Pregnancy Dating & Conception: Understanding the Timeline

Question
My girlfriend and I are expecting a baby.  When we went for an ultra sound and the doctor said she was 9 weeks 5 days. But 9 weeks 5 days is 15 days before we had sex.  She's cheated on me before so now I'm worried this child is not mine.  Should I be worried about a 15 day difference?

Answer
Dear Stephen,

Pregnancy dating is always based on the date of the last menstrual period, NOT the actual conception date. So the 15 day difference actually makes perfect sense. Women usually are the most fertile during the 24 hours surrounding the day they ovulate, which is usually midway through their menstrual cycle OR 14-15 days after the LMP date for most women.

Don't worry - many young women don't even realize this, let alone men! Rest easy as far as the dating is concerned.  :-)

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

Brenda