I Just Had My First OB Appointment Wednesday. I Am

I just had my first OB appointment Wednesday. I am eight weeks pregnant and the baby is measuring good for eight weeks but the sac is only at six weeks. The timing is correct so what causes the sac to not grow fast enough and what are the odds on a miscarriage because of it or will it catch up? Also she said my it was low in the uterus. I had a miscarriage at 13.5 weeks in March because my cervix opened too soon so I had an abdominal cerclage placed in May. Is that a sign of miscarriage too?