" A baby is considered 'term,' or mature, if it is born between 38 and 42 weeks gestation. The due date is estimated as 280 days -40 weeks- from the first day of the woman's last menstrual period and also by ultrasound. Forty weeks is the median of the normal range, explains Philip Hall, MD, professor and director of maternal-fetal medicine at St. Boniface General Hospital in Winnipeg, Manitoba. 'People seem to have thought for a long time that 40 weeks is some magic number, he says. Rather, it is just a midpoint...'Two standard deviations is within the normal range...and that's 13 days on either side.' Thus, a due date would be expressed more accurately as a 'due month.' "
Evidence Based Birth: The Evidence on Due Dates