Due Date Calculator — Pregnancy Due Date

First day of last menstrual period

Conception date

Cycle length (days)

days

Estimated Due Date

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End of 1st Trimester

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End of 2nd Trimester

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Current Week

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Likely Conception

About the Due Date Calculator

This calculator uses Naegele's rule, the standard method used by obstetricians worldwide. It adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a standard 28-day cycle. Our calculator adjusts for cycle lengths between 21 and 45 days.

Pregnancy milestones

Week 12: End of first trimester — risk of miscarriage drops significantly
Week 20: Anatomy scan — detailed ultrasound examination
Week 28: Third trimester begins
Week 37: Baby is considered full term
Week 40: Estimated due date

Always confirm your due date with a qualified healthcare provider. Ultrasound measurements in early pregnancy provide the most accurate dating.

How pregnancy due dates are calculated

Naegele's Rule is the standard method: add 7 days to the first day of the last menstrual period (LMP), then subtract 3 months (or add 9 months). This gives a 40-week (280-day) gestational period measured from the LMP. Only 4% of babies are born on their exact due date; 80% are born within 2 weeks either side.

First trimester — weeks 1-12: organ formation, highest miscarriage risk
Second trimester — weeks 13-26: rapid growth, anomaly scan at 20 weeks
Third trimester — weeks 27-40: lung maturation, positioning for birth
Full term — 39-40 weeks is considered full term; 37-38 weeks is early term

Frequently Asked Questions

How is a due date calculated?

The most common method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle. Your healthcare provider may adjust this based on ultrasound measurements.

How accurate is the due date?

Only about 5% of babies are born on their exact due date. Most births occur within two weeks either side. The due date is a best estimate based on average pregnancy length.

What are the pregnancy trimesters?

The first trimester runs from week 1 to week 13. The second trimester is weeks 14–27. The third trimester is weeks 28–40 (birth).

Can I calculate due date from conception?

Yes. If you know your conception date, add 266 days (38 weeks) to get your estimated due date. However, conception date is often hard to pinpoint exactly, making LMP-based calculation more practical.

What is an estimated due date vs gestational age?

Estimated Due Date (EDD) is calculated from the last menstrual period (LMP). Gestational age counts from the LMP, so at conception (about 2 weeks after LMP) you are already 2 weeks "pregnant" by gestational age. Fetal age (time since conception) is always 2 weeks less than gestational age. Healthcare professionals use gestational age as the standard.

How accurate is the due date calculated from LMP?

Only about 4% of babies are born on the exact EDD. Around 80% are born within 2 weeks either side. A due date based on an early ultrasound (8-13 weeks) is often more accurate than LMP calculation because it directly measures fetal size. If the ultrasound date differs from LMP by more than 7-10 days, the scan date is usually used.