Due Date Calculator
Estimate your baby's due date and see how far along your pregnancy is.
Estimated due date
- Pregnancy today:
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- Trimester:
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- Estimated conception:
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- Days until due date:
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Key dates
How the Due Date Is Calculated
Pregnancy is counted from the first day of the last menstrual period, not from conception, because that date is usually known while the exact day of conception rarely is. A typical pregnancy lasts 280 days, or 40 weeks, from that day. This method, known as Naegele's rule, assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, ovulation usually happens later or earlier, so the calculator moves the due date by the difference: with a 32-day cycle the due date is 4 days later.
If you know the date of conception, for example after IVF or from ovulation tracking, the due date is 266 days (38 weeks) after conception.
Worked Example
If the first day of your last period was 1 March 2026 and your cycle is 28 days, the estimated due date is 6 December 2026. Conception probably happened around 15 March. The first trimester ends on 6 June (13 weeks and 6 days), the halfway point of 20 weeks is on 19 July, and full term begins on 15 November at 37 weeks.
How Pregnancy Weeks Are Written
Midwives and doctors write the length of a pregnancy as completed weeks plus days, for example 12+3, which means 12 full weeks and 3 days. The calculator shows the same format. Pregnancy is divided into three trimesters: the first lasts until 13+6, the second from 14+0 to 27+6, and the third from 28+0 until birth.
How Accurate Is a Due Date?
A due date is an estimate, not an appointment. Only about 4% of babies are born on their due date. Most arrive within two weeks either side of it, and a birth between 37+0 and 41+6 weeks is considered full term. In Finland, Sweden and most other countries, an ultrasound scan early in pregnancy measures the baby and confirms or corrects the due date. If the scan gives a different date, follow the date from your maternity clinic: it is more accurate than any calculation based on dates alone.
Key Stages of Pregnancy
| Weeks | Stage |
|---|---|
| 4–5 | A home pregnancy test usually turns positive around the time of the missed period. |
| 8–14 | First visits to the maternity clinic and the early ultrasound scan are usually arranged. |
| 18–21 | Many countries offer a detailed structural ultrasound scan around the halfway point. |
| 24+ | The baby's movements are usually clearly felt and regular check-ups continue. |
| 37–42 | Full term. The baby can be born at any time. |
Frequently Asked Questions
What if I don't remember the first day of my last period?
Use your best estimate, then rely on the date set at your first ultrasound scan, which is based on the baby's measured size.
Does a long or irregular cycle change the due date?
Yes. A longer cycle usually means later ovulation and a later due date, which the calculator allows for. If your cycles are very irregular, a date based on the last period is less reliable and the ultrasound date matters even more.
Can the due date change during pregnancy?
It is usually set once, after the early scan, and then kept. Later scans check the baby's growth but do not normally move the due date.
Is my information saved?
No. The calculation happens in your browser and nothing you enter is stored or sent anywhere. This calculator gives general information only and does not replace the care and advice of a midwife or doctor.