How the due date calculator works
Pregnancy is counted from the first day of your last period, not from the day you conceived. A typical pregnancy lasts 40 weeks, or 280 days, from that date. This is the same method (often called Naegele's rule) that midwives use at your booking appointment before your first scan:
due date = first day of last period + 280 days
with a longer or shorter cycle: + (cycle length − 28) days
The 280 days assumes a 28-day cycle, where you ovulate around day 14. If your cycle is usually 32 days you probably ovulated about four days later, so the calculator moves your due date four days later too. If you know your conception date, your due date is 266 days (38 weeks) after it.
Worked example
If the first day of your last period was Sunday 1 March 2026 and your cycle is 28 days:
- Due date: 1 March 2026 + 280 days = Sunday 6 December 2026
- On 1 June 2026, 92 days have passed, so you're 13 weeks 1 day pregnant and in your second trimester
- With a 32-day cycle, the due date moves to Thursday 10 December 2026
If you conceived on 10 January 2026, your due date is 266 days later: Saturday 3 October 2026.
How many weeks pregnant am I?
Weeks of pregnancy are counted in completed weeks and days from the first day of your last period. So "13 weeks 1 day" means 13 full weeks plus one day, and you're in your 14th week. It also means that in the first two weeks or so of "pregnancy", you weren't actually pregnant yet.
| Trimester | Weeks |
|---|---|
| First trimester | 0 to 12 weeks |
| Second trimester | 13 to 27 weeks |
| Third trimester | 28 weeks to birth |
Key NHS appointments and dates
- Booking appointment: ideally before 10 weeks. Contact your GP surgery or local maternity service as soon as you know you're pregnant; in many areas you can self-refer online.
- Dating scan (12-week scan): offered at around 10 to 14 weeks. The sonographer measures the baby to check your due date, which may be changed as a result.
- Whooping cough vaccine: usually given at around 20 weeks, though you can have it from 16 weeks.
- 20-week screening scan: also called the mid-pregnancy or anomaly scan, usually between 18 and 21 weeks. It checks how your baby is developing.
- Full term: babies born from 37 weeks are considered full term.
- Going overdue: if you haven't given birth by 41 weeks, your midwife will talk to you about a membrane sweep and induction.
Why your due date is only an estimate
Very few babies arrive on their due date, and most are born within a week or two either side. The calculation assumes regular cycles, and many people aren't sure exactly when their last period started. That's why the NHS offers a dating scan: measuring the baby gives a more reliable estimate, and if it differs from your period-based date, your midwife will use the scan date from then on.
Irregular periods, recently stopping hormonal contraception, or bleeding in early pregnancy can all make the period-based date less accurate. If that applies to you, rely on your scan date.
Planning ahead
Your due date also sets the timetable for maternity leave and pay. To get Statutory Maternity Pay you must tell your employer you're pregnant at least 15 weeks before the week your baby is due. Use the maternity pay calculator to see what you'll get, or the date calculator to count the days between any two dates.
If you have any pain, bleeding or concerns at any stage, contact your midwife, GP or NHS 111 straight away.
Frequently asked questions
How is my due date calculated?
The NHS adds 280 days (40 weeks) to the first day of your last period. For example, if your last period started on 1 March 2026, your estimated due date is 6 December 2026.
When is the 12-week scan?
The dating scan is offered at around 10 to 14 weeks of pregnancy. It confirms how many weeks pregnant you are and can change your due date.
When is the 20-week scan?
The 20-week screening scan, also called the anomaly scan, usually takes place between 18 and 21 weeks of pregnancy.
Can I work out my due date from conception?
Yes. Add 266 days (38 weeks) to the date you conceived. Conceiving on 10 January 2026 gives a due date of 3 October 2026.
What are the chances of giving birth on my due date?
Low. Only around 4% of babies are born on their due date. Most arrive by 41 weeks, and anywhere from 37 to 42 weeks is normal.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.