My best friend's mom didn't know she was pregnant and she had a baby girl!!!?

A couple of weeks ago, my best friend's mother gave birth to a beautiful baby girl. Well, the thing is, she didn't have the slightest clue that she was pregnant. Her mom isn't big at all, and when I first met her, my best friend and I would always joke with her that she pregnant, but none of us really knew she was! I was wondering how does a woman not know that she is pregnant? I know it happens sometimes. Her mother said that during the times the baby was kicking inside of her, that she thought it was stomach pains because it would hurt her stomach, and she always was constipated, and she thought that was going through menopause because she is in her early 40's.