Low rising hcg and just a sac at 5 w 6 days

i did an ultrasound at 5w 1 day (hcg 1100) and 5 w 4 days (hcg 1300) and the doc couldn't see anything.I had another ultrasound today, 5w6 days and th doc saw a sac (hcg 1350). Doc asked me to come back for a scan after two weeks. Has anybody had a positive experience with low hcgs? Is miscarriage inevitable here?