Gestational Age by LMP

Estimate gestational age and due date from last menstrual period (LMP) using the Naegele rule (EDD = LMP + 280 days).

For educational and clinical reference. Not a substitute for medical judgment.
Gestational Age
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Estimated Due Date
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References

  1. ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
  2. Naegele FK. Lehrbuch der Geburtshilfe. 1830.

How to use

  1. Enter the date of the first day of the last menstrual period (LMP).
  2. Reference date defaults to today (change if calculating for a past visit).
  3. Gestational age and estimated due date appear instantly.

Frequently asked questions

What is the Naegele rule?

EDD = LMP + 280 days (40 weeks). It assumes a regular 28-day cycle and ovulation on day 14.

How accurate is dating by LMP?

LMP dating assumes a 28-day cycle with ovulation on day 14. Ultrasound dating (especially in the first trimester) is more accurate when LMP is uncertain.

Why is gestational age in weeks and days?

Pregnancy is conventionally counted in completed weeks plus extra days (e.g., 12w4d means 12 completed weeks and 4 additional days).

When does the first trimester end?

The first trimester ends at the completion of week 13 (13w6d). Second trimester: 14w0d–27w6d. Third trimester: 28w0d–delivery.

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