How low will HCG levels be to miscarry?

I'm having diffculties in my 2nd pregnancy & sadly have been told to expect a miscarriage. My HCG levels have been dropping steadily for almost a month & I'm awaiting my ultrasound results but I sneaked a glance at the screen & saw an empty circle (I'm uming a blighted ovum). My HCG level was 9900 yesterday. How soon can I expect to miscarry? Also, will I need a D&C procedure because I am 10 weeks?