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Pregnancy Due Date (EDD) Calculator

NaegelePregnancy Due Date & Gestational Age

Menstrual History

Last Menstrual Period (LMP)Required

Select the first day of your last normal period.

Cycle LengthDays

Default is 28 days. Adjust for irregular cycles (between 20–45 days).

Naegele's Rule assumes ovulation on Day 14. Cycle correction shifts EDD by +0 day(s) relative to standard 28-day baseline.

Pregnancy Due Date

Select your Last Menstrual Period date. The calculator will automatically display your estimated due date and current gestational age.

Guidelines & Evidence

Verified

Last Review: 2026

When to Use

What Is Gestational Dating?

Gestational dating establishes the estimated due date (EDD) and current gestational age (GA) — both essential for safe pregnancy management. The LMP-based method (Naegele's Rule) counts 280 days (40 weeks) from the first day of the last menstrual period, assuming a 28-day cycle with ovulation on Day 14. Cycle-length correction shifts this by (cycle − 28) days. Where possible, first-trimester ultrasound (crown-rump length, CRL) provides superior accuracy and supersedes the LMP date when there is a discrepancy beyond guideline thresholds.

Primary Clinical Indications

Establishing the EDD at the first prenatal visit — required for all subsequent gestational-age-dependent decisions.

Timing prenatal screening windows — nuchal translucency (11–13⁺⁶ weeks), anatomy scan (18–22 weeks), GDM screen (24–28 weeks), GBS swab (36–37 weeks).

Defining prematurity and post-term thresholds — preterm <37 weeks; post-term ≥42 weeks.

Guiding decisions about induction of labour (IOL) — elective IOL requires confirmed ≥39 weeks; post-dates IOL at 41 weeks (ACOG) or 41⁺⁰–41⁺⁶ (NICE).

Assessing fetal growth — small-for-gestational-age (SGA) and large-for-gestational-age (LGA) diagnoses depend on an accurate EDD.

ART pregnancies — embryo transfer date allows precise calculation; LMP-equivalent is derived from transfer date minus embryo age minus 14 days.

Reconciling LMP vs ultrasound discrepancies — ACOG re-dating thresholds define when ultrasound should replace LMP as the dating standard.

Limitations of LMP-Based Dating

Irregular cycles — women with oligomenorrhoea (e.g., PCOS) or recent OCP use may have unpredictable ovulation, making LMP unreliable.

Recall bias — up to 40% of women are uncertain of their LMP date; this is the leading source of dating error.

First-trimester bleeding — implantation bleeding or a threatened miscarriage may be confused with a period, causing the LMP to be recorded incorrectly.

Cycle-length variability — the standard formula assumes 28-day cycles; cycles outside 20–45 days require correction.

Late presentation — women presenting after 22 weeks for the first time have significantly less accurate LMP-based dating (±3–4 weeks).

ACOG Re-Dating Thresholds (Ultrasound vs LMP)

Gestational Age at US	Biometric Parameter	Re-date if Discrepancy Exceeds	Notes
≤ 8⁺⁶ weeks	Crown-Rump Length (CRL)	5 days	CRL most accurate dating parameter; re-dating nearly always justified if available
9⁰–15⁺⁶ weeks	CRL (or mean sac diameter if no heartbeat)	7 days	CRL still preferred; BPD emerging
16⁰–21⁺⁶ weeks	BPD, HC, AC, FL composite	10 days	Composite biometry; anatomy scan window
22⁰–27⁺⁶ weeks	Composite biometry	14 days	Dating less precise; only re-date if LMP very uncertain
≥ 28⁰ weeks	Composite biometry	21 days	Third-trimester US unreliable for dating; rely on established EDD

Related Scores in Practice

In clinical practice, this assessment is frequently evaluated alongside other validated measures. Depending on the patient's presentation and specific diagnostic requirements, you may also need to utilize the Ivf Dating, Fetal Growth Percentile, Biophysical Profile or the Bishop Score to formulate a comprehensive care plan.

Last Comprehensive Review: 2026