👶 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.

Step-by-Step: Estimate a Due Date

1. Enter the first day of the last menstrual period
2. Enter the typical cycle length, if different from 28 days
3. Review the estimated due date and current trimester

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