Due date calculator
A due date is estimated by adding 280 days (40 weeks) to the first day of your last period, which is known as Naegele’s rule. If your cycle is usually longer or shorter than 28 days, the date moves by the difference. If you know the conception date, add 266 days (38 weeks) instead. A last period that started on 1 March 2026 gives a due date of 6 December 2026. Your midwife or doctor confirms the dating, usually with an early ultrasound scan.
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Worked example
The first day of your last period was 1 March 2026.
- With a 28-day cycle: 1 March 2026 + 280 days = 6 December 2026.
- With a 32-day cycle: add 4 more days (32 − 28) = 10 December 2026.
- From a conception date of 15 March 2026: + 266 days = 6 December 2026, the same as the 28-day result, because conception is counted as two weeks after the last period.
On 1 June 2026 that pregnancy is 13 weeks and 1 day (92 days since 1 March 2026), in the first trimester. The second trimester starts on 7 June 2026 (14 weeks) and the third on 13 September 2026 (28 weeks).
How the calculation works
Pregnancy is counted in weeks from the first day of the last period, not from conception, so at the moment of conception a pregnancy is already “two weeks” along. Naegele’s rule adds 280 days to that date. It assumes a 28-day cycle with ovulation about two weeks in; if your cycles are usually longer, ovulation tends to happen later, so the calculator moves the due date later by the same number of days (and earlier for shorter cycles), as the NHS due date calculator also does.
From a conception date, the calculator adds 266 days, which is 280 minus those first two weeks. Your current week is the number of whole weeks since the start date, plus the extra days, so “13 weeks and 1 day” means 92 days. Trimesters follow ACOG’s definitions: up to 13 weeks 6 days, 14 weeks 0 days to 27 weeks 6 days, and 28 weeks on.
How accurate is this?
A date from the last period is a good first estimate, but it depends on remembering that date and on when you actually ovulated. The NHS notes that pregnancy normally lasts 37 to 42 weeks from the first day of the last period, and that the 12-week scan can estimate more accurately how many weeks pregnant you are. ACOG explains that when the last-period date is uncertain, a first-trimester ultrasound measuring the embryo’s crown–rump length is used to estimate the due date.
In its guidance for clinicians, ACOG recommends settling the due date as soon as the last-period date, the first accurate ultrasound, or both are known, and changing it later only in rare circumstances. So use this result to plan, and go by the date your midwife or doctor gives you once your pregnancy has been dated.
Frequently asked questions
How accurate is a due date?
It is an estimate of the middle of a range. The NHS notes that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period, so a baby arriving a week or two either side of the due date is common.
Why are pregnancy weeks counted from my last period?
Because the first day of the last period is a date most people know, while the day of conception usually is not. ACOG explains that pregnancy is assumed to start about two weeks after that day, so the first two weeks are counted before conception.
What if I do not know the date of my last period?
Your midwife or doctor can estimate the due date from an ultrasound in the first trimester, which measures the length of the embryo or fetus. If you know the conception date, for example from fertility treatment, you can also use the conception option here.
Can my due date change after a scan?
It can be set from the scan when that is more reliable than the last-period date. ACOG recommends deciding the due date once the last-period date and the first accurate ultrasound are known, and changing it after that only in rare circumstances.
When does each trimester start?
Counting from the first day of the last period, ACOG defines the first trimester as up to 13 weeks and 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.
Sources
- How Your Fetus Grows During Pregnancy (FAQ): due date, weeks and trimesters. American College of Obstetricians and Gynecologists (ACOG).
- Methods for Estimating the Due Date (Committee Opinion 700). American College of Obstetricians and Gynecologists (ACOG).
- Due date calculator. NHS.
Written by the Lunation editorial team.
Related guides
- When Am I Most Fertile?Your most fertile days are the six ending on ovulation day. How to find them in your own cycle, why day 14 is only a rough guide, and when to talk to a doctor.
- What Are the Signs of Ovulation?Stretchy cervical mucus, a positive LH test and a small rise in resting temperature are the clearest signs of ovulation. What each one tells you, and when.
More calculators
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- Ovulation calculatorEstimate your ovulation day (about 14 days before your next period) and six-day fertile window for the next three cycles.
- Cycle length calculatorWork out your average cycle length, shortest and longest cycle, and how much it varies from your period start dates.
Medical disclaimer. This calculator gives general wellness estimates, not medical advice. It does not diagnose or treat any condition and is not a method of contraception. If something about your cycle or your health worries you, talk to a doctor or midwife.