Pregnancy Dating Concerns: Ultrasound Accuracy & Conception Timeline

Question
Dear Brenda,

I am hoping you can bring me some peace of mind... I am currently 36 weeks pregnant and am starting to become terribly anxious about the accuracy of my conception dating. My last period was on 2/01/09-- I normally have 31-35 day cycles. I had sex on 2/08/09 with one man (we didn't finish, however) and then I had sex with another man on 2/18/09 (we didn't use protection and he ejaculated inside me).

I had a transvaginal ultrasound done on 3/18/09 and it dated me at exactly 6 weeks 0 days pregnant. The crl was .37 cm and it showed an Edd of 11/11/09.  I went in exactly one week later and had another transvaginal ultrasound done which dated me again at exactly 7 weeks 0 days pregnant with a crl of 9.5mm and Edd of 11/11/09. Both of these point to a conception date of 2/18/09, but my ultrasounds from 12 weeks on have me measuring a little further ahead-- for example, my 20 week ultrasound changed my Edd to 11/07/09 (4-5 days earlier).  

I've done some research and know that sperm can live for 5 days... I'm starting to panic that perhaps I conceived from the 2/08/09 encounter. Can you help me put my mind at ease? From the information I gave you do you think you might be able to answer my question of conception dating?

I appreciate your time!
Regards, Joanne

Answer
Dear Joanne,

With your cycle length history, LMP date, as well as with the results of the two earliest ultrasounds (which would have been the most accurate in dating gestational age), I calculate that your due date would be on/about 11/12/09 and you are approximately 36 weeks, 1 day along today. This would have given you an optimal fertility window from 2/14 to 2/24 and your estimated date of conception with all dates and information provided would have been on or about 2/19/09. This matches pretty closely with the date of the 2nd encounter you described and I would be fairly comfortable in saying that the 2/18 encounter was the one that resulted in the pregnancy.

I hope this has helped you and answered your question. I wish you well and a blessed delivery!

Brenda