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
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.