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Declining values of beta HCG are more consistent with abortion but decline rate is not as can be expected from complete abortion. It should decline its level about 50% in 24 hours if it was complete abortion. But this does not happen in incomplete or threatened abortions.
If there is a viable pregnancy, beta HCG level doubles in almost 72 hours. But this happens in first 11 weeks of pregnancy and after that its level does not rise that much and rather becomes more steady.
I think repeat ultrasound and repeat beta HCG level should clear the situation.