I would wait and see if you can pass naturally, but if your levels are going down, from experience, it is most likely a miscarriage and they should see a heartbeat in a vaginal ultrasound by now.
You can try these pills as well that are to dilate you and cause you to essentially go into labor to pass it. It is painful, but if it works it does take out the risk of a D&C. They did not work for me for my 2 that I needed a D&C for though I did try it first both times.
They will wait a month before getting you to try the pills or a D&C and will monitor things with u/s about every week or so. My first one we did that, but the sac collapsed and started to go bad though I did not get an infection as they gave me antibiotics.
The 2nd one passed within about 2 weeks, but I had horrible bleeding with that one and passed the huge clots before I knew when I was at work.
The last one I was just a week difference between our baby and me and when I was 11 weeks and the baby only 8 we knew for sure we lost it and it no longer had a heartbeat. I elected to do the pills right away since nothing had happened and since the pills did not work I had a D&C as I did not want to wait since I would need to wait 4 months to see a specialist about all this.
If your hcg is declining that's not a good sign. However if you are unsure if your dates it can't hurt to wait a week or two and see what happens. Good luck