Pregnancy Due Date Calculator
Estimate days until estimated due date using Naegele's rule from the date of last menstrual period and cycle length. Returns a single number of days for educational use, not a substitute for prenatal clinical confirmation.
Last updated: May 2026
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About this calculator
Naegele's rule estimates the due date as the first day of the last menstrual period (LMP) + 280 days (40 weeks), with an adjustment for cycle length when not the 28-day reference. This calculator combines the days elapsed since LMP with the constant (280 − cycleLength + 14): result = floor((today − LMP) / day) + (280 − cycleLength + 14). For a 28-day cycle the constant becomes 280 − 28 + 14 = 266; for a 35-day cycle, 280 − 35 + 14 = 259. The formula's output should be interpreted as the day index of pregnancy plus the offset constant — but the result label 'Days Until Due Date' may mislead since the formula adds rather than subtracts elapsed days from a target. For clinical interpretation, the conventional Naegele-rule due date is LMP + 280 days; cycle-length adjustment in practice adds (cycleLength − 28) days to that figure, so a 35-day cycle gives LMP + 287 days, not the formula's implied direction. Edge cases: month input uses 0-indexing (January = 0), as in JavaScript's Date object — passing 1 actually means February. Cycle lengths shorter than 14 days (impossible biologically) would give negative adjustments. Early-pregnancy ultrasound dating (crown-rump length at 6–13 weeks) is the gold standard for estimating gestational age and supersedes LMP-based calculations whenever they disagree by more than 5–7 days.
How to use
Example 1 — average cycle, recent LMP. LMP date 15 January 2025 (month = 0, day = 15, year = 2025), cycle length 28 days. Suppose today is 1 March 2025. Step 1: days since LMP = 45 (16 Jan to 1 Mar inclusive ≈ 45 days). Step 2: constant = 280 − 28 + 14 = 266. Step 3: formula result = 45 + 266 = 311. Note this is not 'days until due date' in conventional terms. The clinical Naegele due date for this LMP is 15 January + 280 = 22 October 2025; days from 1 March 2025 to that due date are ~235 days. The formula's 311 doesn't match this directly — interpret it as a position number along an internal counting scheme rather than a calendar countdown. Example 2 — longer cycle, computing conventional due date manually. LMP 1 June 2025, cycle length 35 days. Standard Naegele adjusted: due date = LMP + 280 + (cycleLength − 28) = 1 June + 280 + 7 = 1 June + 287 days ≈ 16 March 2026. The formula's constant for this case = 280 − 35 + 14 = 259, which doesn't appear to match the standard adjustment direction (cycle length above 28 should add days to the due date, not subtract). For accurate due-date estimation in clinical or personal-planning contexts, use an obstetric pregnancy wheel or a dedicated due-date calculator that follows the standard ACOG Naegele protocol with cycle-length adjustment in the conventional direction.
Frequently asked questions
What is Naegele's rule and how accurate is it?
Naegele's rule estimates the expected due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), assuming a 28-day cycle with ovulation on day 14. It's named after Franz Karl Naegele, a 19th-century German obstetrician who popularised the method. The rule is straightforward and has been a clinical standard for over 150 years, but its accuracy is modest: in studies, only about 5% of pregnancies deliver on the calculated due date; about 60% deliver within ±5 days, and 90% within ±2 weeks. The rule assumes regular 28-day cycles and ovulation on day 14, both of which vary substantially in real populations. For cycle lengths other than 28 days, adjust by adding (cycle − 28) days to the LMP + 280 date. Early-pregnancy ultrasound (crown-rump length at 6–13 weeks) is the most accurate method for dating and is the clinical gold standard; when LMP and ultrasound dates disagree by more than 5–7 days in the first trimester, ultrasound is used as the official due date.
Why is early ultrasound preferred over LMP-based dating?
Early-pregnancy crown-rump length (CRL) measurement at 6–13 weeks is accurate to within ±3–5 days because fetal size at this stage follows a tight standard curve and does not yet vary significantly across populations or by maternal characteristics. LMP-based dating, in contrast, depends on the woman remembering the exact start date of her last period (often imprecise), a regular 28-day cycle (only ~25% of women have consistently regular cycles), and ovulation on day 14 (real ovulation timing varies by 3–7 days even in regular cycles). LMP-based dating tends to date pregnancies slightly later than ultrasound on average. The American College of Obstetricians and Gynecologists (ACOG) recommends using ultrasound dating when first-trimester scans differ from LMP by more than 5 days for 6–9 week scans, 7 days for 9–14 week scans, and 10 days for 14–16 week scans. After 22 weeks, dating accuracy of ultrasound drops to ±10–14 days because fetal growth becomes more variable.
Why does the calculator's month input start from 0 instead of 1?
The formula uses JavaScript's Date constructor, where months are 0-indexed: January = 0, February = 1, March = 2, ..., December = 11. This is a quirk of the underlying Date API and is not a clinical or biological convention. If you enter 1 for the month thinking 'January', you'll actually create a Date for February of that year, shifting your due date estimate by approximately one month. Always subtract one from the calendar month when entering: January 15 = month 0, day 15; July 4 = month 6, day 4; December 25 = month 11, day 25. The result label 'Days Until Due Date' may also be misleading given the formula's structure; verify the result against a conventional Naegele calculator or a pregnancy wheel before relying on it for clinical or personal planning.
What are the common mistakes when estimating due dates?
The biggest mistake here is the 0-indexed month input — entering the calendar month directly without subtracting 1 shifts dates by a month. The second is treating LMP-based estimates as precise; real due-date accuracy is ±2 weeks at best, and obsessing over single-day predictions misses the natural variability of pregnancy length. The third is forgetting to confirm with early ultrasound — ultrasound dating supersedes LMP whenever they disagree significantly. People also incorrectly adjust for cycle length in the wrong direction: longer cycles should give later due dates (more days to ovulation), but some formulas (and possibly this one) adjust in the opposite direction. Irregular cycles, recent oral contraceptive use, breastfeeding, or PCOS make LMP-based dating particularly unreliable, and ultrasound is essential. Finally, treating the EDD as a deadline causes anxiety: only ~5% of babies are born on the exact EDD; most full-term births fall between 37 and 42 weeks, with 39–41 weeks being the modal range. The EDD is a midpoint estimate, not a deadline.
When should I not use this calculator?
Do not use it for clinical pregnancy dating — only confirmed by early-trimester ultrasound or by your obstetric provider. The formula has structural issues (0-indexed months that don't match calendar convention, unclear cycle-length adjustment direction) that make it unreliable for personal planning, let alone medical decision-making. It is not appropriate for women with irregular cycles, recent contraceptive use, breastfeeding-related cycle changes, or PCOS — those populations need ultrasound-based dating from the start. Do not use it for IVF pregnancies, where embryo transfer date is the reference point (typically EDD = transfer date + 263 days for day-5 transfer, 266 days for day-3 transfer). It is unsuitable for ectopic, molar, or non-viable pregnancies, which require clinical management and not a due-date estimation. Avoid it for twin or higher-order pregnancies, which deliver earlier on average (~36–37 weeks for twins, 32–34 weeks for triplets) and need specialised dating and delivery planning. For any actual pregnancy, use an obstetric provider and a clinical pregnancy wheel or app validated against ACOG guidelines.