The number of houses placed under contract fell sharply in November in the first drop in nearly a year, figures released Tuesday show. It was the clearest sign yet that predictions of another downturn in real estate may become a reality.

The National Association of Realtors said that its index of pending home sales plunged to 96 from a revised level of 114.3 in October. Analysts had predicted a drop, but nothing like that.

“We thought it would drop 2 percent,” said Jennifer Lee of BMO Capital Markets. “When you see 16 percent, the first thing you say is, what the heck happened here?”

Since the majority of pending sales become final in six weeks to two months, the index is considered a reliable indicator of where the market is headed. The index is calculated by comparing the number of pending sales with the level of 2001, when the index was formulated.