Pregnancy Due Date Calculator
Estimate your pregnancy due date with Naegele's Rule (LMP), known conception date, or first-trimester ultrasound. Cycle-length adjustment for cycles 21–35 days. Shows current gestational age, trimester, and key prenatal milestones.
Based on 2 official sources↓Estimated due date
2027-02-05
Estimated conception: 2026-05-15
Today you are
0w 0d
Trimester
—
Days to go
280
Full-term window (37–42w)
Not yet
Pregnancy milestones
- End of 1st trimester2026-07-31Risk of miscarriage drops significantly. NT scan window 11-14 weeks.
- Anatomy ultrasound (20 weeks)2026-09-18Detailed anatomy scan; possible to determine fetal sex.
- End of 2nd trimester2026-11-06Glucose tolerance test (24-28 weeks) typically completed by now.
- Viability threshold (24 weeks)2026-10-16Modern NICU survival rates ~50% at 24 weeks, ~80% at 26 weeks.
- Full-term window starts (37 weeks)2027-01-15Delivery from this date is medically full-term.
- Due date (40 weeks)2027-02-05Estimated due date. Only ~5% deliver exactly on this date.
- Post-term (42 weeks)2027-02-19Beyond this point, induction usually recommended (placental aging).
Pregnancy due dates are estimates, not appointments. Only about 4-5% of babies arrive on the predicted date — the rest spread across a roughly 5-week window centred on it. The number you get from this calculator is a planning anchor, not a deadline. Naegele's Rule (LMP + 280 days) is the textbook formula and works fine for most people, but a first-trimester ultrasound trumps it whenever available because cycle length and ovulation timing vary more than the textbook assumes.
This calculator runs three methods so you can pick the one that fits your data: last menstrual period (LMP) with cycle-length adjustment, known conception date (helpful for IVF or fertility tracking), and first-trimester ultrasound (CRL — crown-rump length — is the gold standard between 6 and 13 weeks). Pick the method, give it a date, and the calculator returns the due date plus current gestational age, trimester, days remaining, and a milestone calendar (NT scan, anatomy scan, viability, full-term window).
How due dates are estimated
Naegele's Rule (LMP + 280 days). Take the first day of your last menstrual period, add 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14. Decent default if you track periods reliably.
Cycle-length adjustment. Real cycles run 21-35 days. The rule under-estimates the due date for longer cycles and over-estimates for shorter ones. Adjustment: add (cycle_length − 28) days. Example: 32-day cycle → due date moves 4 days later. ACOG endorses this for cycles in the 21-35 day range; outside that range, ultrasound becomes the only reliable method.
Conception date method. When you know the day of conception (IVF transfer, ovulation tracking, single-event timing), add 266 days (38 weeks). More accurate than LMP because it skips the cycle-length guess.
First-trimester ultrasound (gold standard). Between 6 and 13 weeks the embryo grows at a predictable rate, so the crown-rump length (CRL) measurement gives a gestational age accurate to within ±5 days. Tell the calculator the scan date and the gestational age the sonographer reported (e.g. "11 weeks 4 days"), and it back-calculates the due date. ACOG recommends adopting the ultrasound estimate over LMP if the difference exceeds 7 days in the first trimester.
Why your real delivery date will probably differ. A 2013 study (Jukic et al., NIH) tracked natural conceptions and found the spread of delivery from estimated date was wider than the textbook suggests — roughly half delivered within ±7 days, the rest ranged from −18 days to +13 days. First-time mothers tend to deliver slightly later than the estimate. Don't book non-refundable travel near the due date.
Practical examples
Standard 28-day cycle, LMP method
Setup: LMP was March 1. Regular 28-day cycle.
March 1 + 280 days = **December 6**. Conception ≈ March 15.
Takeaway: Standard estimate. Once your first-trimester ultrasound comes in, swap to that estimate if it differs by more than 7 days.
Long 32-day cycle
Setup: LMP was March 1. Cycles run 32 days consistently.
March 1 + 280 + (32 − 28) = March 1 + 284 days = **December 10**. Four-day shift later than the textbook.
Takeaway: Without the cycle adjustment, you would arrive at the OB four days "early" by their counting. Volunteer your real cycle length at the first appointment.
IVF transfer (day-5 blastocyst)
Setup: Day-5 blastocyst transferred April 10. Conception is the transfer date minus 5 days = April 5.
April 5 + 266 days = **December 27**.
Takeaway: IVF dates are the most precise input you can give. The clinic typically supplies the due date directly; this calculator reproduces their math so you can verify.
First-trimester ultrasound corrects an off cycle
Setup: LMP-based estimate was Dec 1, but the 12-week ultrasound on Sep 15 measured the fetus at "11 weeks 0 days".
280 − (11 × 7 + 0) = 203 days remaining. Sep 15 + 203 days = **April 6 next year** … wait, that is months later than the LMP estimate. The real takeaway: the LMP-based date was off by months because the cycle in question was anovulatory or atypical. Switch to the ultrasound estimate.
Takeaway: When LMP and ultrasound disagree by more than a week in the first trimester, ACOG advises taking the ultrasound. Document both in your records — affects everything downstream (NT scan, anatomy scan, induction timing).
When this calculator is not enough
High-risk pregnancies, multiples, or cycles outside the 21-35 day range need clinical dating, not formula. This calculator is a planning tool, not a substitute for prenatal care.
If you suspect the LMP and ultrasound estimates disagree significantly, do not "average" them — pick the ultrasound estimate (especially first-trimester) and document the discrepancy in your record. Mid-pregnancy redating is generally not recommended once a first-trimester ultrasound has been recorded.
Frequently asked questions
Which method is most accurate?▾
In ascending order of accuracy: LMP < cycle-adjusted LMP < known conception date ≈ first-trimester ultrasound (6-13 weeks). Second-trimester ultrasounds are less accurate (±10-14 days) because growth variability widens. Always prefer the first-trimester scan when you have one.
Why is the result a probability window and not a single date?▾
Because the underlying biology is variable. Even with a perfect first-trimester ultrasound, the actual delivery date has a standard deviation of around 13 days. The calculator shows the 37-42 week "full-term window" precisely to set this expectation.
My cycles are irregular. Can I still use the LMP method?▾
Not reliably. If cycles vary by more than a week month-to-month, LMP-based dating can be off by weeks. Schedule an early ultrasound (8-12 weeks) for an accurate estimate.
What is the difference between gestational age and fetal age?▾
Gestational age is counted from the first day of LMP — what every clinician uses. Fetal age (or "embryonic age") is counted from conception, which is roughly 2 weeks shorter. So a "12 weeks pregnant" fetus is about 10 weeks old in fetal terms. This calculator displays gestational age, the standard.
Are induction and post-term decisions made on this date?▾
Yes — and this is why ultrasound dating matters. Post-term (>42 weeks) carries higher risks (placental aging, oligohydramnios), so most providers schedule induction by 41-42 weeks at the latest. If your dating is wrong by a week, you might be induced unnecessarily or carry too long.
Why does the calculator show milestones at specific dates?▾
Milestones are clinically actionable: end of 1st trimester (NT scan window 11-14 weeks), 20-week anatomy scan (detailed fetal anatomy), 24-week viability threshold (NICU survival rises from ~50% at 24w to ~80% at 26w), 37-week full-term window, and 42-week post-term limit. Marking them on a calendar is more useful than just knowing the due date.
Sources & references
Cross-check every number in this calculator against the primary sources below.
- OfficialACOG Committee Opinion No. 700 — Methods for Estimating the Due Date (reaffirmed 2022)In force: 2022
- AcademicNIH NICHD — Due Date Calculation, Pregnancy and Birth (Jukic et al. 2013)In force: 2013
- OfficialWHO — Pregnancy, Childbirth, Postpartum and Newborn Care (4th ed.)In force: 2022
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