Federal investigators are taking a new look at concerns that a herbicide found in Indiana's drinking water could be linked to birth defects, especially in babies conceived in the summer months.The Environmental Protection Agency limits how much atrazine, a popular weed killer often used on corn crops, can be in drinking water, and many water companies treat to lower the levels, 6News' Joanna Massee reported.Atrazine was effectively banned in Europe a few years ago. Within the last 90 days, the EPA began taking another look at its position on the weed killer.A study by biologists at the University of California, Berkeley, found that exposure to atrazine turned some male frogs into fully functioning females.Some Indiana scientists are now questioning whether the government's acceptable levels of the chemical are truly safe, especially for expectant mothers."We found that women who conceive their babies in the months between April and July, June, are the most likely to have birth defects," said Dr. Paul Winchester, a neonatologist with St. Francis Hospital and the Indiana University School of Medicine.Dubbed the June Effect, Winchester said research shows that the rate of birth defects correlates with spikes of atrazine in the drinking water during the summer growing season.Julie Bertel's daughter, who was conceived in early July, was expected to be healthy. But shortly after birth, she was diagnosed with a serious brain malformation, suffering seizures and other problems.Bertel said doctors have not been able to offer an explanation for what caused the defect."I mean, she was supposed to be perfect, and she wasn't, and nobody could tell me why," Bertel said. "There's just so much information that's been recently presented to us that could link this to (atrazine contamination)."The EPA currently requires levels of atrazine stay below a 12-month average of three parts per billion. Water utilities must treat water to keep levels within that limit.Patrick Carroll, the Drinking Water Branch Chief of the Office of Water Quality at the Indiana Department of Environmental Management, said the agency has sent letters to 31 Indiana water utilities, asking them to participate in a voluntary atrazine monitoring program.There have been no recent violations involving Indiana water utilities. Carroll said IDEM defers to the federal government to dictate acceptable levels of chemicals in drinking water."It's a big business in Indiana. A lot of people apply atrazine, so that's the reason that we see a lot of it in the source waters here," Carroll said. "We don't have any plans for (stricter regulations). We do rely on the EPA for that type of work."Winchester said without human testing, it's difficult to directly link increased atrazine levels in Indiana drinking water to birth defects."Imagine me randomizing a group of pregnant women, giving half of them weed killer and half of them not," he said.A representative for Syngenta, one of the major manufacturers of atrazine, told 6News that the water in Indiana and around the country is safe, and that the company and the EPA have conducted studies showing the herbicide does not cause birth defects.