Expert: Catherine Beier MS CBE - 2/21/2009

Question
My Girlfriend is pregnant and I have to question if it is mine.  We went to the doctor on January 20 and they said she was 20 weeks pregnant.  They gave us a conception date of September 5, 2008.  On January 30 we had our ultrasound and the doctor told us she was 21 weeks and 2 days pregnant.  That giving us a conception date of September 5, 2008.  She says that she started her her menstrual cycle between August 15 and August 20.  My first question is how accurate are these dates?  

Second major issue is that we did not have sex for the first time until September 15, 2008.  With everything I have found online it would appear that my chances of being the father are void.  I do want a child but of course I will not take care of a child that is mine.  There is another potential father but she does not appear willing to allow anyone a paternity test.  The other issue is that she was with her ex up to September 15.  He was out of town from that date until September 21 at which time she was with both of us until mid December.  My second question is what is the likelihood of my being the father?

Thank you for your time and any information you may have.

Distraught in Florida

Answer
Fred,

Ultrasound at that point in pregnancy can be off by as much as a week or more in either direction, making it unreliable.

The best indicator of a true due date is an early transvaginal ultrasound between 8-10 weeks of pregnancy.  If you have information from this stage of pregnancy, it would be a much better predictor of whether or not you're the father.

Given that two estimates gave the same probable date of conception, which was well before you first had intercourse, the likelihood that the child is yours is not good.

The only way to be certain is with a paternity test.  However, prenatal paternity tests are invasive and carry risks of complications and miscarriage.  I understand completely why your girlfriend would refuse a paternity test at this point.

So, the only way to be sure whether or not you're the father would be to have a paternity test once the baby is born.  That type of paternity test doesn't carry risks to you or the baby and will give you the peace of mind you seek.

Best,
Catherine