As the actual date of conception is an estimate, a few days can make a huge difference - i.e., 6 weeks and 0 days or 6 weeks and 6 days. Wait the wait and go back for a vaginal ultrasound. I had two miscarriages, one before each of my healthy sons were born. The fetus in the second miscarriage was characterized by delayed and slow fetal heartbeat. Heartbeat when detected at 8 weeks was slow and worse, irregular. I did not have the evacuation right away, hoping against hope that things would improve, but it didn't. I went for several more ultrasounds before I finally went for the evacuation at around 12 weeks, which gave me some time to say goodbye. I wouldn't recommend that for everyone, but I needed it. When I lost the first fetus, it was a complete shock as he looked perfectly healthy at 8 weeks and was still at the 12th week. Will be praying for your baby. Take care.