Pregnancy Due Date Calculation: Understanding First Day of Last Period

Question
Hi, curious, I finished my period on July 25th, but didnt have sex until July 30th. Do you still calculate from the 1st day of your period?

Answer
Due dates are always calculated using the first day of the last menstrual period, unless the exact date of ovulation is known or an early ultrasound is used to confirm the dating.  Normal gestation averages 38 weeks from the date of ovulation.  Since the average woman ovulates 14 days after her last period, this results in the typical due date 40 weeks after LMP.