worried, please help

Brigit

As of today I'm 7 weeks pregnant. I've had 4 hcg tests that have all come back with my numbers increasing but my Dr says they aren't doubling fast enough. She sent me for an ultrasound last week and we saw the heartbeat but my baby is only 2mm instead of 5-9mm. I only have one ovary and am wondering if that is not helping my hcg levels to grow fast enough. Has anyone ever had this problem? Please help. I'm terrified I'm going to lose this baby.