How to Calculate Your Pregnancy Due Date

📖 5 min read · Updated May 2025 · HealthPregnancy

Your estimated due date (EDD) is calculated using Naegele's rule — the most widely used method, adopted by obstetric guidelines worldwide.

Naegele's Rule

Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Equivalently: take the first day of your LMP, add 1 year, subtract 3 months, add 7 days.

Example: LMP = 1 January 2025. Due date = 1 January 2026 − 3 months + 7 days = 1 October 2025 + 7 days = 8 October 2025.

Adjusting for Cycle Length

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle differs, adjust: for each day your cycle is longer than 28 days, add one day to the due date. For a 32-day cycle, add 4 days. Shorter cycles push the date earlier.

How Accurate Is the EDD?

Only 4–5% of babies arrive on their exact due date. About 80% are born within 2 weeks of the EDD. Early ultrasound (before 14 weeks) provides the most accurate gestational age — better than LMP calculation for women with irregular cycles.

Pregnancy Trimesters

First trimester: weeks 1–12 (conception through 12 weeks)
Second trimester: weeks 13–26 (anatomy scan typically at 18–20 weeks)
Third trimester: weeks 27–40 (rapid growth, birth preparation)

Full-term pregnancy is 39–40 weeks. Early term is 37–38 weeks. Late term is 41 weeks. Post-term is 42+ weeks.

Calculate your estimated due date and trimester milestones.

Use the Due Date Calculator →

Frequently Asked Questions

How is the due date calculated?

Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle. For longer or shorter cycles, adjust by the difference in days from 28.

Is an ultrasound more accurate than LMP for due dates?

Yes — an ultrasound before 14 weeks (first trimester) is the most accurate way to determine gestational age, particularly for women with irregular cycles or who are unsure of their LMP. After 14 weeks, ultrasound accuracy decreases.

What if my due date changes after an ultrasound?

If the ultrasound date differs from the LMP-based date by more than 5–7 days (first trimester) or 10–14 days (second trimester), doctors usually adjust the due date to match the ultrasound measurement.

What is considered full term?

Full term is 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term is 42+ weeks.

Can I calculate a due date from conception date?

Yes — add 266 days (38 weeks) from the conception date. Conception typically occurs about 14 days after the LMP, which is why LMP-based calculations add 280 days (266 + 14).