An estimated due date is calculated from the first day of the last menstrual period using Naegele’s rule, which assumes a 28-day cycle. This calculator adjusts for a different cycle length and also shows current gestational age and the dates each trimester begins.
Only about 4% of babies arrive on the exact date. Roughly 90% are born within two weeks either side, which is why clinicians treat it as the centre of a window rather than a deadline.
Because the date of the last period is usually known and the date of conception rarely is. It means the first two weeks of "pregnancy" precede conception, which is a known quirk of the convention.
Yes. Ovulation timing shifts with cycle length, so a 35-day cycle moves the due date about a week later than the standard 28-day assumption.
Generally yes, especially a first-trimester scan, and clinicians will revise the due date if the scan differs materially from the period-based estimate.
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