How the Estimated Due Date is calculated
The classic method (Naegele's Rule) assumes a 28-day cycle and adds 280 days (40 weeks) to the first day of the last menstrual period (LMP).
Normal values / gestation reference
| Gestational age | Trimester |
|---|---|
| 0 – 13⁺⁶ weeks | 1st trimester |
| 14 – 27⁺⁶ weeks | 2nd trimester |
| 28 – 40 weeks | 3rd trimester |
| 37 – 41⁺⁶ weeks | Term |
Clinical use
Accurate dating underpins the timing of every antenatal milestone — screening windows, growth assessment, and decisions around induction or post-term management. LMP-based dating assumes a regular 28-day cycle with ovulation on day 14; it is inaccurate with irregular cycles, recent hormonal contraception, or uncertain LMP. Where available, a first-trimester crown-rump length ultrasound (ideally 8–13⁺⁶ weeks) is more accurate than LMP dating and should take precedence when the two disagree by more than the accepted discrepancy threshold for that gestational age.
Frequently asked questions
How accurate is a due date calculated from LMP?
Naegele's Rule (LMP + 280 days) is a reasonable population-level estimate but assumes a regular 28-day cycle with ovulation on day 14 — it can be off by a week or more with irregular cycles, so a first-trimester ultrasound is more accurate when available.
What if my menstrual cycle isn't 28 days?
LMP-based dating becomes less reliable the more a cycle deviates from 28 days. In this situation, a first-trimester ultrasound is the preferred, more accurate dating method — see the OB Wheel Calculator for cycle-length-adjusted dating.
Can the due date change during pregnancy?
The EDD is normally set once, at the most accurate dating opportunity available (ideally a first-trimester scan), and isn't routinely changed later just because growth measurements differ — later biometry reflects growth, not dating accuracy.
What's the difference between due date by LMP and by scan?
LMP-based dating counts forward from the reported last period; scan-based dating measures the fetus directly (via crown-rump length in the first trimester) and back-calculates an equivalent LMP. When they disagree significantly, ultrasound dating is generally preferred, especially in early pregnancy.
References
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150–e154.
- Committee on Obstetric Practice, ACOG & AIUM. Method for Estimating Due Date, reaffirmed 2023.