Paternity Possibility: Ultrasound Dates & Conception Timeline

Question
I had affair with woman involving sexual intercourse on March 17.  Not sure when her last menstrual period was. She also was involved and having regular sexual relations with her husband during and after this time. She became pregnant, and had two ultra sounds, the first on June 17, saying she was 12 weeks and 4 days, and a second ultra sound on August 10 saying she was 20 weeks and 2 days. Both ultra sounds predict due date of December 26. Is it possible that I am father?

Answer
Hello,

Based on the information you gave me, the dates do not add up to your sexual encounter on March 17th.  The first ultrasound is key, because at 12 weeks all babies typically grow the same rate.  It is accurate +/- 3 to 5 days on average.  The ultrasound age takes into account when her last period started, so baby was actually 10 weeks 2 days from conception at that first ultrasound.  This puts at conception date of April the 4th based on both ultrasounds.  That is more than 2 weeks from your sexual encounter.  It would be very unusual for a first trimester ultrasound to be off by that much.   So it is likely you are the father?  My opinion is no.  But only a paternity test will let you know 100% if you are the father.  

I hope this helps some.