Due date confusion? help?

Hi there! I am currently pregnant for the 2nd time (1st was early MC), so I'm really nervous this time around! my periods are irregular, always between a week early or late. Last period was from Sept 29 - Oct. 5. I know for a fact conception occurred either October 16 and/or 17 as I was using ovulation tests and keep track of when we have sex. So, here's the thing. Based off LMP, my due date is July 6th. However, I went to the ER around 6.5 weeks for cramping and spotting, and they saw a HB of 174 but baby measured 5.5 weeks. At my 8 week scan I should have been around 8.5 weeks, but baby was measuring 7.5, and had a HB but she didn't tell me the rate for some reason. I'm going in 1 week later, this coming Tuesday, for another US to check on the progress. I am so nervous the baby will have stopped growing or lost its HB. I know there is nothing I can do, but I'm reaching for things to comfort myself. For example, if baby was conceived on 10/16 Or 10/17, wouldn't the gestational age just now be approaching 8 weeks? does anyone have any experience with something like this happening and had a good result?