Unless in cases where the woman may know or not AND keep the facts of being pregnant in secret...which happened with my first daughter. First of all, I am sure most of you would find it very hard to believe that no one could have NOT tell that a certain woman wasn't pregnant...my daughter's mother did NOT show...it's like her stomach didn't expand or protrude outwardly like most pregnant women...honestly, I just had assumed she had gained weight...to make this story short, I did not know she was carrying a baby until 30-45 minutes before she actually gave birth!!...I know this sounds far-fetched and very odd...but looking back, I believe she kept the truth to herself for several reasons...with one of the reason could be simply she didn't want to see me 'run' off if I had known...etc.
Like ^Angel^ indicated, I would find it very hard for a woman NOT to know she wasn't pregnant UNLESS some profound complexity derived which kept the woman's menstrual cycle intact and not disturbed time-wisely....which I'm sure there's a small percentage of such cases whereas a woman doesn't know until much later....