Ultrasound measuring embryo smaller than expected based on LMP

Hi everyone,
I am new to this site, but have really enjoyed reading the posts. It has helped me get through the past couple of days. I have two healthy children ages 6 and 19 months. Both pregnancies were a piece of cake. After my son was born I had thyroid problems, since resulting in normal levels. We just found out in June that we're expecting baby #3, it was our first month of trying. We had our first ultrasound the other night and the embryo was only measuring 5W 6D, when I should actually be 8W 1D based on my LMP. They also could not detect a heartbeat. My Doctor's office called today and said I should schedule a D&C, but I cannot help but think that there is hope. What if my pregnancy didn't follow the textbooks? Could a heartbeat still show up next week in a repeat ultrasound? Anyone have a success story to share? They do not want to monitor my HCG levels either, I feel like I'm being brushed under the rug.