EDD

Estimates the expected date of delivery from the last menstrual period.

Obstetrics & Gynecology ObstetricsEvidence-BasedUpdated 2026-07

Clinical Notes

Formula

EDD = LMP + 280 days

Normal Range

Typically around 40 weeks from LMP

Clinical Notes

The estimated date of delivery is a clinical estimate and may require adjustment based on ultrasound findings.

References

ACOG guidance
Obstetrics references

Calculator Information

Specialty

Obstetrics

📋 Reference Ranges

Normal Range

Typically around 40 weeks from LMP

What This Calculator Measures

Estimates the expected date of delivery (EDD) by adding 280 days to the first day of the last menstrual period (LMP), per the conventional Naegele calculation.

How to Use

Enter the first day of the last menstrual period in the date field.
The calculator adds 280 days and returns the estimated date of delivery.
Treat the result as an estimate: the calculation assumes a regular 28-day menstrual cycle with ovulation at day 14.
Ultrasound dating may supersede LMP dating when clinically appropriate.

Interpretation Guide

The calculator returns a single estimated date of delivery, approximately 40 weeks (280 days) after the LMP. The estimate assumes a regular 28-day cycle; longer or shorter cycles shift the true due date. The result is an estimate — first-trimester ultrasound dating is considered more reliable and may supersede LMP dating when the dates differ meaningfully.

Pregnancy-specific

Clinical Significance

Estimating the due date is one of the first steps in prenatal care, guiding the scheduling of screening, monitoring, and delivery planning; LMP-based estimation is the conventional starting point, with ultrasound used to confirm or correct the date.

When to Use

Estimating a due date from a reliable LMP when ultrasound dating is not available or not required
Educational review of the Naegele rule and pregnancy dating
Supporting discussions about timing of prenatal care milestones

When Not to Use

As the definitive date when ultrasound dating indicates a significantly different date
When the LMP is uncertain or the cycle length is unknown or irregular
To determine pregnancy viability or to time obstetric interventions without clinical assessment

Limitations

Assumes a regular 28-day menstrual cycle with ovulation approximately 14 days before the next menses.
The estimate is based on the date only; it does not incorporate cycle length, menstrual regularity, or assisted-reproduction dating.
Pregnancy dating is typically confirmed or corrected by early ultrasound; the calculator's output is an estimate.

Worked Example

A patient's first day of last menstrual period was January 1, 2026.

Inputs:

lmp: 2026-01-01

EDD = 2026-01-01 + 280 days = 2026-10-08 — reported as the estimated date of delivery.

Evidence

Source: ACOG Committee Opinion
Reference: ACOG Committee Opinion No. 700: Methods for estimating the due date. Obstet Gynecol. 2017;129(5):e150-e154.
Reviewed By: MedCalcHub Clinical Team
Version: 2017
Updated: 2026-08

ACOG Committee Opinion No. 700. Obstet Gynecol. 2017;129(5):e150-e154.

References

ACOG Committee Opinion No. 700: Methods for estimating the due date. Obstet Gynecol. 2017;129(5):e150-e154. (Clinical guideline)

Disclaimer: This calculator is intended for educational and clinical decision support. The expected date of delivery is an estimate based on the LMP and a 28-day cycle assumption; ultrasound dating should be used to confirm or correct the date when clinically indicated.

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