Satya.
Progesterone does fall prior to HCG. So if HCG levels are being done and they double as hoped but the progesterone (which is measured with serial hcg tests) begins to fall this does not augur well for a pregnancy.
Conversley sometimes those HCG levels don't double quite like they shoudl but the progesterone remains stable or rises - there is more hope for the pregnancy then.

Satya is absolutely right - very often no cause can be found for losses in the first trimester (sadly also for later losses and stillbirths this is also true).

Wishing you a sticky sticky baby my love..