Your due date is usually estimated as 280 days, or 40 weeks, from the first day of your last period. For IVF, it is worked out from the embryo transfer date instead: 261 days after a day-5 transfer, or 263 days after a day-3 transfer.
Only a small minority of babies arrive on the due date itself. Think of it as the middle of a window, and expect your midwife or a dating scan to confirm or adjust it.
How a due date is calculated
The usual method is Naegele’s rule: take the first day of your last menstrual period (LMP) and add 280 days. It assumes a 28-day cycle, with ovulation around day 14.
That means pregnancy weeks are counted from your last period, not from conception. You are already “four weeks pregnant” around the time a period is due, even though conception happened only about two weeks earlier.
Dating scans can alter the date, particularly if your cycles are irregular or you are unsure when your last period began. If a scan gives a different date, your midwife will usually go with the scan.
Using ovulation to estimate a due date
If your cycle is longer or shorter than 28 days, ovulation moves with it. Ovulation typically happens about 14 days before your next period, so for a 32-day cycle you would expect it later than day 14.
For a last period starting on 1 October 2026:
| Cycle length | Estimated ovulation | Due date if you conceive |
|---|---|---|
| 28 days | Thu 15 October 2026 | Thu 8 July 2027 |
| 32 days | Mon 19 October 2026 | Mon 12 July 2027 |
With a 28-day cycle the fertile window runs from 10 to 16 October. The Ovulation and due date tool does these sums for you. It is an estimate for a regular cycle and not a form of contraception.
How pregnancy weeks and trimesters work
Pregnancy is described in weeks and days, such as 18+0 (18 weeks and 0 days). Trimesters are three roughly equal stages:
- First trimester: up to the end of week 13
- Second trimester: weeks 14 to 27
- Third trimester: week 28 until birth
Full term starts from 37 weeks. Our Pregnancy week calculator tool also lists scan windows based on NHS guidance, with the dating scan at 10 to 14 weeks and the anomaly scan at 18 to 20 weeks.
For example, a last period starting on 1 June 2026 gives a due date of 8 March 2027. On 5 October 2026 you would be 18 weeks and 0 days pregnant, in the second trimester, with 154 days to go and the anomaly scan window open.
IVF and embryo transfer due dates
With IVF, you know the exact date the embryo was transferred, so the due date is calculated from it. The embryo’s age is subtracted from the transfer date, then 14 days are added to match the usual LMP-based count.
- Day-5 blastocyst transfer: due date 261 days after transfer
- Day-3 embryo transfer: due date 263 days after transfer
For a day-5 transfer on 1 October 2026, the IVF due date tool gives a due date of 19 June 2027. For a day-3 transfer on the same date, it is 21 June 2027. In both cases, the “matching last period” date falls in mid-September 2026.
Follow your clinic’s instructions on when to take a pregnancy test and which dates to use. If you used IUI or insemination, the tool also offers that option.
Tips for the weeks ahead
- Book your appointments as early as you can. Your GP or midwife will explain what is offered.
- Keep your hydration up; see How much water should you drink each day?, though needs differ in pregnancy, so ask your midwife.
- Rest matters. Sleep cycles: how to choose a bedtime or wake-up time explains how sleep is structured.
- Pre-pregnancy weight can influence care. BMI explained: what it tells you and what it doesn't covers how BMI is used and what its limits are.
Note: This is general information, not medical advice. Our calculators give estimates only. Your midwife, GP or fertility clinic will confirm your dates and advise on your care.
Try the calculator
Start with the Ovulation and due date tool if you are planning, the Pregnancy week calculator tool to see how far along you are, or the IVF due date tool if you have had a transfer.
Open the free Ovulation and due date
Worked example 2: a later LMP and a dating scan window
Suppose your last period started on 10 August 2026. On 5 October 2026 the pregnancy week calculator shows:
| Item | Result |
|---|---|
| Pregnancy length on 5 October | 8 weeks, 0 days |
| Trimester | First |
| Due date | Monday 17 May 2027 |
| Days to go | 224 |
| Dating scan window (10 to 14 weeks) | 19 October to 16 November |
| Anomaly scan window (18 to 20 weeks) | 14 December to 3 January |
| Full term from 37 weeks | 26 April 2027 |
The scan windows follow NHS guidance, but your hospital or midwife will give you the actual appointment date, which may fall slightly inside or outside the window. Between 37 weeks and the due date you are described as full term, which is why the “full term” date comes about three weeks before the due date.
Longer and shorter cycles: two more examples
If your cycle is not 28 days, ovulation shifts, and so does the date you would be due if you conceived that cycle. With a last period starting on 1 October 2026, the ovulation calculator gives:
| Cycle length | Estimated ovulation | Fertile window | Due date if you conceive |
|---|---|---|---|
| 24 days | Sunday 11 October 2026 | 6 to 12 October | Sunday 4 July 2027 |
| 28 days | Thursday 15 October 2026 | 10 to 16 October | Thursday 8 July 2027 |
| 35 days | Thursday 22 October 2026 | 17 to 23 October | Thursday 15 July 2027 |
The due date moves by the same number of days as the cycle differs from 28. A 35-day cycle puts it seven days later than a 28-day cycle, and a 24-day cycle puts it four days earlier. These are estimates for a regular cycle. Cycles that vary from month to month make the prediction less reliable, and the tool is not a form of contraception.
A later IVF example
For a day-3 embryo transfer on 10 December 2026, the IVF due date calculator gives a due date of Monday 30 August 2027. The matching last period date is 23 November 2026, and the usual pregnancy test window is 19 to 24 December 2026. Your clinic will tell you its own testing date, and you should follow that rather than the tool. For a day-5 transfer on the same date, the due date is two days earlier, because a day-5 embryo is two days older than a day-3 embryo.
Step by step: working out the due date by hand
- Take the first day of your last period (the first day of full bleeding, not spotting).
- Add 280 days. A shortcut is to add one year, subtract three months and add seven days, which gives the same result within a day or so.
- If your cycle is not 28 days, add or subtract the difference. A 32-day cycle adds four days.
- For IVF, skip steps 1 to 3 and add 261 days to a day-5 transfer or 263 days to a day-3 transfer.
- To count weeks, divide the days since your last period by 7. The remainder is the extra days.
For 1 June 2026, adding one year and subtracting three months gives 1 March 2027, and adding seven days gives 8 March 2027, which matches the date in the guide above.
What happens if the baby is early or late
Due dates are a guide. NHS maternity care describes babies as early term, full term and late term at set weeks, and it offers monitoring and options if pregnancy goes past the due date. Your midwife will explain what applies in your area, and you should contact them if something feels wrong, such as less movement from the baby, bleeding or pain. Do not rely on a calculator for any health decision.
Common mistakes
- Counting from conception. Weeks are counted from your last period, so you are about two weeks further along than the date of conception suggests.
- Using the wrong date. The first day of your last period is the starting point, not the last day.
- Assuming the date is fixed. A scan may move it, and your midwife will usually go with the scan.
- Using the 28-day rule for a very irregular cycle. The ovulation-based date may be a better start, and the scan is better still.
- Mixing IVF and LMP dates. Use the transfer-based date for IVF, and ask your clinic if unsure.
Glossary
- LMP: the first day of your last menstrual period.
- EDD: estimated due date.
- Gestational age: how far along the pregnancy is, counted from the LMP.
- Trimester: one of three stages of roughly 13 weeks.
- Full term: from 37 weeks.
- Blastocyst: a day-5 embryo, the stage commonly used in IVF transfers.
When you plan around the dates, other guides can help. Calories and TDEE: how many calories do you need? explains how energy needs are estimated, though pregnancy changes them and you should ask your midwife for advice. Alcohol units explained: how to count units in your drinks covers how units are counted, but the NHS advice in pregnancy is specific, so follow your midwife’s guidance.
When to check with a professional
Your midwife, GP or fertility clinic will confirm your due date and tell you about appointments, scans, testing and benefits. If you have a concern about your health or the baby’s, contact them directly.
Frequently asked questions
How accurate is a due date?
It is an estimate. Only a small minority of babies are born on the due date, and many arrive in the weeks either side. Scans can adjust it.
Why am I "four weeks pregnant" so soon?
Because weeks are counted from the first day of your last period, about two weeks before conception. That is why a missed period is already around four weeks.
Does a longer cycle change my due date?
It can. If you ovulate later than day 14, the due date moves later by the same number of days, and a dating scan may confirm it.
How is an IVF due date different?
It starts from the embryo transfer date and the age of the embryo, not the last period, so it is generally more precise.
Is the due date the same as the date I will give birth?
No. It is an estimate in the middle of a window, and only a small minority of babies are born on it. Many arrive in the weeks either side.
Which date does a dating scan use?
The scan measures the baby and gives its own estimate. If it differs from your LMP date by more than a few days, your midwife will usually use the scan date. Ask them how it works in your area.
Do twins have a different due date?
Twin pregnancies often end earlier, and your care team will plan the timing with you. The calculator works from the LMP, so check with your midwife.
Can I use the ovulation tool if I have irregular periods?
It gives a guide, but it assumes a regular cycle. If your cycles vary, the estimate is less reliable, so ask your GP or midwife about other ways to find the date.