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Under the Gregorian calendar, century years not divisible by 400 would not be leap years.
Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7].
In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. American College of Obstetricians and Gynecologists.
Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period).
Despite all efforts, the Julian calendar still required further reform, since the calendar drifted with respect to the equinoxes and solstices by approximately 11 minutes per year.
By 1582, this resulted in a difference of 10 days from what was expected.