How the due date is calculated
Most doctors and midwives start with Naegele's rule: the estimated due date (EDD) is 280 days, or 40 weeks, after the first day of your last menstrual period (LMP). Counting from the period rather than from conception sounds odd, but the first day of a period is a date most people know, while the day of conception usually is not.
Due date = first day of last period + 280 days + (cycle length − 28)
Due date = conception date + 266 days
The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer, you probably ovulated later, so the calculator moves the due date later by the same number of days. A 32-day cycle adds four days; a 26-day cycle takes two away.
Worked examples
From the last period
Your last period started on 1 January 2026 and your cycle is 28 days. 1 January + 280 days = 8 October 2026. With a 32-day cycle the estimate becomes 12 October 2026.
From a known conception date
If conception happened on 15 January 2026, add 266 days (38 weeks): the due date is again 8 October 2026. Pregnancy is still described in weeks from the last period, so on the conception day you are already counted as about 2 weeks pregnant.
How pregnancy weeks are counted
| Stage | Weeks (from LMP) |
|---|---|
| First trimester | 0 to 13 weeks + 6 days |
| Second trimester | 14 to 27 weeks + 6 days |
| Third trimester | 28 weeks to birth |
| Early term | 37 weeks to 38 weeks + 6 days |
| Full term | 39 weeks to 40 weeks + 6 days |
| Late term | 41 weeks to 41 weeks + 6 days |
| Post-term | 42 weeks and later |
"8 weeks 3 days" means eight full weeks plus three days have passed since the first day of the last period. The calculator also lists approximate dates for common milestones, such as the mid-pregnancy anatomy scan (usually 18–22 weeks) and glucose screening for gestational diabetes (often 24–28 weeks). Your own care team may schedule them differently.
Why the due date is only an estimate
Only a small minority of babies, roughly one in twenty, are born on their exact due date. Most arrive in the weeks around it, and a birth anywhere between 37 and 42 weeks is common. Several things make the estimate uncertain:
- Ovulation varies from cycle to cycle, even in people with regular periods.
- Remembering the date of the last period is not always easy, especially if it was light or unusual.
- Irregular cycles, recent hormonal contraception or breastfeeding make the period-based method less reliable.
An ultrasound in the first trimester, which measures the baby from crown to rump, is the most accurate way to date a pregnancy. If the scan and the period-based date differ by more than a few days, your care provider will usually go by the scan. Once a due date is set this way, it is normally not changed later.
IVF pregnancies
With IVF the date of conception is known precisely. If you know the transfer date, the usual estimate is 266 days from the egg retrieval (fertilisation) date, which is 263 days after a day-3 embryo transfer or 261 days after a day-5 blastocyst transfer. Your fertility clinic will give you the official date.
Common mistakes
- Using the last day of the period instead of the first day. Always use the day bleeding started.
- Using the date of a positive test. A test turns positive about two weeks after conception, so it gives a date that is far too late.
- Ignoring a long cycle. With a 35-day cycle, the plain 280-day rule can be a week too early.
This calculator gives an estimate for information only. Your doctor or midwife will confirm the due date, usually with an early ultrasound.
Frequently asked questions
How accurate is a due date calculator?
It gives the same estimate a doctor makes with Naegele's rule, but only about one baby in twenty is born on that exact day. Most births happen within two weeks either side.
Why are weeks counted from my last period, not from conception?
Because the first day of the last period is usually known, while conception is not. Counting from the period adds about two weeks before conception.
What if my cycle is irregular?
The period-based estimate is less reliable. An early ultrasound gives a more accurate date, and your care provider will use it instead.
Can the due date change?
It may be adjusted once after an early ultrasound if the scan disagrees with the period-based date. After that it normally stays the same.
When is a baby considered full term?
Full term is from 39 weeks to 40 weeks and 6 days. Births from 37 weeks are called early term, and from 42 weeks post-term.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion 700: Methods for Estimating the Due Date (2017).
- ACOG Committee Opinion 579: Definition of Term Pregnancy (2013).
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.