Calculating a Due Date: Conception Date & Accuracy

Question
IF BABY IS DUE OCTOBER 29 2008 WHEN DID THE MOTHER HAVE SEX

Answer
A due date of October 29, 2008 works back to a conception date of February 5, 2008.  However, whether or not this date is correct depends very strongly on how that due date was determined.  If the due date was determined from a very early (<10 weeks) ultrasound, then that conception date should be correct within 3-5 days.  But if the due date was determined using only the date of the last period, or if it was determined using a later ultrasound, then that conception date could be off by 2-3 weeks or even more.