Due Date Calculation: Naegele's Rule and Why Only 5% Are On Time

A Date That Predicts Your Baby's Arrival with About 5% Accuracy

Only about 5% of babies are born on their calculated due date. Roughly 80% arrive within two weeks of it, either direction. The due date is the midpoint of a probability window, not a fixed delivery appointment. Yet it drives every clinical decision in prenatal care, scheduling anatomy scans, timing genetic screening, deciding when a pregnancy is "overdue" and induction becomes necessary.

The standard calculation method, Naegele's rule, adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The 280-day figure comes from 19th-century observations by German obstetrician Franz Karl Naegele, updated by later studies showing the average pregnancy from conception runs 266 days, or about 38 weeks from fertilization, which equals 40 weeks from LMP if ovulation occurred on day 14 of a 28-day cycle.

This guide explains Naegele's rule, the difference between gestational age and fetal age, why first-trimester ultrasound gives a more accurate date than LMP alone, and the clinical decisions that hinge on gestational age at specific weeks.

Naegele's Rule: How the Standard Calculation Works

Gestational age counts from the first day of the last menstrual period, not from conception. This convention exists because LMP is a known date while the exact day of ovulation and fertilization usually is not. The formula:

EDD = LMP + 280 days (or: LMP + 9 months + 7 days)

The "+9 months +7 days" shortcut gives the same result as adding 280 days for most months. If LMP was January 1, add 9 months (October 1) and 7 days: October 8 is the estimated due date.

Gestational age counts weeks from LMP. At 20 weeks gestational age, the pregnancy is at the halfway point of the standard 40-week term. Fetal age (also called embryonic age or conception age) counts from fertilization and runs about 2 weeks less than gestational age. A fetus at 20 weeks gestational age has a fetal age of approximately 18 weeks.

Clinicians use gestational age as the standard because it anchors to a known date. Fetal age appears in embryology research but almost never in prenatal clinical notes.

How the Math Works Step by Step

Example: LMP is March 15.

Step 1: Add 7 days to the LMP day. March 15 + 7 = March 22.

Step 2: Add 9 months to the LMP month. March + 9 months = December.

Result: Estimated Due Date = December 22.

Verification by day count: March has 16 days remaining after March 15. April through November = 30+31+30+31+31+28+31+30 = 242 days. Plus 22 days in December = 16 + 242 + 22 = 280 days. Confirmed.

The calculation assumes a 28-day cycle with ovulation on day 14. A woman with a 35-day cycle ovulates later, meaning her conception occurred later and the due date should shift about 7 days forward. Most obstetric calculators include cycle length correction for this reason.

Common Misconceptions

• The due date is a fixed delivery date. The due date marks 40 weeks gestational age, the statistical center of normal delivery timing. Full-term pregnancy spans 39 weeks 0 days through 40 weeks 6 days. A birth at 38 weeks is early term; at 41 weeks is late term. Most deliveries fall within a 5-week window centered on the due date.
• LMP gives an accurate due date for everyone. Naegele's rule assumes a 28-day cycle with ovulation on day 14. Women with irregular cycles or cycles longer than 30 days have LMP-based dates that can be off by 1-2 weeks. First-trimester ultrasound measurement of the embryo (crown-rump length) produces due dates accurate to within 5-7 days for most pregnancies.
• Ultrasound gives a perfect due date at any stage. Ultrasound accuracy for gestational age dating decreases as pregnancy progresses. In the first trimester (before 14 weeks), crown-rump length dating carries a margin of ±5-7 days. In the second trimester, head and femur measurements carry ±10-14 days. Third-trimester dating is far less reliable, sometimes ±3 weeks.
• Going past the due date means something is wrong. About 10-15% of pregnancies continue past 41 weeks in healthy women. Providers typically begin discussing induction options at 41 weeks (7 days past due) because the risk of stillbirth begins to rise modestly, not because post-dates pregnancy is automatically pathological.