Lost final Invisalign tray, don't have another?

So I recently lost my final invisalign tray for my top teeth, it was tray 39/39. I had started it about a week ago. I haven't saved my previous trays, I just throw them away. I contacted my ortho and we're ordering a new tray; should I order 38 because my teeth will move back, or should 39 be okay? I have no idea how long the replacement will take to arrive.

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