It really depends on your body shape, tone etc! I was about 18 weeks when I got a real round belly not just the weight I had put on due to ms in the first trimester! Peoples perceptions are funny too - I was at the shops yeserday and this gorgeous older lady asked when i was due - I said Tuesday, she took one look at my belly and said, Oh my you arent very big!! Ha ha - must have been what I was wearing!