My hcg levels were 3900 when i was 5 weeks pregnant and were only 4,400 two weeks later. at 7wd5 my baby had grown appropriately and i heard a heartbeat of 160. what is the honest likelihood that i'll miscarry within the next few weeks?

Difficult prediction: The abnormal rise of your betaHCG level probably reflects "abnormal placentation" (if those 2 values are accurate) and an increased risk of pregnancy loss despite the detection of a "heartbeat of 160". It is quite difficult to offer you an "honest likelihood that you will miscarry within the next few week". A repeat sonogram and repeat betaHCG level may be the best way to answer your concerns.