Quick answer
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last period. If your period started on 1 January 2026, your estimated due date is 8 October 2026. With a longer cycle, add the extra days; from conception add 266 days, and from a day-5 IVF transfer add 261 days.
- LMP method: LMP + 280 days (+ cycle length − 28)
- IVF: day-5 transfer + 261 days, day-3 transfer + 263 days
- Trimesters: weeks 1–13, 14–27, 28–40
What is Naegele’s rule?
Naegele’s rule estimates the due date by adding one year, subtracting three months and adding seven days to the first day of your last period. That is the same as adding 280 days.
Example: last period on 1 January 2026, 28-day cycle
1 January 2026 + 280 days = 8 October 2026. With a 32-day cycle, the due date moves 4 days later, to 12 October 2026.
How do you calculate a due date from conception or IVF?
If you know when you conceived, add 266 days (38 weeks). This skips the roughly two weeks between your period and ovulation. A conception date of 24 March 2026 gives a due date of 15 December 2026.
For IVF, the embryo’s age at transfer is known exactly, so the due date calculator counts from the transfer date:
- Day-5 (blastocyst) transfer: add 261 days. A transfer on 15 May 2026 gives 31 January 2027.
- Day-3 transfer: add 263 days. The same date gives 2 February 2027.
How many weeks pregnant are you?
Pregnancy weeks are counted from the first day of your last period, not from conception. The calculator counts the days from that date to today and divides by 7. With conception or IVF dates, it works back to the matching start date first.
If your last period began on 10 March 2026, on 21 October 2026 you are 32 weeks and 1 day pregnant. That is the third trimester, with a due date of 15 December 2026. With a 35-day cycle, the due date moves to 22 December 2026.
Your timeline also shows when each scan window and trimester begins, and when early term (37 weeks) and full term (39 weeks) start.
Frequently asked questions
How is the pregnancy due date calculated?
Doctors usually add 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele’s rule. If your cycle is longer or shorter than 28 days, the date moves by the difference.
Can I calculate my due date from the conception date?
Yes. Add 266 days (38 weeks) to the date of conception. The calculator does this when you choose “Conception date”.
How is the due date calculated for IVF?
For a day-5 (blastocyst) embryo transfer, add 261 days to the transfer date. For a day-3 embryo transfer, add 263 days.
How accurate is the due date?
It is an estimate. Only about 5% of babies are born on their due date, and most arrive between 37 and 42 weeks. An early ultrasound gives the most reliable date.
When does each trimester start?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.
What is full term in pregnancy?
A pregnancy is full term from 39 weeks 0 days to 40 weeks 6 days. Weeks 37 and 38 are early term. The calculator’s timeline shows the date each of these begins for you.
Can a due date calculator replace a dating scan?
No. The calculator uses the same rules doctors start with, but an early ultrasound measures the baby and gives a more reliable date. If the two differ, follow the date your doctor or midwife gives you.