Due Date Calculation and Pregnancy Weeks
Learn how estimated due dates use the last menstrual period or conception date, why pregnancy weeks start earlier and when scans revise dating.
An estimated due date gives patients and clinicians one consistent reference for pregnancy dating. This guide explains the two common calendar inputs, why gestational weeks begin before conception, and how early ultrasound is used to confirm or revise the estimate.
Naegele's rule: the classic formula
The standard method is named after Franz Naegele, the 19th-century obstetrician who popularized it. The rule: take the first day of the last menstrual period (LMP), add 7 days, subtract 3 months — which is arithmetically identical to LMP + 280 days (40 weeks). Worked example:
| Step | Date |
|---|---|
| First day of last period | January 5, 2026 |
| + 7 days | January 12, 2026 |
| − 3 months | October 12, 2026 ← due date |
If your cycle isn't the textbook 28 days, the estimate shifts: providers typically add the difference. A 30-day cycle moves the due date two days later; a 25-day cycle, three days earlier. Our Pregnancy Calculator applies this adjustment automatically.
From conception instead of LMP
When the conception date is known (fertility treatments, for instance), pregnancy is dated at 266 days (38 weeks) from conception rather than 280 from LMP. The two methods agree by design: LMP dating bakes in roughly two weeks of "pre-conception" time. That's also the famous week-numbering quirk — week 1 and week 2 of pregnancy technically occur before conception, because the clock starts at the last period. Every expectant parent discovers this and does a double take.
How ultrasound changes the picture
Calendar formulas are useful starting points, but they depend on remembered dates and assumptions about cycle timing. ACOG states that ultrasound measurement during the first trimester, through 13 weeks and 6 days, is the most accurate method for establishing or confirming gestational age.
If menstrual dating and ultrasound dating differ, a clinician applies established criteria and the available clinical history. Once an estimated due date has been selected, later changes should be uncommon and documented. A calculator cannot make that decision because it has no ultrasound measurements or medical context.
What the pregnancy week number means
Gestational age is counted from the first day of the last menstrual period. That convention means the first roughly two weeks usually precede conception. A result such as 12 weeks, 3 days means 12 completed weeks plus 3 additional days on that clinical calendar; it is not an estimate of time since conception.
Worked comparison: LMP and conception dates
| Known starting point | Standard calendar offset | What it provides |
|---|---|---|
| First day of LMP | 280 days | An initial estimated due date |
| Known conception date | 266 days | An estimate using the conception date |
| First-trimester ultrasound | Clinical measurement | Confirmation or redating by a clinician |
The two calendar offsets differ by 14 days because LMP dating starts before conception. If your cycle is longer or shorter than 28 days, a simple calculator may offer a cycle-length adjustment, but that adjustment remains an estimate.
How to use a due date responsibly
- Enter the most reliable date you know and check that it was entered correctly.
- Save the result as a planning estimate for appointments, leave and general preparation.
- Take the result to your prenatal clinician, particularly if cycles are irregular or the date is uncertain.
- Use the clinician-documented due date for medical decisions and future comparisons.
For a quick calculation, use the Pregnancy Calculator. It shows the calculation method and current gestational week, while keeping the result clearly separate from clinical advice.
Key takeaways
- LMP dating commonly uses 280 days; a known conception date commonly uses 266 days.
- Gestational weeks begin with the LMP, so the count typically starts before conception.
- First-trimester ultrasound is the most accurate clinical method for establishing or confirming gestational age.
- A due date is a reference estimate. Your healthcare professional interprets it using clinical information.
Sources & scope
Pregnancy dating is clinical information. The calculator is an orientation tool and never replaces an ultrasound or prenatal professional.
- American College of Obstetricians and Gynecologists: estimating the due date
- NHS: pregnancy due-date guidance
Sources accessed: August 8, 2026 · Links are provided for verification and further reading.