How the due date is estimated
Naegele's rule adds 280 days — 40 weeks — to the first day of your last menstrual period. A period starting June 26, 2026 gives a due date of April 2, 2027. It assumes a regular 28-day cycle; the ovulation calculator can help if your cycle differs. If you are still trying to conceive, the ovulation calculator estimates the fertile window.
Why it is only an estimate
Only about 1 in 20 babies arrive exactly on the due date — most come within two weeks either side. An early ultrasound often gives a more precise date. This is a general wellness estimate, not medical advice. Talk to a healthcare professional for personal guidance.
Full term itself spans several weeks, generally counted from 37 to 42, so arriving days either side of the date is entirely normal. To see how long there is to go, use the days until date calculator.
Cycle length matters
Naegele's rule assumes a 28-day cycle with ovulation around day 14. Longer or shorter cycles shift the real date. Check your fertile window with the ovulation calculator for context.
A simple adjustment helps when your cycle is not 28 days: add the difference for a longer cycle and subtract it for a shorter one, so a regular 32-day cycle pushes the date about four days later. First-trimester dating scans measure the baby directly and are usually accurate to within a few days, which is why clinics often revise a period-based date after the first ultrasound. Counting gestational weeks from a known date is easiest in the weeks between dates calculator.
Frequently asked questions
- How is the due date calculated?
- Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period.
- How accurate is the due date?
- It is an estimate — most babies arrive within two weeks before or after it.
- What if my cycle is not 28 days?
- The formula assumes a 28-day cycle, so longer or shorter cycles shift the true date.
- Is an ultrasound more accurate?
- An early ultrasound often dates a pregnancy more precisely than the last-period method.
- How many weeks pregnant am I on this system?
- Counting starts from the first day of your last period, so you are already counted as about two weeks pregnant at conception. That is why 40 weeks is full term even though a pregnancy lasts roughly 38 weeks from conception.