MODEL RELEASED. Gestation calculator. Pregnant woman points at a gestation calculator held by a midwife. This is a device for accurately determining the expected date of delivery for pregnant women. Gestation is the period from conception to birth and lasts about 270 days (9 months). However, because of the difficulty in determining the precise date of conception it is usually timed as 40 weeks from the first day of the last menstrual period.