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Pregnancy Due Date Calculator
A due date calculator estimates when your baby is due by adding 280 days (40 weeks) to the first day of your last period, adjusted for your cycle length. If your last period started on 1 January 2026 and your cycle is 28 days, your estimated due date is Thursday 8 October 2026.
How to use the pregnancy due date calculator
- Choose what you know. Most people use the first day of their last period. If you know your conception date or had IVF, choose that instead.
- Enter the date. Pick the date from the calendar. For periods, use the first day of bleeding, not spotting.
- Set your cycle length. If your cycles are longer or shorter than 28 days, change the number. The due date moves by the same number of days.
- Read your timeline. See your due date, how many weeks pregnant you are today, your trimester and the dates of key scans.
How due dates are calculated
Doctors and midwives count pregnancy from the first day of your last menstrual period (LMP), about two weeks before conception. That is why a full-term pregnancy is described as 40 weeks even though the baby develops for about 38.
The standard method is Naegele’s rule: add 280 days to the LMP. It assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you probably ovulated later, so the due date moves later by the same number of days:
| Last period | Cycle length | Estimated due date |
|---|---|---|
| 1 January 2026 | 25 days | 5 October 2026 |
| 1 January 2026 | 28 days | 8 October 2026 |
| 1 January 2026 | 32 days | 12 October 2026 |
Using it as a conception date calculator
If you know when you conceived, the due date is 266 days (38 weeks) after conception. The tool also works backwards: from your last period it estimates conception as your cycle length minus 14 days after the LMP. For a period starting on 1 January with a 28-day cycle, the likely conception date is around 15 January 2026.
Sperm can survive up to five days, so conception can happen a few days after sex. The estimate shows the likely date of ovulation and fertilisation, not necessarily the day you had sex.
IVF due dates
With IVF the age of the embryo is known exactly, so the due date is very precise:
- Day-5 transfer (blastocyst): transfer date + 261 days
- Day-3 transfer: transfer date + 263 days
A day-5 transfer on 20 January 2026 gives a due date of 8 October 2026, the same as a natural pregnancy with an LMP of 1 January. Your fertility clinic will give you the official date.
How far along am I? Weeks and trimesters
The calculator shows how many weeks and days pregnant you are today, counted from the LMP date. Pregnancy is split into three trimesters:
| Trimester | Weeks | What usually happens |
|---|---|---|
| First | 0 to 13 weeks 6 days | Organs form; dating scan at 10–14 weeks |
| Second | 14 to 27 weeks 6 days | Anomaly scan at 18–20 weeks 6 days; movements start |
| Third | 28 weeks to birth | Rapid growth; more frequent antenatal checks |
As a pregnancy week calculator it is handy for following weekly guides and planning time off work, but always go by the dates your midwife gives you.
Early term, full term and post-term
Few babies arrive exactly on the due date. The American College of Obstetricians and Gynecologists defines:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
In the UK, if labour has not started by around 41 weeks your midwife will usually discuss a membrane sweep or induction.
Why your scan date may be different
An early ultrasound measuring the baby’s crown–rump length is the most accurate way to date a pregnancy, especially if your cycles are irregular, you were on hormonal contraception recently or you are unsure of your LMP. If the scan date and your LMP date differ by more than a few days, your care team will usually use the scan date. Use the due date it gives you from then on.
Due date calculator FAQs
How accurate is a due date calculator?
It gives the same estimate your midwife would make from your last period. Fewer than 1 in 20 babies are born on the exact day, but most arrive within two weeks either side.
What if I don't know the date of my last period?
Book an appointment with your GP, midwife or doctor. A dating scan can estimate how far along you are from the baby’s size.
Why is pregnancy 40 weeks and not 9 months?
Counting starts from your last period, about two weeks before conception. 40 weeks is about 9 months and one week.
Can my due date change?
Yes. It is often adjusted after the first dating ultrasound. After that it should not change, even if later scans suggest the baby is larger or smaller.
Does this work with irregular periods?
Only roughly. If your cycle length varies a lot, rely on a dating scan instead.
Sources and methodology
Due date = first day of last period + 280 days + (cycle length − 28) days (Naegele’s rule with cycle adjustment); conception + 266 days; day-5 embryo transfer + 261 days; day-3 transfer + 263 days. Gestational age counts from 280 days before the due date. Milestones follow ACOG term definitions and NHS scan windows. All example dates were produced by this calculator.
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