Naegele's rule: 280 days from the last period
Doctors and midwives count a pregnancy from the first day of the last menstrual period (LMP), not from the day of conception. The reason is practical: most people know when their period started, but very few know the exact day an egg was fertilized. Counted this way, a typical pregnancy lasts 280 days, or 40 weeks.
Naegele's rule, named after the German obstetrician Franz Naegele, turns this into a quick calculation: take the first day of the last period, add one year, subtract three months and add seven days. It gives the same result as adding 280 days, or one or two days off around February, so counting days is the exact method. For a period that started on 1 January 2026, the estimated due date is 8 October 2026. For 15 March 2026 it is 20 December 2026.
The calculator on this page does the counting for you. Choose Last period, enter the first day of your last period and keep the cycle length at 28 days unless your cycle is usually different.
| First day of last period | Estimated due date (28 day cycle) |
|---|---|
| 1 January 2026 | 8 October 2026 |
| 1 February 2026 | 8 November 2026 |
| 15 March 2026 | 20 December 2026 |
| 1 May 2026 | 5 February 2027 |
| 1 August 2026 | 8 May 2027 |
Try it here: Due Date Calculator
Open the full tool pageAdjusting for a longer or shorter cycle
Naegele's rule assumes that ovulation happens on day 14 of a 28 day cycle. The second half of the cycle, from ovulation to the next period, is fairly constant at about 14 days, so in a longer cycle ovulation simply happens later. With a 32 day cycle it is around day 18, four days later than the rule assumes, so the due date also moves four days later. With a 25 day cycle it moves three days earlier.
That is how the cycle length box in the calculator works: it adds the difference from 28 days to the due date. It accepts cycles from 21 to 45 days. If your cycles vary a lot from month to month, the date based on your period is less reliable, and an early ultrasound gives a better estimate.
- 28 day cycle: no change.
- 32 day cycle: due date 4 days later.
- 35 day cycle: due date 7 days later.
- 25 day cycle: due date 3 days earlier.
From a conception date or an IVF transfer
If you know the day of conception, for example from ovulation tests, add 266 days (38 weeks). This is the same as counting 280 days from a period two weeks earlier. Conception on 15 January 2026 gives 8 October 2026, the same date as a last period on 1 January.
With IVF the age of the embryo is known precisely. A day 5 embryo (blastocyst) is counted as 2 weeks 5 days of pregnancy on the day of the transfer, so the due date is 261 days after the transfer. A day 3 embryo is counted as 2 weeks 3 days, so the due date is 263 days after the transfer. A day 5 transfer on 10 March 2026 gives 26 November 2026; a day 3 transfer on the same day gives 28 November 2026. Frozen embryo transfers are counted the same way, by the age of the embryo when it was transferred.
In the calculator, choose Conception date or IVF transfer, enter the date and, for IVF, pick Day 3 or Day 5.
Pregnancy weeks and trimesters
Pregnancy weeks are written as completed weeks plus days. At 12 weeks 3 days you have completed 12 weeks and are in your 13th week. Because counting starts at the last period, you are already about 2 weeks pregnant on paper at conception and about 4 weeks when a period is missed, which is when a home test can usually show a positive result.
The pregnancy is divided into three trimesters. The calculator uses the common definition and shows the dates of each one along with the current week, the days to go and a progress bar.
| Stage | Weeks | Day after the last period |
|---|---|---|
| First trimester | 0 to 13 weeks 6 days | Day 0 to 97 |
| Second trimester | 14 to 27 weeks 6 days | Day 98 to 195 |
| Third trimester | 28 weeks until birth | From day 196 |
| Early term | 37 to 38 weeks 6 days | Day 259 to 272 |
| Full term | 39 to 40 weeks 6 days | Day 273 to 286 |
| Late term | 41 to 41 weeks 6 days | Day 287 to 293 |
| Post-term | 42 weeks and later | From day 294 |
How accurate is a due date?
A due date is an estimate of the middle of the normal range, not a deadline. Only about one baby in twenty-five is born on the due date itself. Most are born between 37 and 42 weeks, which is why the calculator also shows the term window, from 37 weeks to the end of week 41, as a range of dates.
An ultrasound in the first trimester, which measures the length of the baby, is the most precise way to date a pregnancy that did not start with IVF. If it differs from the date based on your period by more than about a week, your midwife or doctor will usually use the ultrasound date. After that, the due date normally stays the same for the rest of the pregnancy.
This guide and the calculator use standard formulas and give general information. They are not medical advice; your midwife or doctor confirms the due date and follows the pregnancy.
Key dates to note
Besides the due date, the calculator lists dates that many parents like to plan around: the likely day of conception, the week a missed period and a positive test are expected, the start of the second and third trimesters, the usual window for the mid-pregnancy scan (weeks 18 to 22), and the start of early, full and late term. Past dates are greyed out and the current one is highlighted.
Your dates are calculated in your browser. They are not saved, not sent anywhere and not added to the share link, so you can use the calculator privately on your phone.