Data Source & Methodology

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The Formula Explained

LMP method (Naegele’s rule, 28-day cycle):

$$ \mathrm{EDD} = \mathrm{LMP} + 280 \ \text{days} $$

Adjusted for cycle length \(C\) (days):

Ovulation \(= \mathrm{LMP} + (C - 14)\) days; then $$ \mathrm{EDD} = \mathrm{Ovulation} + 266 \ \text{days}. $$

Conception known: $$ \mathrm{EDD} = \mathrm{Conception} + 266 \ \text{days}. $$

IVF transfers: 3-day: \( \mathrm{EDD} = \mathrm{Transfer} + 263\ \text{days} \) ; 5-day: \( \mathrm{EDD} = \mathrm{Transfer} + 261\ \text{days} \).

Ultrasound dating: Given scan date \(D_s\) and gestational age at scan \(GA_s\) (days),

$$ \mathrm{EDD} = D_s + (280 - GA_s)\ \text{days}. $$

Glossary of Variables

  • LMP: First day of the last menstrual period.
  • Cycle length (C): Days from first day of one period to the day before the next (default 28).
  • Conception/Transfer date: Date of fertilization or embryo transfer (IVF).
  • EDD: Estimated Due Date.
  • GA: Gestational Age in weeks+days since LMP-based start.

How It Works: A Step-by-Step Example

Scenario: LMP = 2025-02-01, cycle length = 30 days.

  1. Approximate ovulation = LMP + (30 − 14) = 2025-02-17.
  2. EDD = ovulation + 266 days = 2025-11-10.
  3. On 2025-10-30 (today), GA ≈ 37w4d, approaching early term (≥37w0d).

Frequently Asked Questions

Which method is most accurate?

First-trimester ultrasound is generally most precise. Known conception/IVF is also reliable. LMP assumes regular ovulation timing.

My cycles are irregular—what should I use?

Prefer ultrasound dating. If using LMP, understand the margin of error may be higher.

Why is the EDD a single day if birth rarely happens that day?

EDD is the center of a normal distribution; most births occur within ±2 weeks of the EDD.

Can the due date change after a later scan?

Professionals avoid changing EDD after reliable early ultrasound unless the discrepancy is large.

What are term categories?

Early term: 37w0d–38w6d; Full term: 39w0d–40w6d; Late term: 41w0d–41w6d; Postterm: ≥42w.

Is this medical advice?

No. This tool is educational and should not replace professional clinical judgment.

Tool developed by Ugo Candido. Content medically reviewed by the CalcDomain Editorial Board.
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