From my experience, at 39-40 weeks they sometimes send you to the Obstetric Assessment Centre just to do a closer check on baby's position, heartbeat, and to assess if you are progressing toward labour, not necessarily because something is wrong. It does cater to higher risk cases but also routine late-term monitoring, so try not to worry too much lah, it is quite common at that stage. I would just call KKH directly to confirm what your specific appointment is for so you go in feeling prepared.