Understanding a Missing Fetal Heartbeat at 13 Weeks: Seeking Expert Insight

Question
I wrote to you a few weeks ago about an ultrasound at 7wks with no fetal heartbeat but movement seen. You were very reassuring, so I want your opinion about my lastest ultrasound. My Dr. said the fetus measures 13wks 3days and was very active, but we still can not detect a heartbeat. He moved my due date up by 3 weeks, having previously thought I was only 10 wks. He still offered no explanation about the lack of heartbeat and has not tried to hear it with a doppler. He just laughs and says the baby is fine. I know he's probably right I just dont understand why the heartbeat cant be seen. I heard the heartbeat very early in my first pregnancy. I would really appreciate your input so I can {possibly} stop worrying.  

Answer
If the baby is moving, there HAS to be a fetal heartbeat. At 12 weeks gestation, you should definitely hear a heartbeat on doppller. Do not take no for an answer and see if it is heard on Doppler. When did you have the sonogram that showed 13 weeks 3 days>  You did not give me any dates so I can't calculate your due date or the accuracy of the sonogram.However, everything sounds fine.