At that certain point, she starts wearing a corset to assist in this hiding of her condition.
So... Anything and everything you know about wearing corsets while pregnant, especially wearing them tightly enough to severely diminish the appearance of being pregnant.
Also, is it likely to severely damage the child in any way? I want her to come out whole and be a reasonably intelligent young girl as she grows up.
If it's not feasible, then alternative suggestions for hiding a full-term pregnancy from people constantly in close contact with you is much appreciated.