Understanding Your Pregnancy Due Date: Calculating and Sonogram Accuracy

Question
Hi,
I am a 35 year old woman who is pregnant.  My last period was on May 4, 2005.  According to my last period, my due date should be February 8th.  In March my doctor performed a vaginal sonogram and say it looks like my due date may be February 22nd.  How could this be?  How do I know which one is correct?

Answer
If you have a normal 28 day cycle, and your last period started on May 4, then you are due on February 11, 2006.  What was the date of the sonogram? You said that you had a sonogram in March. You were not pregnant until May. Depending on when the sonogram was done, there is an error of + or - one week or two weeks, which could explain the error. I would rely on your due date based on your last period.