MODEL RELEASED. Gestation calculator. Health care worker (at right) shows a gestation calculator to a pregnant woman. 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. Turning the dial on this calculator to the date of the last period results in a reading for the expected date of delivery.