Pregnancy due date calculator
Enter the first day of your last period. The calculator applies Naegele's rule — 280 days — to estimate your due date, along with the estimated conception date and the start of the third trimester.
Use it to estimate a due date from your last period's first day.
Estimated due date
Oct 8, 2026
- Estimated conception
- Jan 15, 2026
- Third trimester starts
- Jul 9, 2026
The result updates as you choose. The headline is the estimated due date; the others give the estimated conception date and when the third trimester begins. The due date is an estimate, not medical advice. Your healthcare provider may adjust it based on ultrasound measurements, cycle length, or your health.
How does it work?
Naegele's rule assumes a regular 28-day cycle. Only about 4% of births land exactly on the due date; it marks the middle of a likely range. An ultrasound gives a more precise date.
Naegele's rule
- last period
- First day of the last menstrual period.
- 280 days
- 40 weeks of an average pregnancy.
A last period starting 1 January gives a due date around 8 October, with conception about two weeks after the period began.
Expert tips
- The due date is an estimate, not medical advice. Your healthcare provider may adjust it based on ultrasound measurements, cycle length, or your health.
Method & sources
The due date is 280 days (40 weeks) after the last period's first day. Conception is estimated about 14 days after that day. The third trimester is taken to start around 27 weeks.
Sources
Where this method comes from — use these references to understand the formula, assumptions, and limits.
- Methods for estimating the due date — American College of Obstetricians and Gynecologists, verified 2026-06-10
- Naegele's rule — estimated due date (last menstrual period + 280 days) — StatPearls / NCBI Bookshelf, verified 2026-06-10
- Due date calculator — NHS — NHS (UK National Health Service), verified 2026-06-10
How we calculate
- The due date is 280 days (40 weeks) after the last period's first day.
- Conception is estimated about 14 days after that day.
- The third trimester is taken to start around 27 weeks.
- Naegele's rule assumes a regular 28-day cycle; an ultrasound is more precise.
Limitations
- The due date is an estimate, not medical advice. Your healthcare provider may adjust it based on ultrasound measurements, cycle length, or your health.
Rounding
Dates are exact day counts. The calculation is done in UTC.
What this calculator does
The most common way to date a pregnancy is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This calculator does that and also estimates the conception date and the start of the third trimester.
How to use it
- Enter the first day of your last period.
- Read the estimated due date.
- Note the estimated conception date.
- See when the third trimester starts.
A worked example
A last period starting 1 January gives a due date around 8 October, with conception estimated about two weeks after the period began.
How accurate is the due date?
Only about 4% of babies arrive exactly on the due date; it marks the middle of a likely range, usually within two weeks either side. An early ultrasound can give a more precise date, especially with irregular cycles.
Common mistakes
- Entering the period's end date instead of the first day.
- Expecting the baby to arrive exactly on the date.
- Relying on it instead of a clinician's dating for irregular cycles.
When it's useful
A quick due-date estimate early in pregnancy, planning around the trimesters, or a starting point before an ultrasound.
FAQ
- 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 it?
- It marks the middle of a likely range. Only about 4% of births land exactly on the date; an ultrasound is more precise.
- What if my cycle isn't 28 days?
- Naegele's rule assumes 28 days, so a longer or shorter cycle shifts the real date. A clinician can adjust the dating.
- How is conception estimated?
- About 14 days after the last period's first day, which is roughly when ovulation occurs in a 28-day cycle.
- When does the third trimester start?
- Around 27 weeks into the pregnancy, which this calculator marks from the last period.
- Can I share a calculation?
- Yes. Use Share to copy a link that reopens the calculator with the same date.
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