Expected date of confinement (EDC) or due date should be determined by your doctor.
The gestational weeks and days at 40W0D (280th day) is the EDC. Pregnancy lasts about 38W0D (266 days) since fertilization.
Then, the beginning of the last menstrual period is the basis of calculation as 0W0D.
But, the early stage of embryo size measured by ultrasound or the date of embryo transfer or the blastcyst transfer is more accurate.
The delivery is classified as abortion (-21W6D), preterm delivery (22W0D-36W6D), term delivery (37W0D-41W6D), postterm delivery(42W0D-).
NOTE; The gestational month counting system in Japan is different from others. Traditionally, the first 4 weeks (0W0D-3W6D) is counted as the 1st month in Japan, though zero month in other culture.
The Japanese counting system is used when you run in Japanese laguage system (Settings->General->International->Language). The western system is used when you run in English laguage system.
The EDC input is available from Jan 01, 1904 to Jan 01, 3000.
For EDC decision support tool for doctors, EDCpro, is for sale in iTunes Store.