J.,
I am so sorry to hear of your situation. Chances are, however, that if you have had two ultrasounds determining that there was no heartbeat, then you do not have a viable pregnancy.
I have known some people who wanted to wait a bit for the miscarriage to happen on its own. This largely depends on the hormone signals in the body. From my friends, I can tell you that the longer you wait to have a D&C, the harder it may be for both your physical and emotional health.
If it were me, I would go back to the doctor and it's likely they will check once more to make sure that they didn't make any mistakes. Then, if they are still confirming a non-viable pregnancy, please do go ahead and schedule the procedure. This is done to ensure that you maintain good reproductive health for future pregnancies.
Everyone I have ever spoken to has said nothing but good things about the doctors and nurses they interacted with during their D&Cs. I have lost three pregnancies myself, and I know how much you were hoping for a baby. It's very hard. Consider talking to your doctor about getting some medical help (good painkillers for after the procedure) and a possible referral to a counselor who specializes in this sort of loss. It IS a huge disappointment when a baby is wanted and not able to grow. But it doesn't mean you aren't able to get pregnant or have a baby-- it was just that *this* one stopped growing.
I'm so sorry. Please, take good care of yourself, get help in keeping your body healthy. If it's any consolation, I know quite a few women who had very successful pregnancies after losses such as these. I ended up getting pregnant with our son shortly after our last miscarriage. So please, take care to make sure your body will healthy and able to manage a pregnancy when you do become pregnant again. For what it's worth, in hindsight, I would have rather had a D&C than having the days of intense cramping which comes with a 'natural' miscarriage. You will have to do what is best for you. Please make sure to take care of your health right now. That is what is most important at this point. So sorry. It's a hard spot to be in.