How do you not know that you're pregnant?

Yesterday night, I was watching tv and came across a show on the discovery health channel. The show was "I didn't know I was pregnant". When I saw that title, I thought to myself, now how in the world do you not know that you're pregnant? Obviously, you'd feel the baby moving in your stomach, or you'd gain weight, or have all these signs out there telling you that you are pregnant. Apparently, it could happen. There were so many stories of women having the case of not knowing they were even pregnant. One of them was about a women who was petite, and stayed petite even when she had her baby. There wasn't even a bump on your stomach to show she was even pregnant. I was completely amazed when the baby was born, because it was pretty big, so why didn't it show on her belly? She also had her monthly "period" that she thought was her monthly period, when really it was her pregnancy spots of bleeding. I really thought it was interesting that this could even happen to a women. What if she didn't intend to be pregnant, when in reality, she was.