Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman’s last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks. Infants born before 37 weeks are considered premature.
You must know::
1- the date of first day of last menstrual period (menstruation)
2- your cycle length (counted from the first day of one period to the first day of the next)
Also this online calculator will calculate Estimated date of delivery .