👶 How Pregnancy Due Dates Are Calculated (Naegele's Rule)

A due date is an estimate, not a prediction of the exact day a baby will arrive — and understanding the simple rule behind it explains both why it's useful for planning and why so few babies actually arrive on that specific day.

Naegele's Rule

This assumes a typical 28-day cycle with ovulation on day 14 — a reasonable average, though real cycles vary, which is why the estimate gets adjusted for a different cycle length when that information is available.

Why Cycle Length Matters

A longer or shorter cycle shifts the estimated ovulation date, and therefore the estimated conception date and due date along with it — someone with a 35-day cycle ovulates later than someone with a 21-day cycle, so the standard 280-day-from-LMP estimate needs adjusting to stay accurate.

Pregnancy Trimesters

Why Due Dates Are Estimates, Not Predictions

Only about 5% of babies are actually born on their calculated due date — full-term birth is normal anywhere from about three weeks before to two weeks after, which is why due dates are best understood as the center of a range, not a fixed appointment.

Always confirm with an OB-GYN or midwife — this is an estimate for general planning, not a medical diagnosis.

Worked Example: Calculating a Due Date

If the first day of the last menstrual period (LMP) was 10 January 2026, with a regular 28-day cycle:

1. Add 280 days to 10 January 2026.
2. The estimated due date is 17 October 2026.

A quick shortcut that gives the same result, Naegele's rule, is: take the LMP date, add one year, subtract three months, and add seven days (10 January 2026 → 10 January 2027 → 10 October 2026 → 17 October 2026).

If the cycle is usually 32 days instead of 28, ovulation tends to happen about four days later, so the estimate moves about four days later too — to around 21 October 2026.

Ultrasound Dating

An early ultrasound, especially in the first trimester, measures the size of the embryo and is often a more accurate way to estimate the due date than the LMP alone. If the ultrasound date and the LMP date differ noticeably, your doctor or midwife may adjust your official due date based on the scan.

Key Terms

• Gestational age: the length of pregnancy counted from the first day of the LMP, which is why pregnancy is described as 40 weeks.
• Full term: 39 weeks 0 days to 40 weeks 6 days.
• Early term: 37 weeks 0 days to 38 weeks 6 days; late term: 41 weeks 0 days to 41 weeks 6 days.

Frequently Asked Questions

What if I don't know the date of my last period?

Your healthcare provider can estimate the due date with an ultrasound instead.

Does the due date change during pregnancy?

It's usually set early — often after the first ultrasound — and then kept the same, even if the baby measures bigger or smaller later on.

This article is for general information and is not medical advice. Always follow the guidance of your doctor or midwife.