i carried my twins to 40 weeks! the only problems i had were small contrx at about 22 weeks due to dehydration and a uti. my dh always went aronund saying 'well, they are due oct 22, but you know, twins always come early!'. i kept my mouth shut, but even i didn't think i would keep em in that long! the day before i had them i was digging up raspberry bushes and planting bulbs and walking the dog around the neighborhood. sigh, i miss those days.
oh, and a woman in my LLL group carried her twins 41+weeks. so no, twins don't have to come early. or with complications.
relax and enjoy your pg. it really is a wonderful time.