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estimated due date
The estimate adds 280 days to the first day of your last period, shifted by your cycle's difference from 28 days since ovulation moves with cycle length. The milestone rows convert the same start date into the appointments that matter: the 8–14 week dating-scan window, the 20-week anomaly scan, and the start of full term at 37 weeks.
Due dates are the centre of a wide distribution — about one baby in twenty arrives on the day, with most births falling within two weeks either side and first babies trending slightly late. A first-trimester ultrasound measures the embryo directly and replaces the calendar estimate when they differ by more than a few days; from that point, the scan date is the official one your maternity notes carry.