About This Calculator
The Pregnancy Due Date Calculator uses Naegele's Rule — the standard clinical method for estimating a baby's due date. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), adjusted for cycle lengths other than 28 days.
How Naegele's Rule Works
Naegele's Rule is based on an average human gestation of 280 days (40 weeks) from the LMP. If your cycle is longer or shorter than 28 days, the estimated due date is adjusted accordingly:
EDD = LMP + 280 days + (Cycle Length − 28) days
Worked example: an LMP of 1 June 2026
If your last menstrual period began on 1 June 2026 with an average 28-day cycle, the calculator returns the following milestones:
| Milestone | Date |
|---|---|
| Estimated conception (LMP + 14 days) | 15 June 2026 |
| End of 1st trimester (≈13 weeks) | 31 August 2026 |
| End of 2nd trimester (≈27 weeks) | 7 December 2026 |
| Full term (39 weeks) | 1 March 2027 |
| Estimated due date (LMP + 280 days) | 8 March 2027 |
Enter your own last period date and cycle length above to see your due date, current week and trimester instantly.
Understanding Trimesters
| Trimester | Weeks | What happens |
|---|---|---|
| First trimester | Weeks 1–13 | Major organ development begins; the risk of miscarriage is highest in the early weeks. |
| Second trimester | Weeks 14–26 | The baby grows rapidly; many parents learn the sex and have the anomaly scan around week 20. |
| Third trimester | Weeks 27–40 | Final growth and preparation for birth; antenatal visits become more frequent. |
Key Appointments and Milestones
- Booking appointment (weeks 8–12) — your first detailed antenatal visit with a midwife.
- Dating scan (weeks 11–14) — confirms the due date and checks for a multiple pregnancy.
- Anomaly scan (around week 20) — a detailed scan of the baby's development.
- Viability milestone (week 24) — the point from which a baby has a chance of surviving outside the womb.
- Full term (from week 39) — the baby is fully developed and ready for birth.
Important Notes
Only about 5% of babies are born exactly on their due date. A baby is considered full-term from 39 weeks. Your midwife or doctor will typically confirm your EDD using an early ultrasound scan (dating scan), which is more accurate than LMP-based calculations.
Frequently Asked Questions
How is the due date calculated? Naegele's rule: first day of the last menstrual period + 280 days (40 weeks). The calculator also supports the conception-date method (+ 266 days) when the conception date is known, and adjusts for cycle length.
How accurate is the estimated due date? Only about 4–5% of babies arrive on the exact predicted date, but around 80% arrive within two weeks of it. First-trimester ultrasound dating is more accurate than an LMP-based estimate.
What if my cycle isn't 28 days? Cycles longer or shorter than 28 days shift the conception window. The calculator adjusts the estimate by adding the difference between your cycle length and 28 days, so the due date stays accurate.
How many weeks pregnant am I? Pregnancy is counted in weeks from the first day of your last menstrual period, not from conception — so at conception you are already about 2 weeks pregnant. The calculator shows your current week and day, for example "Week 8 + 3 days", from today's date and your LMP.
Can I calculate my due date from the conception or IVF transfer date? Yes. From a known conception date, the due date is roughly conception + 266 days. For IVF, add 266 days to the fertilisation date, or use embryo age — a day-5 transfer is due 261 days after transfer. An early dating scan remains the most accurate confirmation.
What is the difference between full term and 40 weeks? The 40-week due date is an estimate, not a deadline. A pregnancy is full term from 39 weeks 0 days to 40 weeks 6 days, "early term" at 37–38 weeks and "late term" at 41 weeks. Birth from 37 weeks is generally considered safe.
Last reviewed: June 2026. Estimates based on Naegele's rule only — not medical advice. Your midwife or obstetrician will confirm your estimated delivery date from an early ultrasound scan.