TORONTO (Reuters) - Like parents who forget to pick up their child at school, the usually-bountiful months of November and December seem poised to leave investors out in the cold this year.

Many have been left scratching their heads as the Toronto Stock Exchange's main index bucks a trend that has lasted for a decade. It has tumbled 7.5 percent midway through November, a month during which it has risen by an average 1.6 percent for the past 10 years.

The index's unusual weakness could signal a broader turning point, even though lingering anxiety over global credit markets has played what may be a unique role in waterlogging stocks in the second half of the year.

"The fact that this period — which has normally been the best — has been so challenging is cause for concern," said Matt Blackman, senior market analyst at TradingEducation.com in British Columbia. "Everything has been turned on its head."

Less than three weeks ago the S&P/TSX composite index was within a whisper of a record high following a gradual ascent through September and October. But since Halloween, it has closed lower in eight of 12 sessions.

Only December and January have proved better months than November for the TSX index. It has logged gains for the past 10 Decembers, a pattern that could be set to change.

Glenn MacNeill, vice-president of investments at Sentry Select Capital Corp in Toronto, said the stock market is paying the price now for a good first half of the year. "Credit issues certainly rule the day, and the market is very nervous," he said.

At its peak, the index was up 13.3 percent on the year. It has shed about half of that gain in November alone.

TSX financial stocks are down four percent on the year as headlines peel layers off the subprime mortgage mess in the United States and reveal a crisis in global credit markets, including Canada's asset backed commercial paper market.

Bank of Montreal shares slipped Friday after Canada's fifth-largest bank by market cap said charges from its ABCP exposure would hit fourth-quarter earnings.