Please help! 7 weeks 2 days??

Hell everyone! Today is September 28, I'm supposably 7 weeks 2 days pregnant. I had my period a long (for me) 4 day period On the 17th of august. I got several negative test and then tested on the 17th of September and positive! I went to the doctor and she said my gest. Sac was measuring 7 weeks 1 day, and there is no baby. Being so early I didn't know if I would see a baby so she telling me I was 7 weeks I was shocked. I have to go back next week to see if there is a baby.. this maybe a stupid question but twins run in my fiancé's family. Could it be twins just really early and that is why the sac is measuring big? That could be why I see no baby? Or is that wishful thinking. Please help