That is correct, studies in viable intrauterine pregnancies have reported the following changes in serum hCG:
-The mean doubling time for the hormone ranges from 1.4 to 2.1 days in early pregnancy (hence the repeating ~ every 48hrs until a diagnosis made).
-In 85 percent of viable intrauterine pregnancies the hCG concentration rises by at least 66 percent every 48 hours during the first 40 days of pregnancy; only 15 percent of viable pregnancies have a rate of rise less than this threshold.
-The slowest recorded rise over 48 hours associated with a viable intrauterine pregnancy was 53 percent.
Based upon what you report, your hCG seems to be rising okay. Given this is a spontaneous pregnancy it is unclear if supplemental progesterone will be helpful. In my patients I don't continue to check Progesterone levels in these circumstances as they are not any more helpful than following the hCG.
I believe most doctors want your hcg to rise 66% - the "doubling" is a myth. At some point in time, I believe your hcg levels off a bit more.
I would call your doctor and inquire about progesterone. My doctor had me on prometrium 3xday until the 10th week.