How are the weeks of gestation mentioned above calculated?
The length of pregnancy is typically about 40weeks counting from the first day of the last menstrual period (LMP). Gestational age as determined by ultrasound measurements is based on this same system. So, if by LMP your pregnancy is 10 weeks, most likely the fetus will measure 10 weeks or so by ultrasound. Since not everyone has 28 day cycles and conception can occur earlier or later than average within a given cycle. Ultrasound in the first trimester is usually the most accurate way to determine gestational age.
