How to use the Pregnancy Due Date Calculator
- Choose how to calculate: last period, conception date or IVF transfer.
- Enter the date (and your usual cycle length for the last-period method).
- See your due date, current week, trimester and key dates.
How the due date is calculated
Doctors usually use Naegele’s rule: the due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP), assuming a 28-day cycle. For longer or shorter cycles the date is shifted by the difference. From a known conception date the due date is 266 days later; for IVF it is 263 days after a day-3 transfer or 261 days after a day-5 (blastocyst) transfer.
Only about 1 in 20 babies arrive on the due date
A due date is an estimate. Most babies are born between 37 and 42 weeks. Your doctor may adjust the date after an early dating ultrasound, which is more accurate than calculation alone.
Trimesters
| Trimester | Weeks |
|---|---|
| First | Week 1 – 13 |
| Second | Week 14 – 27 |
| Third | Week 28 – birth |
Frequently asked questions
Why is pregnancy counted from the last period, before conception?
The date of the last period is usually known while the exact conception date is not, so by convention pregnancy weeks are counted from the LMP. This means you are considered about 2 weeks pregnant at conception.
Is this a replacement for a doctor?
No. Use it for planning, and follow your doctor’s dates and advice for antenatal care.