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). Assumes a regular 28-day cycle with ovulation on day 14.

How accurate is dating by LMP?

LMP dating assumes a 28-day cycle. First-trimester ultrasound dating is more accurate when LMP is uncertain or cycles are irregular.

Why is GA in weeks + days?

Pregnancy is conventionally counted as completed weeks plus extra days (e.g., 12w4d = 12 completed weeks + 4 days).

When does the first trimester end?

First: ends at 13w6d. Second: 14w0d–27w6d. Third: 28w0d to delivery.

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