. In 2014, only about 150 [microcephaly] cases were reported in Brazil in a year - a surprisingly small amount for a large country with nearly 3 million births a year. The United States, with about 4 million births a year, has an estimated 2,500 cases of microcephaly a year, said Margaret Honein, a CDC epidemiologist. The birth defect can be caused by factors such as genetics, malnutrition or drugs. Brazilian officials said the babies with the defect and their mothers are being tested to see if they had been infected. Six of the 270 confirmed microcephaly cases were found to have the virus. Two were stillborn and four were live births, three of whom later died, the ministry said.