Pregnancy Timeline Discrepancy: Understanding Ultrasound & Period Dates

Question
Hello

This may sound a silly question but the story goes, I had intercourse on the 6th sept and had what I thought was a period starting on the 21st/22nd sept.I came off the pill after this period. I didnt come on my period in october so I did a pregnancy test on 19th Oct which was negative. I then found out on 13th November that I was pregnant and I had a ultrasound scan on the 27th Nov which said I was 6 weeks and 5 days. Which didnt add up to when my period was due, but would this be due to the fact I just came off my pill? Also I had three ultrasounds after this the first two said I was due 17th July and the 2nd two said I was due on 14th July. At my 6 week 5 days the CRL on scan was 7.1mm is that normal for that age?
Also as I said this may sound like a silly question but what is the likelyhood that I conceived on 6th september can a scan be that far out?

Thank you

Answer
Dear Michelle,

You would not have conceived on 9/6/07 if your scan on 11/27/07 had you at 6 5/7 weeks. The average CRL for that fetal age would be approximately 7-8mm. That would calculate to an estimated conception date of 10/25/07 and an estimated due date of 7/17/08.

Also, if you had conceived on 9/6/07, you would likely have delivered a full term baby on/about May 29, 08.

I hope this has helped you and answered your question. I wish you well.

Brenda