Paternity & Conception Timeline: Understanding Pregnancy Timing

Question
i had sex with a woman on Dec. 20th and again on Jan. 21st.  she informed me she is pregnant.  she had a vaginal ultrasound done on Mar. 4th, the results of which say she is 8wks and 2 days pregnant.  that is 2 wks and 2 days less than the first sexual encounter, and 2 wks and 1 day after the second encounter.  could it be my child from either of these encounters?  i see your responses to other similar questions about the accuracy of early ultrasounds.  2 weeks off seems to open the possibility that this is another man's child.

please help Brenda.

Answer
Dear John,

If a trans-vaginal ultrasound on 3/4 put her at 8 weeks, 2 days, that calculates back to a LMP date (last menstrual period) of 1/6 and an estimated ovulation/conception date of 1/20. Here is where your confusion lies: pregnancy dating is done by the LMP rather than the actual conception date. Even ultrasounds take this into consideration and the LMP is assumed. So...in all actuality, the ultrasound confirms the probable conception date of the 1/21 encounter. Unless the woman had unprotected sex with another man within the 24 hour period that she had unprotected sex with you, more than likely it was your encounter that resulted in conception of the baby.

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

Brenda