first scan worries

Went for my first midwife/scan appointment today. This is my 2nd baby. According to dates im 12 weeks today but after getting a scan it seems that my uterus is measuring 12 weeks but baby is only measuring 8+3. Can anyone explain why my uterus is measuring larger, is this normal? I know i should have asked the midwife but my emotions were all over the place. Feeling very greatful that my jellybean is still there after what seemed like forever of searching. Im due back in 3 weeks for another scan.