Since she mentioned it went to the neck I focused there as well and felt I could faintly make out the seam of the suit. In action it won't be noticeable though.
Maybe its more of a two piece instead of bodysuit, as some said, that sure looks like her navel. Still not sure where the seams would be next or where the break in the bodysuit would occur, as some suggested to reveal the navel. At the scales perhaps? That way the seam is disguised there?
That's more thought than I care to put into this "costume" really. I just want an awesome DOFP movie!!! The costume I really want would be Mystique in her more traditional white dress or even white body suit, both with skull emblazoned belt. Why can't we get that? We have variations of other "true" comic outfit translations.