Naomi Watts was so terrified -- she accidentally conceived!

The thesp was shooting new fear flick horror Funny Games - the redo of a very nasty 1977 German thriller - when Naomi was so terror stricken she turned to long time BF Liv Schreiber for comfort from her mounting psychological shakes.

Naturally, the two got busy and soon Naomi's dread of night was one helluva morning glory  -- she learned she was pregnant with son Alexander Pete.

"I have to say that I conceived during this film," Naomi admitted.

"I think I was creatively fulfilled.''

Watts is expecting the spawn of terror . . . soon!