Waterloo road fans: Is it possible not to know your pregnant?

I was just watching an episode of Waterloo Road on BBC iplayer. It was the one where that girl Allie has a baby and abandons it at the school. She said that she didn't know she was pregnant until a couple of months before. How is that possible? Surely she would have noticed that she was gaining weight, and that her periods stopped and that she felt ill sometimes. Her parents and friends also didn't know she was pregnant until she actually had the baby! I think it's pretty obvious when a woman is pregnant, for example...they have a baby bump! I was just wondering if this is actually possible or if it was just a rubbish story line?