Calc FYI

Naegele's Rule

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

Variables

Symbol Name Unit Description
$EDD$ Estimated Due Date date The estimated date of delivery.
$LMP$ Last Menstrual Period date The first day of the last menstrual period.

What Is Naegele's Rule?

Naegele's Rule is the standard obstetric formula for estimating a pregnancy's due date (Estimated Delivery Date, EDD):

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

Equivalently, the rule is often applied as:

Add 1 year, subtract 3 months, add 7 days to LMP

These two methods produce the same result when the LMP occurs in a month with 28–31 days.

Why 280 Days?

280 days (40 weeks) is the average duration of human pregnancy measured from the last menstrual period, not from conception. Since ovulation and fertilisation typically occur ~14 days after LMP, the actual embryonic age at birth is ~266 days (38 weeks), but obstetric gestational age starts from LMP for practical reasons (the LMP date is usually known; conception date often is not).

The 40-week count encompasses: - 2 weeks pre-conception (follicular phase) - 38 weeks of embryonic/fetal development

Clinical Utility and Limitations

Naegele's Rule is accurate to within ±2 weeks for about 70% of pregnancies. It assumes: 1. A regular 28-day menstrual cycle 2. Ovulation on day 14 3. A single fetus

For women with irregular cycles, PCOS, or uncertain LMP dates, ultrasound dating (especially first-trimester crown-rump length measurement before 14 weeks) is more accurate and increasingly preferred as the primary dating method.

Full-Term Range

Clinicians define full-term pregnancy as 39–40 weeks (6/7), and "early term" as 37–38 6/7 weeks. Deliveries before 37 weeks are preterm; after 42 weeks are post-term.

Derivation & History

Franz Karl Naegele (1778–1851), a German obstetrician, published the rule in 1812 based on the Hippocratic observation that pregnancy lasts approximately 10 lunar months (10 × 28 days = 280 days). He proposed counting 280 days forward from the first day of the last menstrual period.

The rule was adopted globally because of its simplicity and reasonable accuracy before ultrasound technology existed. Hermann Boerhaave (1668–1738) and other earlier physicians had noted similar gestational durations, but Naegele standardised the LMP-based calculation that remains in use today.

Modern studies confirm that the median human gestation is 280–283 days from LMP, validating the historical estimate within a margin of a few days.

Worked Examples

LMP on 1 January

  1. Method 1: Add 280 days to 1 January 2025
  2. January has 31 days, so 280 days later = 8 October 2025
  3. Method 2: +1 year → 1 Jan 2026; −3 months → 1 Oct 2025; +7 days → 8 Oct 2025

Result: EDD = 8 October 2025

LMP on 15 March

  1. +1 year → 15 March 2026; −3 months → 15 December 2025; +7 days → 22 December 2025

Result: EDD = 22 December 2025

Edge Cases & Limitations

Irregular cycles longer than 28 days: EDD should be adjusted later (e.g., 35-day cycle → EDD + 7 days). Ultrasound is preferred.

IVF pregnancies: Embryo transfer date is known precisely; gestational age is calculated from a standardised offset from transfer date, not LMP.

Twin/multiple pregnancies: Mean gestational age at delivery is shorter (~37 weeks for twins), so the standard EDD is used for monitoring but delivery planning differs.

Uncertain LMP: Approximately 30% of women have uncertain LMP dates; first-trimester ultrasound crown-rump length measurement is the gold standard in such cases.

Real-World Applications

Naegele's Rule is the default EDD calculation in every obstetric information system worldwide. Midwives use it for birth planning appointments. Government maternity leave entitlements (e.g., in Korea, EU countries) are calculated from the EDD. Prenatal screening schedules (NT scan at 11–14 weeks, anomaly scan at 18–22 weeks) are timed relative to the gestational age established by this rule.

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Due Date Calculator