After you know you are pregnant, the next thing you most want to know is the date around which you will be holding your baby in your arms. Technically, only 5% of the babies are born on the exact due date, which falls after 280 days or 40 weeks from the first day of your last menstrual cycle. However, around 80 % of the babies are born in the window of ten days around the due date.
Since it is very unlikely to know when exactly you conceived, a due date just gives an idea of the time when your baby is likely to see the light of the world. If you are able to figure out the ovulation date, which will generally be after about two weeks from the first day of the last period in a 28 days cycle, the pregnancy period is about 38 weeks. A normal pregnancy would last about 38-40 weeks, but one could also have a pre-term labor which would be less than 38 weeks, or even post term to about 42 weeks. A pregnancy lasting 38-40 weeks is also known as a full term pregnancy.
Please note that this is just an estimation. Your doctor might change it during your scan, if it is found out that the baby is too small or too big for the number of weeks calculated. Also, this estimation may not hold true in certain cases of twins, untreated thyroid, smoking, severe infection during pregnancy, or some other medical condition.