Expert: Dorothy H, LCCE - 4/2/2010

Question
my gf and i have been together since dec 5th 09. we just went for our first ultra sound and found out that shes's 20 w 1 d...she was with her ex bf and had intercourse on the 14th of nov 09. im really worried i might not be the father and so is she. she usually gets her period at the beginning of the month but she cant remember if she got her dec one. now the ultra sounds leave a 2 week window so she could be at 18 weeks. is there a chance that the ultra sound could be off by a bit being as all they got to go on is weight and measurements, the baby weighed in a 1lb already is that large? i've done some research online and found a conception calculator and when i put in for her LMP as dec 1st i get a conception date of dec 11-19th witch would set me as the father, but then the weeks count of the ultra sound doesn't quite add up. if i go for nov 1st, it lines right up to the time she was with her ex, i'm so worried that the child wont be mine. is there any advice or statistics that you could give....

Answer
Hi Geoff,

Assuming she had the ultrasound done on April 2, that puts her last period around November 12 and conception around November 26, which would give her a due date of August 19, 2010.

Unfortunately, ultrasound is not that accurate in dating a pregnancy at 20 weeks - it can be 1-2 weeks off, which means the likely dates that your girlfriend conceived is anywhere from November 13 - December 10 - which doesn't narrow it down at all.

If she had any ultrasounds in her first trimester in the doctor's office, the information from those ultrasounds can help pinpoint when conception occurred.

The only way to know for sure is to have a paternity test done after the birth.  However, if she is having amniocentesis done for any reason during her pregnancy, they can also do a paternity test at that time as well; let them know that you also want to test for paternity if she is having an amnio done. Her doctor will probably not want to do an amnio solely to find out paternity because there is a small risk of miscarriage with an amnio.

I'm sorry I can't give you a more definitive answer.  I wish you and your girlfriend the best.

Sincerely,

Dorothy H, LCCE