Pregnancy Due Date Calculator
Estimated delivery date based on Naegele's rule.
Naegele's Rule
The standard due-date formula used worldwide by obstetricians is Naegele's Rule, named after German obstetrician Franz Karl Naegele (1778-1851). It works in three steps:
- Start with the first day of the last menstrual period (LMP)
- Subtract 3 months
- Add 7 days and 1 year
Mathematically equivalent: add 280 days (40 weeks) to the LMP. The rule assumes a 28-day cycle with ovulation on day 14. For different cycle lengths, the conception estimate shifts accordingly. The Wikipedia article on Naegele's Rule details the history and current limitations.
What the data actually shows
A landmark 2013 study in Human Reproduction (Jukic et al., peer-reviewed) tracked 125 healthy single-baby pregnancies with precise ovulation dating and found:
| Delivery Outcome | Probability |
|---|---|
| Delivered on the exact EDD | ~4% |
| Within ±3 days of EDD | ~30% |
| Within ±7 days of EDD | ~60% |
| Within ±14 days of EDD | ~85% |
| Preterm (before 37 weeks) | ~10% |
| Post-term (after 42 weeks) | ~5-7% |
Same study found natural pregnancy duration varied by up to 37 days between individuals even in healthy, dated-by-ovulation pregnancies. The EDD is a target — not a deadline.
How Accurate Is the Due Date?
Only about 4-5% of babies are born on their exact due date. About 60% arrive within one week before or after; 85% within two weeks. First-trimester ultrasound (especially crown-rump length measurement between weeks 7-13) is more accurate than LMP-based dating and becomes the official EDD if it disagrees with LMP by more than 7 days. Per ACOG guidelines, ultrasound dating before 14 weeks is the most reliable method.
The Three Trimesters
| Trimester | Weeks | Characteristics |
|---|---|---|
| First | 1-13 | Highest morning sickness, fatigue, miscarriage risk drops after week 12 |
| Second | 14-27 | "Honeymoon" phase — most people feel best; anatomy scan around week 20 |
| Third | 28-40+ | Rapid weight gain, frequent visits, third-trimester monitoring intensifies |
Conception Date vs LMP Date
If you know your exact conception date (rather than LMP), add 266 days instead of 280 — conception occurs about 14 days into the cycle for someone with a regular 28-day cycle, so the gestational age is 2 weeks ahead of the embryo's actual age. This calculator handles both inputs.
When to Recalculate
Your EDD may be adjusted by your provider after the first ultrasound. Following ACOG's Committee Opinion 700 guidelines:
- Ultrasound before 9 weeks: replaces LMP if they differ by > 5 days
- Ultrasound at 9-13 weeks 6 days: replaces LMP if they differ by > 7 days
- Ultrasound at 14-15 weeks 6 days: replaces LMP if they differ by > 7 days
- Ultrasound at 16-21 weeks 6 days: replaces LMP if they differ by > 10 days
- Ultrasound at 22-27 weeks 6 days: replaces LMP if they differ by > 14 days
- After 28 weeks: ultrasound dating is less reliable than the original EDD
Reliable Pregnancy Resources
- American College of Obstetricians and Gynecologists (ACOG)
- UK NHS Pregnancy Guide
- CDC Pregnancy Resources
- WHO Maternal Health
Important
This is an estimation tool only. Always confirm dates and pregnancy progress with a qualified healthcare provider, who has access to ultrasound and clinical markers far more accurate than any calendar-based formula.
Frequently Asked Questions
What if my cycle isn't 28 days?
Adjust the cycle-length field in the calculator. For a 35-day cycle, conception happens around day 21 instead of day 14, so the due date shifts about a week later than the standard Naegele rule predicts.
Can I be induced if I'm overdue?
Per ACOG, induction is typically discussed at 41 weeks if no other concerns; routine induction at 39 weeks (the ARRIVE trial outcome) is now offered in many practices for low-risk first pregnancies. Discuss timing with your provider.
Does the calculator account for IVF?
For IVF pregnancies, the actual transfer date is known precisely. Add 266 days to the date of fertilization (or adjust based on embryo age at transfer). IVF EDDs are more accurate than naturally-conceived ones because ovulation timing isn't a guess.
Why are second babies often "earlier"?
Research shows the average pregnancy duration is about 4-5 days shorter for second and subsequent babies vs first. The mechanism isn't fully understood — possibly cervical readiness from the first labor.