What Causes Low HCG Levels During Pregnancy?

I am currently pregnant. My hcg levels are increasing but not doubling in 48hrs. My LH surge was measured and occurred on 03/14. My hcg level on 03/27 at 14:30 was 98, on 03/30 at 07:30 is was 216, which is an increase of ~80%. Another hcg was conducted on 04/06 at 14:30 and was 1518, which is a 70% rate increase from the last reading. On 04/10 the gestational sac was seen via ultrasound. I have a history of missed miscarriages, the first was a blighted ovum (no fetal pole at ~7 weeks), the second was a partial mole, the third had no heartbeat at 7-8 weeks. I then had a healthy pregnancy where the hcg levels more than doubled in a 48hr period. My doctor seemed happy with the readings thus far but the low levels of increase in hcg have me worried that I will have another missed miscarriage. What is the likelihood that this will be a successful pregnancy?