Pregnancy Due Date Discrepancy: Understanding Gestation & Ultrasound

Question
my last period was october 12th 2007. they set my due date at july 15th 2008. when i went in to get my sonagram and the nurse told me im not as far along as my doctor said she said its about a week and a half to two weeks off. she gave me an estimated due date of july 25th 2008. when i went in to my next doctor appt they still didnt change my due date or how far along i was, but i also didnt tell them about what the nurse said i just assumed that they would change it. ive had 2 sonagrams and each time they told me im due later in july then what my doctor says. so when was the baby concieved if im due on the 15th of july and whats the conception date if i was due on the 25th? i know that your pregnant for so many days before you really are truly pregnant i just dont understand it all. this is my first. by the way its a girl if that helps any!! thank you so much!

Answer
A lot of this depends on when you had your ultrasounds, and how they calculated your original due date of July 15.  That date does not quite line up with your last period (which gives a due date of July 18 on my calculator).  If you had an early ultrasound when you were given the July 15th date, that is most likely the correct due date, and you would have conceived sometime the week of October 22.  Any ultrasound performed after 10-13 weeks gestation cannot be relied upon for dating purposes.  Ultrasound "dates" a pregnancy by using average size charts.  But just like adults and children come in many different sizes, so do babies, and these differences start to show around 13 weeks gestation.  So if your baby is measuring a little smaller or a little bigger than average, the machine will want to "change" your due date.  That doesn't change the age of your baby, though.  That would be like a doctor measuring a 4-year-old, finding her a little shorter than average, and deciding that from now on she is only 3.  Does this make sense?

At any rate, it's also important to remember than normal gestation lasts anywhere from 37 to 42 weeks.  That's a five-week timespan when a normal baby can decide to be born.  Due dates are only an estimate, based on an average.