What a due date really is
A due date is the centre of a window, not a deadline. It marks 40 weeks from the first day of the last menstrual period, the standard way pregnancy length is counted, even though conception usually happens about two weeks after that date. Most babies arrive within a couple of weeks on either side, and only about 1 in 20 are born on the exact estimated date. Knowing the date is still useful: it schedules scans and tests, tracks growth, and helps everyone plan, as long as it is held as an estimate.
This calculator offers the two standard starting points. If you know the first day of your last period, it adds 280 days. If you know a conception date, for example from fertility treatment, it adds 266 days and works backwards to the implied last period. Both routes should land on the same due date for the same pregnancy, give or take the uncertainty in the input dates themselves. This page is for general information only and is strongly not a substitute for medical advice or antenatal care; please use it alongside, never instead of, your midwife, doctor or clinic.
Naegele's rule
The traditional form, Naegele's rule, takes the first day of the last period, adds a year, subtracts three months and adds seven days. For a last period of September 15, 2026 that gives June 22, 2027, exactly the same as adding 280 days, which is the arithmetic this page performs in UTC to avoid calendar and timezone slips. The conception method adds 266 days to the conception date, reflecting the roughly 38 weeks from conception to birth.
Trimester milestones shown here
The breakdown lists the conception estimate (last period plus 14 days in the LMP method), the end of the first trimester at 13 weeks (last period plus 91 days) and the end of the second trimester at 27 weeks (last period plus 189 days). Trimester boundaries are conventions and sources differ slightly, some placing the second trimester's end at 26 or 28 weeks; the dates here follow the 13- and 27-week marks stated on the page so you can reproduce them exactly.
Worked example
Last period starts September 15, 2026. Due date = Sep 15 + 280 days = June 22, 2027 (Naegele: +7 days, −3 months, +1 year gives the same date). Conception estimate = Sep 15 + 14 days = September 29, 2026. First trimester ends Sep 15 + 91 days = December 15, 2026; second trimester ends Sep 15 + 189 days = March 23, 2027.
How to use this calculator
- Choose the method: last period or conception date.
- Enter that date as accurately as you know it.
- Read your estimated due date in the headline.
- Note the conception estimate and the two trimester end dates in the table.
- Confirm dating at your first ultrasound and antenatal appointment, which overrides a calendar estimate.
Why ultrasound dating can change the date
Calendar methods assume a 28-day cycle with ovulation on day 14. Longer or shorter cycles shift conception, and with it the true due date, by days or even a week or more. An early ultrasound measures the developing baby directly and is generally more accurate for dating, especially when cycle dates are uncertain or irregular, so clinics often set or revise the official due date after the first scan. A revised date from your care team is not a contradiction; it is better information replacing an estimate.
Whatever date you are given, plan flexibly around it: have essentials ready a few weeks early, and do not treat passing the date as something going wrong, since a large share of first pregnancies run past 40 weeks. Decisions about monitoring or induction after your due date belong to you and your care team, based on your full situation. If you have bleeding, pain, reduced movements or any worry at any stage, contact a health professional promptly rather than consulting a calculator. Information only, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.