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So, the doctor told us it is a boy?

Sure, but in his notebook, he wrote: "it will be a girl".
So, if it is a boy, you'll remember that is what the doctor said.
But if it is a girl, you will tell the doctor that he was mistaken, and he will show you his notebook and tell you that you did not remember correctly his prediction.

Haha just giving my 50 percent guess!