Anxious about dates please help

I had an early scan on the 6th December and I measure 5+4 weeks what does this mean? The problem is I made a mistake with a guy on the 15th September could this be him or is this date too far fetched???am I being paronoid if it's not this date then is it Defo my Bf which I am praying for. I have a "12" week scan tomorrow but I'm too paronoid of the out come to go with my bf. I need to know when I conceived I also believe I had 2 periods sep and oct but I'm scared now that they was not normal periods. Am I worrying over nothing is September to late

[AnnieBrooke]

Don't worry, sweetheart, you are not pregnant by anyone from the 15th of September.  You wouldn't even be pregnant by someone from the 15th of October.  Take your boyfriend to the scan, everything is all right.  

The pregnancy date count begins on the first day of your last period (or if you don't know what that date was, by an assigned first day of your last period), based on the size of the embryo when seen and measured during the scan.  To say that again, medical dating of pregnancy does not start with the presumed date of conception, it begins with the presumed date of your last period.  When you had a scan on December 6, the ultrasound saw your baby and said "Rosie's pregnancy is 5 weeks 4 days along."  To get to a conception date, subtract two weeks.  I.e., a 5 w 4 d pregnancy is 3 days and 4 weeks after conception.  (That definitely was an early ultrasound.  The embryo isn't even visible if you're in your 4th week.)  

Anyway, counting back 3 weeks 4 days from your December 6th ultrasound puts conception at around November 17.  

Take the guilty conscience and blow it to the four winds, it is not the September guy's baby.

[AnnieBrooke]

When I said, "that definitely was an early ultrasound, the embryo isn't even visible if you're in your 4th week," I meant that any much earlier than 5 weeks, people sometimes go for an ultrasound and can't even see the baby yet.  It was nice that you got a good clear reading at 5 weeks 4 days, because the accuracy that early is usually pinpoint to the day.