How pregnancy due dates are calculated (and why they're usually wrong)

The standard pregnancy due date is calculated using Naegele’s rule: take the first day of the last menstrual period (LMP), subtract three months, add seven days. The result is 280 days — exactly 40 weeks — from LMP. It’s simple, memorable, and based on assumptions that rarely match reality. Only about 4% of births happen on the predicted day. About half happen within ±5 days. The rest sit outside even that window.

Where 280 days comes from

Franz Karl Naegele published the rule in 1812. He assumed:

• A regular 28-day menstrual cycle.
• Ovulation on day 14 of the cycle.
• Conception immediately at ovulation.
• Gestation of 266 days from conception, which equals 280 days from LMP.

Two centuries later we know each of those assumptions has a meaningful error bar. Cycle lengths vary; ovulation timing varies within a single cycle; the conception window can span 6 days; gestation duration itself has a spread of ±2 weeks even after controlling for the LMP date.

What the data shows

A 2013 study at Cedars-Sinai (Jukic et al., Human Reproduction) measured gestation directly using hormone-confirmed conception dates rather than LMP. The median gestation was 268 days from conception — about 2 days longer than Naegele assumed. The 90% confidence interval was ±17 days. That means a baby arriving any time between 37 weeks and 42 weeks from LMP is statistically unremarkable.

The CDC’s National Vital Statistics System data on US births shows:

Why the due date is still useful

Even though only 4% of babies are born on the date, the date is the reference point for almost every prenatal clinical decision: ultrasound timing, fetal anatomy scans, glucose-tolerance testing, group B strep screening, induction policy if gestation exceeds 41 weeks. The number is a clinical anchor, not a prediction.

Modern obstetrics typically refines the LMP-based estimate with a first-trimester ultrasound (crown-rump length is accurate to ±5 days at 8-13 weeks). If the ultrasound disagrees with LMP by more than 7 days, ACOG (American College of Obstetricians and Gynecologists) recommends switching to the ultrasound date.

Why first pregnancies run later

Multiple studies (Jukic 2013, Mongelli 1996, Smith 2001) agree: first pregnancies last about 5 days longer than subsequent ones, on average. The biological mechanism is unclear — possibly cervical-tissue conditioning, possibly hormonal-feedback differences. The practical implication: first-time mothers should plan for a delivery window skewed past 40 weeks rather than centered on it.

What to ask instead

The honest version of the question “when’s the baby coming?” is “what’s the 90% delivery window?” — typically 37+0 to 41+6 weeks from LMP. That’s a 35-day spread. Maternity leave planning, relative travel arrangements, and finished-nursery deadlines should be tied to the start of that window, not the center.

Use our due date calculator to get the Naegele estimate plus the 90% window from your LMP date. For LMP-from-conception conversion (helpful with IVF or known conception dates), the calculator does the offset for you.

Edge cases the rule doesn’t handle

• Irregular cycles. Naegele assumes 28 days; cycles routinely span 21-35. A 35-day cycle ovulates ~7 days later than the rule assumes, so the due date should be pushed back by 7 days. Many calculators include a cycle-length adjustment.
• IVF.Conception date is known precisely. Standard adjustment: due date = transfer date + 263 days (for a day-5 blastocyst transfer). LMP-based calculators don’t apply here.
• Twins. Twin gestations are typically ~36-37 weeks rather than ~40. The Naegele-derived 40-week due date is essentially never reached by a twin pregnancy.

The pragmatic takeaway

Treat the due date as the midpoint of a 5-week window, not a prediction. First pregnancies skew late; cycle-adjusted due dates are more accurate; ultrasound-confirmed dates are more accurate still. The exact date matters far less than the window, and the window is wider than most first-time parents are told.

Sources: Jukic AM et al., Human Reproduction 2013; ACOG Committee Opinion 700 (Methods for Estimating the Due Date, 2017); CDC National Vital Statistics Reports Vol. 72, No. 1, 2023.