How Due Dates Are Calculated
The estimated due date (EDD) marks 40 weeks of gestation. Three routes reach it, depending on what you know:
| Starting point | Formula |
|---|---|
| Last menstrual period | LMP + 280 days (Naegele's rule), cycle-adjusted |
| Known conception date | Conception + 266 days |
| IVF transfer | Day-3 embryo: transfer + 263 days · Day-5: transfer + 261 days |
IVF dating is the most precise of the three — the embryo's age is known to the day. LMP dating assumes ovulation at mid-cycle, which the cycle-length adjustment refines.
Worked Examples
From LMP
LMP September 4, 28-day cycle → EDD = September 4 + 280 days = June 11. With a 33-day cycle, add 5 more days → June 16.
From conception
Known conception September 18 → EDD = September 18 + 266 days = June 11 (consistent with the LMP math — conception ≈ LMP + 14).
From IVF
Day-5 blastocyst transferred September 23 → EDD = September 23 + 261 days = June 11. All three routes converge; they're one clock read from different points.
What "Estimated" Really Means
- Roughly 1 in 20 babies arrives on the due date itself.
- About 70% arrive within ±10 days.
- First-trimester ultrasound (±5–7 days) outranks calendar math and re-dates the pregnancy when they disagree.
- Delivery between 37–42 weeks is within normal range; 39–40 is "full term."
Read your EDD as the center of a fortnight-wide window and plan leave, travel, and logistics accordingly.
After the Date: What It Anchors
The EDD isn't just a countdown — it schedules the entire prenatal calendar:
| Milestone | Timing |
|---|---|
| Dating/viability scan | ~7–9 weeks |
| NT screening | 11–13 weeks |
| Anatomy scan | 18–22 weeks |
| Glucose screening | 24–28 weeks |
| GBS swab | 36–37 weeks |
| Post-term monitoring | from 41 weeks |
For the live week-by-week view of where you are today, continue to the Pregnancy Calculator; to estimate the fertile window behind a date, the Conception Calculator.
The Bottom Line
One remembered date — period, conception, or transfer — yields your due date in seconds, and the three formulas agree because they're the same 40-week clock. Hold the result loosely (it's a window, not a booking), confirm with first-trimester ultrasound, and let your care team translate the estimate into decisions.
Informational only — not a substitute for prenatal care.
Naegele's Rule: The 200-Year-Old Formula Behind Your Due Date
Almost every due date starts with Naegele's rule, named after the German obstetrician who popularised it in the early 1800s. Take the first day of your last menstrual period (LMP), add one year, subtract three months, then add seven days. An LMP of 10 January gives: +1 year → 10 January next year, −3 months → 10 October, +7 days → 17 October. That is the same estimate as adding 280 days (40 weeks) to the LMP, which is what the calculator above does.
The rule rests on two assumptions: a textbook 28-day cycle with ovulation on day 14. Real cycles vary, which is why a due date is best treated as the centre of a window, not an appointment — only about 4–5% of babies arrive on the exact day, while the large majority are born within two weeks either side of it. Our article on understanding your due date walks through the full timeline of what the date means trimester by trimester.
When Due Dates Move: Ultrasounds and Irregular Cycles
If your cycle is longer or shorter than 28 days, the LMP-based date shifts by the difference: ovulation in a 35-day cycle happens around day 21, not day 14, so the true due date sits about a week later than Naegele's rule suggests. A quick correction is to add (your cycle length − 28) days to the calculated date — and if your cycle is irregular, treat any LMP estimate as provisional until a scan weighs in.
That scan is the tiebreaker. A first-trimester ultrasound (ideally 8–13 weeks) measures the embryo's crown–rump length and dates a pregnancy to within about 5–7 days, which is more reliable than period maths. Standard obstetric practice (including ACOG guidance) is to move the official due date when the early ultrasound disagrees with the LMP date by more than about a week; after the first trimester, babies grow at increasingly individual rates, so later scans are not used to re-date. Once your date is set, counting forward to milestones — the anatomy scan around week 20, full term at 39–40 weeks — is where a tool like our age calculator becomes useful in reverse after the birth, for tracking your baby's exact age in weeks and days.