Checked 15 October 2026. What was checked. How these pages are maintained.
How due dates are estimated
Naegele’s rule: due date ≈ first day of the last menstrual period + 280 days (40 weeks). From conception, many charts add 266 days. Real pregnancies vary by one or two weeks. Ultrasound dating can replace this estimate.
Worked example
Naegele’s rule: last period 1 January 2026, due date about 8 October 2026 (add 1 year, subtract 3 months, add 7 days). A conception date is typically about 2 weeks later than LMP on a 28-day cycle.
Cycles are not all 28 days. Ultrasound dating can disagree with LMP. This is an estimate for planning, not medical advice. Confirm with a clinician.
Two hundred and eighty days from the last period
Naegele’s rule, as this page applies it, adds 280 days to the first day of the last menstrual period. From 15 January 2026 that lands on 22 October 2026. The shortcut people memorise is the same date: add one year, subtract three months, add seven days. 15 January plus seven days is 22 January, minus three months is 22 October, and the year stays 2026. From a conception date the page adds 266 days instead, about two weeks less, because gestational age on a chart is counted from the last period, not from conception. A 28-day cycle is the assumption under both numbers.
The page also shows a rough gestational age if the start date is already in the past: whole weeks and leftover days from that start until today. That line uses the computer’s date. It is not a clinical measurement.
When the chart and the ultrasound disagree
Cycles that are not 28 days shift the relationship between the last period and conception. An ultrasound dating scan often replaces the LMP estimate, and the clinician’s date is the one appointments follow. One or two weeks of variation is ordinary. This page is a planning sketch for a calendar. It does not diagnose a pregnancy, and it does not adjust for IVF or for a known long cycle. Bring the result to a prenatal visit rather than treating 22 October as a booking you cannot move.
Common questions
Why 40 weeks if conception is at week 2?
Gestational age counts from LMP, not from conception. That is why 40 weeks is the usual chart.
Irregular cycles?
This page assumes a 28-day cycle. Ask a clinician if your cycle is much longer or shorter.