Naegele's rule, step by step
The rule is named for the 19th-century obstetrician who formalized it, and it is still the starting point for every due date: due date = first day of last menstrual period + 280 days. That is 40 weeks, the standard length of a pregnancy as dated from the last period.
There is a classic shortcut for doing it without counting 280 days: take the first day of the last period, add 7 days, subtract 3 months, add 1 year. Both versions give the same answer; the 280-day addition is easier to verify, so this guide uses it throughout.
Worked example: 28-day cycle
Last period: January 15, 2026. Add 280 days. January has 31 days, so January 15 plus 16 days reaches January 31, leaving 264 days. February 2026 has 28 days (2026 is not a leap year), leaving 236. March 31, leaving 205. April 30, leaving 175. May 31, leaving 144. June 30, leaving 114. July 31, leaving 83. August 31, leaving 52. September 30, leaving 22. Twenty-two days into October is October 22, 2026.
The shortcut confirms it: January 15 plus 7 days is January 22, minus 3 months is October 22, plus 1 year is October 22, 2026. Same answer both ways.
The cycle-length adjustment
The 280-day rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer or shorter, ovulation shifts, and the due date shifts with it. The adjustment: add (your cycle length minus 28) days to the result.
Worked example: last period January 15, 2026, 32-day cycle. Base date from the rule: October 22, 2026. Adjustment: 32 minus 28 is 4 days. Due date: October 26, 2026.
Another: 24-day cycle, last period January 15, 2026. Adjustment: 24 minus 28 is negative 4 days. Due date: October 18, 2026. The logic: shorter cycles mean earlier ovulation, so the pregnancy is effectively further along relative to the last period.
The conception form of the rule
Equivalently, the due date is about 266 days (38 weeks) after conception. If you know the conception date, perhaps from fertility tracking, add 266 days directly and skip the cycle adjustment, because conception already pins down ovulation.
Example: conception on January 29, 2026. Add 266 days: January 29 plus 2 days reaches January 31, leaving 264; then the same month walk as above lands on October 22, 2026. Notice this matches the 28-day-cycle result from the last period, which makes sense: January 29 is 14 days after January 15.
What to do with irregular periods
When cycle length varies, the period-based formula is shaky because the ovulation day is unknown. Three workarounds, in order of reliability:
1. Use the earliest reliable anchor. If you tracked ovulation with test strips or temperature, date from that: due date = ovulation date + 266 days. A known ovulation date beats any cycle-length guess.
2. Use the average, then verify by ultrasound. Take your average cycle length over the last 6 months, run the formula with the adjustment, and treat the result as provisional until the first scan. Tell your provider the cycles are irregular so the scan gets proper weight.
3. Skip the calendar entirely. If you truly do not know the last period date, the first-trimester ultrasound becomes the primary dating method. A crown-rump length measurement in weeks 8 to 12 dates a pregnancy within about a week, which is better than a guessed last period.
How accurate is the result
The formula is precise; the biology is not. Only about 1 in 20 babies arrives on the exact due date. About 8 in 10 arrive within the two weeks around it, from 38 to 42 weeks. The due date is best understood as the midpoint of that window, not an appointment.
What the date is genuinely good for is scheduling: screening windows, scan timing, and term definitions (early term at 37 weeks, full term at 39, late term at 41, post-term at 42) all hang off this number. Your provider may revise it once after the first ultrasound, and after that the revised date is the one everything follows. The pregnancy calculator runs Naegele's rule with the cycle adjustment, plus the conception, IVF, and known-due-date modes.