Naegele's rule: calculating a due date from your last period

Nearly every pregnancy wheel and due-date calculator is built on one simple idea called Naegele's rule. Named after the 19th-century German obstetrician Franz Naegele, it estimates the due date by counting forward from the last menstrual period.

The formula

In its plainest form:

Estimated due date = first day of last period + 280 days

That's 40 weeks. A common shortcut phrases it as “add one year, subtract three months, add seven days,” which lands on the same date for a 28-day cycle. Our calculator uses the exact day count instead, so leap years and month lengths never throw it off.

Why 280 days?

Dating starts from the last period rather than from conception because the last period is a date most people can actually identify, whereas the moment of conception usually isn't known. On an idealised 28-day cycle, ovulation happens around day 14, so 280 days from the LMP is roughly 266 days from conception. If you know the conception date instead, our conception mode counts those 266 days directly.

Adjusting for cycle length

The rule's biggest assumption is a 28-day cycle with ovulation on day 14. Cycles vary, and that matters:

• A longer cycle means later ovulation, so the due date moves later.
• A shorter cycle means earlier ovulation, so the due date moves earlier.

The adjustment is one day for each day your cycle differs from 28: EDD = LMP + 280 + (cycle − 28). A 32-day cycle pushes the estimate four days later. The calculator makes this adjustment automatically once you set your cycle length.

When Naegele's rule is least reliable

Menstrual dating struggles when the LMP is uncertain, when cycles are irregular, or when bleeding in early pregnancy is mistaken for a period. In all of those situations a first-trimester ultrasound is more accurate, and guidelines recommend using the scan to date the pregnancy when the two disagree by more than a small margin. We cover that trade-off in how accurate is your due date and in crown-rump length dating.

Reading the result

Once you have a due date, the same LMP gives you the current gestational age — the weeks-and-days figure that drives most clinical decisions. If you're unsure how gestational age differs from the baby's actual age since conception, gestational age vs. fetal age explains it.

Naegele's rule remains the backbone of pregnancy dating not because it's perfect, but because it's simple, transparent, and good enough as a starting estimate. Treat the number it gives you as a well-founded guess to be confirmed with a clinician, not a fixed date.