Liquideyes, I had same experience with #1. According to LMP I was like, 8 weeks or something. But at the scan, no heartbeat. Gynae said it could be the embryo stopped growing. I had to go back 2 more times before heartbeat was detected. Turned out at my first scan I was only 4 weeks. Using LMP not accurate - I must have had a late ovulation or whatever. If you use the online EDD calculators, they have you put in the length of your cycle as well. Counting from LMP, I am now 8 weeks, but according to online calcuator, I am 6 weeks only.
Now I look at my 7 year old son, I think back of that time when I was so worried, it's quite funny really.