Your baby's due date,
three ways.

Field guide

How to calculate your due date.

A pregnancy is counted in gestational weeks from the first day of the last menstrual period (LMP), not from conception. The convention exists because the LMP is something most people can pin down on a calendar, while ovulation is harder to know precisely. The estimated due date (EDD) is exactly 40 weeks (280 days) after the LMP.

Naegele's rule

Named after the 19th-century German obstetrician Franz Karl Naegele who proposed it in 1812, Naegele's rule is the standard EDD formula:

That's mathematically identical to a much simpler statement: EDD = LMP + 280 days. Either form gives the same answer for any starting date.

The cycle-length adjustment

Naegele's rule assumes a 28-day cycle with ovulation on day 14. Real cycles vary. If yours runs longer or shorter, ovulation (and therefore conception) shifts — and the EDD shifts by the same amount.

A 32-day cycle adds 4 days to the EDD. A 25-day cycle subtracts 3 days. This calculator does the adjustment automatically when you enter a non-default cycle length.

From an ultrasound report

First-trimester ultrasounds (before 14 weeks) measure the embryo's crown-rump length (CRL) and produce a gestational age accurate to about ±5–7 days. To turn that into an EDD:

Use the “Ultrasound” mode in the calculator above and enter both the scan date and the gestational age the report quoted. ACOG guidelines say the ultrasound date supersedes the LMP date if they disagree by more than 5–7 days in the first trimester (or 10–14 days in the second).

From a known conception date

If you know your conception date directly, typically from IVF or careful fertility tracking — that's an even cleaner anchor than LMP. Pregnancy is 266 days (38 weeks) from conception, with the extra 14 days of “gestational age” being the two pre-ovulation weeks of the cycle.

Is the due date 100% accurate?

No. Only about 5% of babies arrive on the EDD itself. Roughly half arrive within a week of it, and about 90% arrive between 38 and 42 weeks. The EDD is the median of a probability distribution, not a deadline.

The reasons for variation are biological, not statistical:

• Cycle and ovulation variability. Even within a single person, ovulation can drift by several days month-to-month.
• LMP recall error. Most people's memory of an exact period date is off by 1–3 days.
• Individual gestation length. Healthy, uncomplicated pregnancies range from 37 to 42 weeks at term, a five-week window.
• First vs. subsequent pregnancies. First pregnancies tend to be slightly longer than subsequent ones — by an average of about 5 days.

The clinically important categories used by the American College of Obstetricians and Gynecologists (ACOG):

• Preterm: before 37 weeks.
• Early term: 37⁰ to 38⁶ weeks.
• Full term: 39⁰ to 40⁶ weeks (the lowest-risk window for elective delivery).
• Late term: 41⁰ to 41⁶ weeks (induction usually discussed).
• Post-term: 42⁰ weeks and beyond (induction routine).

Worked example

LMP of 1 January 2026, standard 28-day cycle:

• Conception ≈ 15 January 2026 (LMP + 14 days)
• End of first trimester ≈ 1 April 2026 (week 13⁺⁶)
• Anatomy scan ≈ 21 May 2026 (week 20)
• Viability ≈ 18 June 2026 (week 24)
• Full term ≈ 1 October 2026 (week 39)
• EDD = 8 October 2026 (week 40)

Shift to a 32-day cycle and every date moves four days later. Shift to a 25-day cycle and they move three days earlier.

Disclaimer

This calculator is for educational use only and is not a substitute for professional prenatal care. Pregnancy timelines are highly individual, and a small percentage of pregnancies don't fit the standard pattern at all. Use this page as a planning landmark; let your obstetrician, midwife, or family physician be the authoritative source on every clinical question.