Need help!

[Cynthiaaaa]

Hello! If I had unprotected sex on January 9 and January 17 what week was it that I could have gotten pregnant? I had an ultrasound done March 20 I was 11 weeks and 3 days my ultrasound measurement came out to 10 weeks and 6 days what week could I have gotten pregnant? The gave me my due date to be October 8 .

[AnnieBrooke]

Your ultrasound in your late tenth / early eleventh week gave you the due date of October 8?  Did you have any earlier ultrasounds?

Ultrasounds in the eleventh week are not accurate enough to pin down the day with the exactness you need.  The margin for error using an ultrasound in the eleventh week to try to determine conception is +/- 6 days.

Keeping in mind that you have to say "plus or minus 6 days" from the following dates -- if you run a conception calculator, it gives a possible conception date of January 15 for the due date of October 8.  Though that in a way sounds closest to the sex on the 17th, it is actually equally close to sex on the 9th because sperm can live 5 (some research says 6) days in your reproductive tract.  This means that from the sex on the 9th, the sperm could have gotten you pregnant up to the 14th or 15th.  

Then you look at the fact that the ultrasound has a margin for error, and factor in +/- 6 days or so, this means that sperm from the 9th could have lasted until the 14th and then the margin for error only has to be one day, or if you got pregnant from the guy on the 17th, the margin for error only has to be two days.

Basically, only a DNA test will tell.  I'm sorry it doesn't point clearly to one of the events and not the other.