HCG level is decreasing.

I recently did artificial insemination and my physician put me on Endometrium (a progesterone suppository), I became pregnant and all was going well. I had my blood drawn and hcg levels taken at 4 weeks and they were normal as expected, and 5 weeks normal. This week, the 6th week my HCG is only at 5200. I can't recall my exact numbers last week or during week 5, i think they were at 1600.  I have an ultrasound tomorrow, but am very worried. Is there a chance that this can move on to normal HCG increases?