TORONTO, Dec 18 (Reuters) - The Canadian dollar weakened to a session low on Wednesday after the U.S. Federal Reserve announced plans to trim its economic stimulus program. The central bank said it would reduce its monthly asset purchases by $10 billion to total $75 billion. It trimmed equally from mortgage and Treasury bonds. The Canadian dollar was at C$1.0678 to the greenback, or 93.65 U.S. cents, weaker than Tuesday's close of C$1.0610, or 94.25 U.S. cents. The currency traded as low as C$1.0692.