Naegele's Rule: The 280-Day Math Behind a Due Date

A due date is an estimate built on a 19th-century rule of thumb, not a precise prediction — Naegele's rule, still the standard starting point in obstetric dating today, adds 280 days (40 weeks) to the first day of the last menstrual period. The Pregnancy Due Date Calculator runs that calculation and layers on the two adjustments that make it more accurate for an individual: cycle length and ultrasound dating.

The base calculation

280 days assumes ovulation on day 14 of a standard 28-day cycle, so the calculation is dueDate = lastMenstrualPeriod + 280 days, adjusted by (actualCycleLength − 28) days for anyone whose cycle isn't exactly 28 days — a longer cycle shifts the estimated due date later, a shorter one earlier, since ovulation timing shifts with cycle length.

Ultrasound adjustment

Early ultrasound measurements (typically first trimester) can produce a more accurate estimate than LMP-based dating alone, particularly for irregular cycles — the tool accepts a manual ultrasound-based day adjustment on top of the LMP calculation, letting a clinician's more precise estimate override the calendar-based one without discarding the rest of the timeline math.

The trimester and milestone timeline

Beyond the single due date, the tool tracks gestational age in weeks and days from a corrected LMP, and marks the standard trimester boundaries: second trimester beginning at week 14, third at week 28, full term beginning at week 37, with the due date itself landing at week 40. Progress percent is computed as elapsed days over the total days from corrected LMP to due date — a simple ratio, but useful for seeing at a glance where a given week sits in the overall timeline.

An estimate, not a guarantee

Only a small fraction of births land exactly on the calculated due date — the number is a statistical center point, useful for planning and milestone tracking, not a guarantee. For everyday planning around a pregnancy, the Sleep Cycle Calculator helps with the sleep disruption that shifts throughout the trimesters, and the Habit & Streak Tracker is useful for tracking daily routines — medication, water intake, walks — across the whole timeline.