Enter the first day of your last menstrual period (LMP) and the calculator returns an estimated due date 40 weeks (280 days) later. This is Naegele's rule, the standard obstetric estimate used since 1812. About 4 % of babies arrive on the predicted date; 80 % arrive within two weeks of it; the remaining 20 % go earlier or later. The calculator also shows your current gestational age in weeks and days. For broader pregnancy resources see your healthcare provider — this is a math tool, not medical advice. For other "what date is N days from today" questions, see days from today.
Common use cases
- Pregnancy planning and milestones. The estimated due date anchors prenatal-visit scheduling, the timing of ultrasounds and blood tests (each tied to a specific gestational week), and the planning of maternity leave. Most healthcare apps and clinic intake forms ask for both the LMP and the calculated due date.
- Tracking gestational age week by week. The calculator shows current gestational age (weeks + days since LMP) so you can match against developmental milestones — heartbeat audible at ~6 weeks, anatomy scan at 18–22 weeks, viability at 24 weeks, full-term at 37+ weeks. Update the calculator each visit if your provider revises the due date based on early ultrasound.
- Maternity leave and HR planning. Many countries grant maternity leave starting some weeks before the due date. Use the calculator to find the due date, then count back the appropriate number of weeks for your jurisdiction's leave start.
- Trying-to-conceive cycle math. Knowing the typical 280-day timeline helps with planning conception around target birth months — for example, conceiving in March generally produces a December baby. This isn't precise (cycle lengths vary, ovulation isn't always day 14), but it's the right order of magnitude.
How it works
The calculator implements Naegele's rule: estimated due date = first day of last menstrual period + 280 days. The 280-day figure assumes a 28-day cycle with ovulation on day 14, and counts 266 days of gestation from ovulation. The classical mnemonic is "+1 year, -3 months, +7 days" but the calculator just adds 280 days directly. Modern practice often refines the estimate based on first-trimester ultrasound (crown-rump length); when that revision happens, switch to the ultrasound-based date and discontinue Naegele.
Worked examples
New-Year LMP
LMP 2026-01-01.
Result: Due 2026-10-08.
280 days after Jan 1 lands in early October. The 40-week mark is 9 months and 1 week — the famous "9 months" figure understates the actual gestation length.
Mid-summer LMP
LMP 2025-06-15.
Result: Due 2026-03-22.
Mid-June LMP gives a late-March due date. The Northern Hemisphere "spring baby" pattern.
Edge cases & gotchas
- Naegele assumes a 28-day cycle. If your typical cycle is shorter or longer than 28 days, the actual ovulation day shifts and the LMP-based due date is biased. Long cycles → due date estimate is too early; short cycles → too late. First-trimester ultrasound (crown-rump length measurement) gives a more accurate estimate that's independent of cycle length.
- Implantation bleeding can be confused for LMP. Some people mistake light bleeding around implantation (~6–12 days after conception) for a normal period. Using that bleeding as the "LMP" gives a due date that's about 2 weeks too late. Ultrasound resolves the ambiguity if there's any doubt.
- Predicted vs actual delivery date. Only ~4 % of births occur on the predicted due date. ~80 % arrive within ±2 weeks; the remainder go earlier (preterm, < 37 weeks) or later (post-term, > 42 weeks). The "due date" is a midpoint estimate, not a deadline.
- IVF and assisted-conception cycles use a different formula. For IVF pregnancies, the embryo transfer date (or known fertilization date) is the anchor, not the LMP. The standard formula is transfer date + 266 days for day-5 blastocyst transfer, or + 263 days for day-3 cleavage transfer. Naegele's rule does not apply.
Frequently asked questions about Pregnancy Due Date Calculator
Is Naegele's rule accurate?
It's a population average, not an individual prediction. About 4 % of babies arrive on the predicted date; 80 % arrive within two weeks of it. For an individual pregnancy, first-trimester ultrasound is more accurate (typically within 5 days vs Naegele's ±2 weeks).
How is gestational age different from fetal age?
Gestational age is counted from the first day of the LMP, so it's about 2 weeks longer than the actual age of the fetus (which is counted from conception). When your provider says "you're at 12 weeks gestational age," the embryo is about 10 weeks old. Almost all clinical references use gestational age.
When should I use ultrasound instead of LMP for the due date?
When LMP is uncertain (irregular cycles, recent contraceptive use, postpartum, breastfeeding) or when first-trimester ultrasound disagrees with LMP-based estimate by more than 5–7 days. Most providers re-date the pregnancy after the dating ultrasound (at 8–13 weeks) and use the ultrasound date thereafter.
Does the calculator work for IVF pregnancies?
Not directly. For IVF, anchor on the embryo transfer date or known fertilization date. Day-5 blastocyst transfer + 266 days = due date. Day-3 cleavage transfer + 263 days = due date. The LMP-based Naegele rule does not apply.
What does "term" pregnancy mean?
Full-term: 37 weeks 0 days through 41 weeks 6 days. Preterm: < 37 weeks. Post-term: ≥ 42 weeks. Within full-term, "early term" is 37–38+6, "full term" is 39–40+6, and "late term" is 41–41+6. Most providers will recommend induction at 41 weeks if labor hasn't started spontaneously.
What's the legal validity of the calculated date?
For maternity-leave applications, payroll dates, and insurance forms, the due date your healthcare provider records is what matters — not the calculator output. If they re-date based on ultrasound, use that. The calculator is an unofficial estimator only.
Glossary
- LMP
- Last menstrual period. The first day of the most recent menstrual period before pregnancy. Used as the anchor date for Naegele's rule.
- Naegele's rule
- A 1812 formula for estimating due date: LMP + 280 days. Equivalently, "+1 year, -3 months, +7 days." Standard obstetric estimate when no ultrasound is available.
- Gestational age
- Time elapsed since the first day of the LMP, measured in completed weeks and days. About 2 weeks longer than the actual age of the fetus, which is counted from conception.
- Estimated due date (EDD)
- The midpoint estimate of when delivery will occur, based on Naegele's rule (LMP + 280 days) or refined by first-trimester ultrasound. Only ~4 % of babies arrive on this date.
- Term
- A pregnancy that has reached 37 completed weeks. Subdivided into early term (37–38+6), full term (39–40+6), and late term (41–41+6). Below 37 weeks is preterm; ≥ 42 weeks is post-term.