Its hard to tell really, one you can tell is definitely a pregnancy sac but the other Im not sure, did he say you are definitely 6 weeks along? Twins is only more likely if it runs in the maternal side of the family by the way as it is caused by the womens chance of releasing more then 1 egg or her eggs splitting which she inherits from the women in her own side of the family so it makes no difference if dad or anyone in dads family is a twin x