Pregnancy Due Date Calculator
Calculate your estimated due date from your last menstrual period.
See your current week, trimester, and key pregnancy milestones.
Naegele’s Rule is the internationally accepted standard for estimating a pregnancy due date (Estimated Date of Delivery, or EDD) from the first day of the last menstrual period (LMP). Developed by German obstetrician Franz Karl Naegele in the early 1800s, the rule has remained the clinical standard for over two centuries.
Core formula:
EDD = Last Menstrual Period + 280 days (40 weeks)
Equivalent calculation:
EDD = LMP − 3 months + 7 days + 1 year
Variable definitions:
- LMP: first day of the last menstrual period (the starting reference point, even though conception occurs ~2 weeks later)
- 280 days: the standard assumed pregnancy length: 40 weeks from LMP = 38 weeks from conception
- Gestational Age: the age of the pregnancy counted from LMP (not from conception)
- Trimester: pregnancy is divided into three 13-week periods
Worked example: LMP: January 15, 2026. Add 280 days = October 22, 2026 (estimated due date). Using the shortcut: Jan 15 − 3 months = Oct 15, + 7 days = Oct 22, 2026.
At any given date: Gestational weeks = (Today’s date − LMP) / 7
Pregnancy milestone timeline:
| Gestational Week | Milestone |
|---|---|
| Week 6 | Heartbeat first detectable by transvaginal ultrasound |
| Week 10–12 | End of first trimester; miscarriage risk drops significantly |
| Week 18–20 | Anatomy scan (mid-pregnancy ultrasound) |
| Week 24 | Viability threshold — survival outside womb becomes possible |
| Week 28 | Third trimester begins |
| Week 37 | Early term |
| Week 39–40 | Full term |
| Week 42 | Post-term — induction typically recommended |
Accuracy notes:
- Only about 5% of babies are born on their exact due date
- The full-term window spans weeks 39–40 (or 37–42 for most normal deliveries)
- Naegele’s Rule assumes a 28-day menstrual cycle with ovulation on Day 14
- Women with cycles shorter or longer than 28 days may have an EDD that is off by the difference
- A first-trimester ultrasound (8–10 weeks) provides the most accurate dating and can adjust the EDD by up to 5–7 days
This calculator is for informational and planning purposes only. Always confirm your due date with your healthcare provider.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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