How this calculator works
Cycle-adjusted Naegele dating adds 280 days to the first day of the last menstrual period, then adjusts for a cycle longer or shorter than 28 days. This is for pregnancies with regular cycles and a known LMP. Gestational age counts from LMP, including roughly two weeks before conception. First-trimester ultrasound dating takes precedence when clinically indicated; IVF transfer dating needs a different method. The assessment date is explicit so the displayed gestational age is reproducible. The result is an estimate for information only and is not medical advice.
Formula
EDD = LMP + 280 + (cycle length - 28) calendar days. Gestational age = assessment date - LMP, expressed as weeks and days.
Worked example
For LMP 2026-01-08 and a 28-day cycle, the estimated due date is 2026-10-15. On that date, LMP gestational age is 40 weeks 0 days.
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Frequently asked questions
Is the due date a prediction of the birth day?
No. It is a dating estimate, and births occur over a range of dates.
Why count before conception?
Clinical gestational dating starts at the last menstrual period rather than the uncertain day of fertilisation.
What if I know the conception date instead?
Add 266 days to the conception date for the 38-week convention. This page dates from the last menstrual period and has no conception input.