how to calculate due date naegele rule

How to Calculate the Due Date Using Naegele's Rule

Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate delivery. This page explains the logic, assumptions and limitations of the calculation.

Medical notice: this calculation is educational, based on population-level averages, and does not replace prenatal care, ultrasound, diagnosis or guidance from a doctor or obstetrician.

Last menstrual period (LMP)First day

Estimated due date (EDD)

Sunday, January 17, 2027252 days until estimated due date

4 wks & 0days1st trimester

Gestational progress10%

DUM40 weeks

Pregnancy milestones

Wk 4β-hCG high enough for a positive test; primitive heart begins beating; implantation complete
Wk 5Neural tube closing; yolk sac visible; neural plate formed
Wk 6Heartbeat detectable by transvaginal ultrasound; limb buds visible; CRL ~4 mm
Wk 7Distinct brain divisions (forebrain, midbrain, hindbrain); hand plates visible
Wk 8All major organs forming; embryo is now called a fetus; lips and nose take shape
Wk 9External ears developing; eyelids covering eyes; neck visible; CRL ~22 mm
Wk 10Fingers and toes separated; external genitalia beginning to differentiate; fetus can swallow

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Why 280 days?

Naegele's Rule assumes ovulation on day 14 of a 28-day cycle. Since the egg takes about 266 days from fertilization to delivery (38 weeks), and fertilization occurs on day 14 from LMP, the total is 14 + 266 = 280 days.
Shorter or longer cycles shift the actual ovulation. That is why a first-trimester ultrasound — ideally between 8 and 12 weeks — is the best method to confirm or correct the individual due date.
ACOG recommends revising the EDD by ultrasound when the difference between the LMP calculation and fetal biometry exceeds 5–7 days.

Practical examples

28-day cycle

Input: DUM: 1º de janeiro · Ciclo: 28 dias
Expected output: DPP: 7 de outubro

Standard case — aligns perfectly with Naegele's Rule.

Long cycle (35 days)

Input: DUM: 1º de janeiro · Ciclo: 35 dias
Expected output: DPP estimada: 14 de outubro (7 dias depois)

Ovulation occurs on day 21, not day 14 — the real due date is shifted forward.

Full tool FAQ

What is Naegele's Rule and how does it work?

Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the actual time of ovulation, which is why ultrasound is more accurate for confirming or revising the individual EDD.

Is the calculated EDD an exact date or an estimate?

Can an ultrasound change the calculated due date?

What does each trimester of pregnancy mean?

What is gestational age and why does it start before conception?

What is post-term pregnancy and when is it a concern?

Are IVF pregnancies calculated differently?

Does this calculator replace prenatal care?

Frequently asked questions

Is the calculated EDD the actual delivery date?

No. Only about 5% of births happen on the exact EDD. Most occur within a two-week window before or after.

Can an ultrasound change the EDD?

Yes. If the difference between the LMP-based EDD and ultrasound measurements is relevant, ACOG recommends adjusting the EDD.