Billings to architecture firms returned to growth territory in April after falling in March, the American Institute of Architects said in a release. The AIA Billing Index for Iowa registered 50.5 last month, up from 47.8 in March. Any reading above 50 indicates an increase in billings. Jessica Reinert, executive director of AIA Iowa, said it is difficult to predict how long the index will remain positive. “In contrast to years past, the continuous up and downs of the index leaves the market unsettled,” Reinert said in a statement. “Though we may not be at our lowest point, the next few months of data will really tell us the health of the industry for the future.” Most regional indexes registered no-growth scores, with the Midwest at 49.3, the West at 49 and the Northeast at 45.1. In the South, the index registered 51.6.