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Frequently Asked Questions

What is Naegele's rule?

Naegele's rule is the most standard method for calculating a due date, established by the 19th-century German obstetrician Franz Karl Naegele. It is used by obstetricians worldwide.

Calculation

Reference point: the first day of the last menstrual period (LMP)

Due date = LMP + 280 days = LMP + 40 weeks

On the calendar: take the LMP, subtract 3 months and add 7 days.

Why 280 days?

It comes from the statistical observation that an average pregnancy lasts about 40 weeks (280 days). The 280 days assume:

  • A menstrual cycle of 28 days (the average)
  • Ovulation occurring 14 days after the start of the period
  • Fertilization happening shortly after ovulation
  • Actual pregnancy duration of about 266 days from conception

Counting from the LMP is practical because mothers typically remember the start of their last period but not the exact day of ovulation or conception.

How accurate is it?

Only about 5% of mothers give birth exactly on the EDD. Statistically:

  • Within ±1 week of EDD: about 65%
  • Within ±2 weeks of EDD: about 90%
  • Normal delivery range: 37–42 weeks of pregnancy

So Naegele's rule is not an exact date but an estimate of "around when labor is likely".

What are the limitations?

The estimate is less accurate when:

  • The menstrual cycle is not 28 days (longer cycles shift the EDD later)
  • The LMP is not remembered precisely
  • Pregnancy was achieved through IVF or other assisted reproduction — the embryo transfer date is more accurate
  • Variables such as maternal age, ethnicity, and number of previous pregnancies

How is it different at a clinic?

Obstetricians combine Naegele's rule with ultrasound measurements. In early pregnancy (8–13 weeks), the fetus's crown-rump length (CRL) is strongly correlated with gestational age, which allows for a more accurate EDD.

Therefore, if your doctor's EDD differs from this calculator, trust your doctor's date.

About this calculator

Our due date calculator uses the pure Naegele's rule (LMP + 280 days, 28-day cycle assumed). Cycle-length input will be added in a future update.