Expert: Dorothy H, LCCE - 11/13/2010

Question
Hi Dorothy,

I have a quick question regarding my date of conception...

Basically I had sex on September 5th (early morning September 6th). He wore
a condom, the first one broke right in the beginning of things. He
immediately stopped and put a new one on.

Thereafter September 8th (off and on) until the end of September, I was with
my ex-boyfriend having unprotected sex.

I had a transvaginal ultrasound on October 27th and I was dated at 8 weeks
and 1 day (1.66cm) which would put my date of conception around
September 14th. My question is, who is likely to be the father and are the
transvaginal ultrasounds pretty accurate? What is the margin of error? My
periods have always been very irregular and there is no way that I could date
it based in my LMP.

Thank you for your time and I am hoping that you can put my mind at ease!

Answer
Hi,

I apologize for the delay in answering your question.

Transvaginal ultrasounds at 8 weeks are accurate within +/- 3 days. According to the ultrasound, you conceived somewhere between 9/11 - 9/17.

Based on the ultrasound, which is highly accurate for determining date of conception at 8 weeks, you did not conceive September 6.

If you have a second ultrasound in your first trimester which also confirms the 9/14 conception date, it will further confirm that you conceived well after September 6.

If you have an ultrasound in the 2nd or 3rd trimester, it would be much less accurate for dating the pregnancy.

I hope that helps to put your mind at ease!

Dorothy H, LCCE