Pregnancy Due Date Calculator
Estimate due date from last menstrual period.
Formula
Due = LMP + 280 days
Example
LMP Jan 1 → Due Oct 8.
Understanding the Pregnancy Due Date
Pregnancy calculations operate on a 40-week clock counted from the first day of the last menstrual period - a convention rather than biology, since conception itself usually happens about two weeks into that count. The pregnancy due date calculator works with these conventions used by obstetric practice.
How it actually works
Estimate due date from last menstrual period.
The formula is straightforward arithmetic once the inputs are correct; the value of the calculator is in handling the algebraic manipulation reliably and removing transcription errors. Plug in your specific inputs above and the result appears as you type, so you can immediately see how each variable affects the answer.
What the numbers really say
A pregnancy with LMP March 1 has Naegele's-rule due date December 8 - exactly 40 weeks later. Most spontaneous births occur between 37-42 weeks; only about 5% happen on the calculated due date itself. The number is a planning tool, not a deadline.
The deeper context most users miss
Pregnancy calculations are an interesting case of convention overriding biology. The 40-week count from LMP is purely arithmetic convention - real biological pregnancy duration averages closer to 38 weeks from conception. This works clinically because LMP is something most women know precisely while conception date is rarely known. The mismatch creates the 'I am 4 weeks pregnant and just found out' confusion - the woman is actually 2 weeks past conception, but conventionally counted as 4 weeks. Modern ultrasound can correct dating if the convention does not match measured fetal development, which happens in about 10% of pregnancies with irregular cycles or other timing complications.
What people get wrong
- Confusing conception date with LMP date. Pregnancy weeks by convention include the two weeks before conception. A 4-week pregnancy is actually 2 weeks past conception.
- Treating the due date as a deadline. Only about 5% of births happen on the due date itself; 80%+ happen within two weeks of it.
- Using LMP alone for irregular cycles. Naegele rule assumes a 28-day cycle. Longer or shorter cycles shift dating. Get an early ultrasound for confirmation when cycles are irregular.
- Misinterpreting "weeks". Obstetric weeks are weeks of gestation from LMP, not weeks since conception or weeks of physical symptoms.
When this calculator helps most
The pregnancy due date calculator is most useful when you are making a real decision - comparing options, sizing a commitment, sanity-checking a quote, or planning ahead. The output is precise to your inputs; the inputs themselves are the place to slow down. Spend extra time on the assumptions you are making about rate, term, timing, or context-specific variables - those swing the answer far more than the formula's arithmetic does. A 5% change in the input often produces a 10-20% change in the output, which means small input errors compound into large output errors.
Where the math comes from
Naegele rule (Franz Naegele, 1830s) is the classical formula. ACOG (American College of Obstetricians and Gynecologists) and SMFM (Society for Maternal-Fetal Medicine) publish current pregnancy dating recommendations, recommending first-trimester ultrasound when available.
Questions and answers
How accurate is the due date?
Within 2 weeks for most pregnancies. Only about 5% deliver exactly on the calculated date. First-trimester ultrasound is more accurate than LMP-only calculation.
What if I do not know my LMP?
Early ultrasound (before 20 weeks) is the most accurate alternative. After 20 weeks, dating becomes less precise.
Are pregnancy weeks counted differently in different countries?
Most use Naegele rule. France uses 41 weeks from LMP. Some sources distinguish 'gestational age' (from LMP) and 'fetal age' (from conception, about 2 weeks less).
How does this affect prenatal scheduling?
Standard prenatal visits, screenings, and tests are timed by gestational age in weeks. Your provider's chart reflects the agreed dating; that is the schedule that drives appointments.
What about IVF or known conception date?
IVF dating is more accurate than LMP since conception event is exactly known. Add 266 days to conception (vs 280 from LMP) for the same due date.
Sources & References
Authoritative references consulted in building this calculator and educational content. These are primary sources — check directly for the most current figures.
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