A worked example: LMP of January 1
A last menstrual period (LMP) starting January 1, 2026 gives an estimated due date of October 8, 2026 — 280 days later. On that same January 1 starting date, the pregnancy is at week 0, day 0, in trimester 1, with 280 days remaining.
Naegele's rule: due date = LMP + 280 days
280 days is 40 weeks, the standard estimate for a full-term pregnancy counted from the first day of the last period (not from conception, which typically happens roughly two weeks later). This single addition is the entire due-date calculation.
Current week and trimester: a running count from the same start date
Checking in on May 1, 2026 against that same January 1 LMP shows 17 weeks and 1 day along, now in trimester 2, with 160 days remaining until the same October 8 due date — the due date itself never moves; only the "how far along" figures update as the "as of" date changes. Trimesters split at week 13 (end of first) and week 27 (end of second), a common clinical convention.
Why the same LMP always gives the same due date
Because Naegele's rule is a fixed 280-day addition to one starting date, checking the calculator on any day of the pregnancy against the same original LMP always returns the identical due date — only the current-week/trimester/days-remaining figures change as time passes, which is a useful sanity check that the LMP entered hasn't been mistyped.
Why an ultrasound date can override this estimate
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14 — an assumption that doesn't hold for everyone. An early ultrasound measures the fetus directly and is generally considered more accurate when it disagrees with the LMP-based estimate, especially for irregular cycles.