After five months of positive readings, the American Institute of Architects' billings index fell into negative territory in April, the group reported Wednesday.

The index, which fell to 48.4 in April from 50.4 in March, reflects the nine-to-12-month lag between architecture billings and construction spending.

"Considering the continued volatility in the overall economy, this decline in demand for design services isn't terribly surprising," said Kermit Baker, the architects' chief economist.

"Favorable conditions during the winter months may have accelerated design billings, producing a pause in projects that have moved ahead faster than expected," he said.

— Alan J. Heavens#