Understanding Gestational Age and Preterm Birth: A Patient's Question

Question
My name is Nicole.  I am 23 years old and now have 2 children. The first day of my last period was december 24, 2003.  My son owen was born June 30 at 26 weeks.  He was 2lbs 2oz, 14 inches long.  He is doing pretty good.  My question is that my period was normal, so that would give me a due date of september 29 or 30., but I had an ultrasound on may 17 and they gave me a due date of october 6.  thats an 8 day difference.  I had sex on the 5 of january and the 17 of january, and thats a 12 day difference in conception. I know when the baby is conceived you have already calculated 2 weeks.  So how old is my son and when did we concieve him.  the week or two would make a big difference in chances of complications for him!  Please try to answer before 8am tommrrow my time because i won't have access to a computer for awhile.  it is 12:50 am here

thank you so much worried mother nicole

Answer
If your last period started on December 24, you probably ovulated on January 6. YOu could only have conceived if you had sex on January 5,6,7, or 8.  You did not conceive when you had sex on January 17.  We count a pregnancy as 40 weeks from the first day of your last period, or 38 weeks from the day of conception. The results would be the same. When your baby was born, your were 26 weeks 5 days pregnant. using either method of calculation.