The formula — and why it starts before conception
Naegele's rule estimates a due date as the last menstrual period (LMP) date plus 280 days (40 weeks). This intentionally starts counting from the LMP rather than from conception, because conception's exact date usually isn't known with certainty, while the LMP date is something most people can identify. Since ovulation and conception typically happen around 2 weeks after the LMP, the "40 weeks" of a full-term pregnancy actually includes roughly 2 weeks before conception occurred at all.
A worked example
For an LMP of 1 January 2026, the estimated due date is 8 October 2026 (280 days later). Checked partway through, on 13 September 2026, this pregnancy would be at week 36, day 3 of gestation, in the third trimester, with 25 days remaining until the estimated due date.
How the trimester boundaries are set
The first trimester covers weeks 0-12, the second covers weeks 13-26, and the third covers week 27 through the estimated due date around week 40. These boundaries are conventional groupings used for tracking developmental stages and screening schedules, not sharp biological cutoffs — a pregnancy doesn't fundamentally change on the exact day it crosses from week 12 to week 13.
Why the actual due date is an estimate, not a guarantee
Naegele's rule assumes a standard 28-day menstrual cycle with ovulation on day 14 — real cycles vary in length, and ovulation timing varies with them, so the calculated due date is a statistical estimate rather than a guaranteed delivery date. Only a small percentage of babies are actually born exactly on their estimated due date; healthcare providers use it as a planning reference point and typically refine the estimate further using an early ultrasound, which can measure fetal size directly rather than relying solely on LMP-based counting.
Why this matters for anyone using an online due-date calculator
Any due-date calculator using this method inherits its underlying assumption of a standard cycle — if your cycle is meaningfully longer or shorter than 28 days, or if you aren't certain of your LMP date, the estimate carries more uncertainty than the single date it presents might suggest. This is exactly why the result is informational, and why confirming timing with a healthcare provider (ideally backed by an ultrasound) matters more than treating the calculated date as precise.