The due date (estimated date of delivery, EDD) is most often worked out with Naegele's rule: add 280 days — 40 weeks — to the first day of your last menstrual period (LMP). Pregnancy is dated from the LMP rather than conception because that date is usually known, and conception happens about two weeks later at ovulation. If your cycle isn't 28 days, the estimate shifts by the difference. It is genuinely an estimate: most babies arrive within a couple of weeks either side of the date.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Naegele's rule (LMP + 280 days). Not medical advice.
The due date rules
Adding 280 days to the LMP is the same as the classic shortcut "subtract 3 months and add 7 days" (then advance a year). The cycle adjustment matters because ovulation — and therefore conception — happens later in a long cycle and earlier in a short one, moving the due date by the same number of days. Gestational age is always measured from the LMP, so at conception you are already considered "2 weeks pregnant", and the trimesters are defined by those gestational weeks.
Worked example — a January LMP
Scenario: last period began January 1, 2026, with a regular 28-day cycle.
The estimated due date is October 8, 2026, with conception around mid-January. Other start dates follow the same +280-day rule: an LMP of March 15, 2026 points to December 20, 2026, and June 1, 2026 to March 8, 2027. Remember this is a central estimate — a full-term birth is anything from 37 to 42 weeks, and only about 4% of babies actually arrive on the predicted day. A first-trimester ultrasound usually gives a more precise date, especially for irregular cycles.
Frequently Asked Questions
Naegele's rule: LMP + 280 days. Jan 1, 2026 → Oct 8, 2026. Adjusted for cycle length.
The LMP is known; conception (≈ 2 weeks later) usually isn't. So 40 weeks ≈ 38 weeks of development.
First: to week 12. Second: weeks 13–27. Third: week 28 to birth.
An estimate — ~4% deliver on the date; full term is 37–42 weeks. Ultrasound is more precise.
Use an early dating ultrasound; treat the LMP estimate as a rough guide and confirm with your provider.