Pregnancy Due Date Calculator

What this calculator does

• Estimates a due date from the first day of the last period and the average cycle length.
• Adjusts for cycle lengths other than 28 days, which the classical rule does not.
• Estimates the conception date and the trimester boundaries.
• Is a calendar calculation, not a clinical assessment.

How the calculation works

The estimate uses Naegele's rule: 280 days, or forty weeks, from the first day of the last menstrual period. That rule assumes ovulation around day 14, which is true for a 28-day cycle and progressively less true as cycles get longer or shorter. This calculator adjusts for the cycle length you enter, so a 35-day cycle moves the estimate a week later than the classical rule would place it — a correction that matters and that many due date calculations simply omit. Trimester boundaries are then derived from the same starting point. What the method fundamentally cannot do is observe the pregnancy. It works from dates alone, and it assumes the date you give is accurate and that ovulation followed your usual pattern that month. A dating scan, usually offered at around twelve weeks in the UK, measures the pregnancy directly and is more accurate than any calculation from a calendar.

Step by step

1. Start from the first day of the last menstrual period.
2. Add 280 days.
3. Adjust by the difference between your cycle length and 28 days.
4. Estimate conception as roughly two weeks after the period began, adjusted the same way.
5. Derive the trimester boundaries from the same starting date.

Worked example

Someone whose last period began on 1 February 2026, with an average 28-day cycle.

What was entered

The arithmetic

1. The cycle is 28 days, so no adjustment to the classical rule is needed.
2. 280 days from 1 February 2026 gives an estimated due date of 8 November 2026.
3. Conception is estimated at around 15 February 2026, roughly two weeks after the period began.
4. The first trimester ends on 9 May 2026 and the second on 15 August 2026.
5. With a 35-day cycle instead, the estimate would move a week later — which is why the adjustment matters.

What the calculator returns

Key assumptions

• The date entered is the first day of the last menstrual period, not the day it ended.
• Ovulation followed the usual pattern for the cycle length given.
• Cycle length is reasonably regular.

Limitations

• This is a calendar calculation, not a clinical assessment, and it is not a pregnancy test.
• A dating scan measures the pregnancy directly and is more accurate than any date-based estimate. In the UK one is usually offered at around twelve weeks.
• Very few babies arrive on the estimated date. A pregnancy is considered to run normally anywhere from 37 to 42 weeks, so the estimate is the middle of a wide range rather than a prediction.
• Irregular cycles, recent hormonal contraception, breastfeeding and uncertainty about the date of the last period all reduce reliability considerably.
• It does not apply to IVF pregnancies, where dating is worked out from the transfer date instead.
• For anything concerning — bleeding, pain, reduced movements, or simply worry — contact a midwife, a GP or NHS 111 rather than a calculator.

Common questions

Related calculators

• Pregnancy Calculator — Estimate the fertile window from the same cycle information.
• Date Calculator — Count the days between any two dates, if you want to check an interval yourself.

Official sources

Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.

• Due date calculator — NHSPregnancy normally lasts from 37 to 42 weeks from the first day of the last period; a 12-week scan estimates gestation more accurately
• Your antenatal care — NHSAntenatal appointments and scans offered during pregnancy in the UK