HIDDEN TWINS ! is this poss ???

im 11 weeks pregnant and altho we are just getting over the shock as i had implannon in my arm ( yes we have been told there more chance of being hit by lightening by the doc!) so we had a scan last week baby looks huge and very clear but my partner has twins in his family his granparents had twins then his uncle same side of family !! my parnet has a "feeling " that thers one hiding behind the other !!! and people are commenting on how big i look already charming i know !!! is it possible that this could be true ? would it not have been detected ?? we have another scan in coupke of weeks !! please put my mind at rest thanx x x x