The duration of a pregnancy is measured by gestational age.
Gestational age is basically a measure of the length of time that a baby spends in the womb.
It is generally calculated as the time from the first day of the last menstrual period to the day of birth. Conception occurs on about day 14 after the first day of the last menstrual period. However, this date is not usually known. The first day of the last menstrual period usually is known. Therefore the beginning date for the pregnancy is the first day of the last menstrual period.