Last Menstrual Period (LMP) Due Date Calculator | Estimated Delivery Date Reference
This calculator performs purely calendar and arithmetic date calculations based on the last menstrual period (LMP) dating method. It implements the modified Naegele formula with cycle-length adjustment (range 21–45 days, default 28 days). The 280-day mean gestational duration reference constant is used as the population-level baseline from LMP. Outputs include the estimated due date (EDD), approximate ovulation/conception date, implantation window range, 24-week viability threshold date, and the calendar dates corresponding to the standard preterm / term / post-term classification cutoffs. Results are mathematical reference values only.
All calculations and data on this website are for informational reference only. This page performs
calendar arithmetic only and does not provide medical dating, fetal assessment, or
delivery predictions. Gestational age assignment and dating accuracy should be discussed with a
qualified prenatal care clinician. Values do not replace obstetric dating via ultrasonography. No
guidance or recommendation is made regarding prenatal care, testing, or management.
LMP → EDD Calculation
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Calculation Method: Modified Naegele Formula
Arithmetic Steps Implemented
Given an LMP date (first day of last menstrual period) and a cycle length C days (default 28, range 21–45):
- EDD (modified Naegele): LMP + 280 days − (C − 28) days
- EDD (classic Naegele): LMP + 7 days − 3 months + 1 year (equivalent to LMP + 280 days when C = 28)
- Approximate ovulation/conception: LMP + ⌊C/2⌋ − 14 days (approximates cycle day 14 as ovulation anchor adjusted by C/2 shift)
- Implantation window: Ovulation date + 6 days to Ovulation date + 12 days
- 24-week viability date: LMP + 24 × 7 days = LMP + 168 days
- 37-week preterm/term cutoff: LMP + 37 × 7 days = LMP + 259 days
- 41w6d end-of-term cutoff: LMP + 42 × 7 − 1 days = LMP + 293 days
- 42-week post-term cutoff: LMP + 42 × 7 days = LMP + 294 days
Worked Example
InputLMP = Jan 1, 2026; Cycle Length = 28 days
Step 1: Classic NaegeleJan 1 + 7 d = Jan 8; Jan 8 − 3 mo = Oct 8 (2025); + 1 y = Oct 8, 2026
Step 2: Modified formula checkJan 1 2026 + 280 d − (28−28) d = Jan 1 2026 + 280 d = Oct 8, 2026
Step 3: OvulationJan 1 2026 + (28/2) − 14 d = Jan 1 2026 + 14 − 14 d = Jan 15, 2026
Step 4: Implantation windowJan 15 + 6 d = Jan 21, 2026 … Jan 15 + 12 d = Jan 27, 2026
Step 5: 24-week viabilityJan 1 2026 + 168 d = Jun 18, 2026
Step 6: Preterm (37w) cutoffJan 1 2026 + 259 d = Sep 17, 2026
Step 7: Term window end (41w6d)Jan 1 2026 + 293 d = Oct 21, 2026
Step 8: Post-term (42w) cutoffJan 1 2026 + 294 d = Oct 22, 2026
EDD OutputOctober 8, 2026 (Thursday)
Classification Thresholds: Preterm / Term / Post-term
| Classification | Gestational Age Range (from LMP) | Calendar Date (relative to LMP) |
|---|---|---|
| Pre-viable (before viability threshold) | < 24 weeks 0 days | Before LMP + 168 days |
| Preterm | < 37 weeks 0 days | Before LMP + 259 days |
| Term (early term) | 37 weeks 0 days – 38 weeks 6 days | LMP + 259 days … LMP + 272 days |
| Term (full term) | 39 weeks 0 days – 40 weeks 6 days | LMP + 273 days … LMP + 286 days |
| Term (late term) | 41 weeks 0 days – 41 weeks 6 days | LMP + 287 days … LMP + 293 days |
| Post-term | ≥ 42 weeks 0 days | LMP + 294 days and after |
Reference Sources
- [1] World Health Organization (2008). Gestational age classification: ICD-10 NA coding reference. WHO Department of Reproductive Health and Research. Preterm <37w, term 37w0d–41w6d, post-term ≥42w0d standard classification.
- [2] American College of Obstetricians and Gynecologists (2017, reaffirmed). Committee Opinion on Methods for Estimating the Due Date. ACOG. Naegele formula historical basis and LMP as first-line dating method, superseded by first-trimester ultrasound when available.
- [3] Society of Obstetricians and Gynaecologists of Canada (2021). SOGC Clinical Practice Guideline: Pregnancy Dating Methods. LMP-based dating, cycle-length adjustment principles, and comparison with ultrasonographic dating.
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Frequently Asked Questions
-
The classic Naegele formula estimates the estimated due date (EDD) by adding 7 calendar days to the date of the last menstrual period (LMP), then subtracting 3 calendar months, then adding 1 calendar year. This produces a date approximately 280 days (40 weeks) from the LMP, assuming a 28-day menstrual cycle with ovulation occurring on cycle day 14. The modified Naegele formula with cycle-length adjustment generalizes this: EDD = LMP + 280 days − (Cycle Length − 28). For cycles longer than 28 days, the EDD is shifted later; for shorter cycles, earlier.
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The cycle-length adjustment accounts for the relationship between cycle length and the presumed ovulation day. Under the 28-day reference cycle, ovulation is assumed to occur on cycle day 14, i.e., 14 days before the next period. For a cycle of length C days, ovulation is assumed to occur on approximately cycle day (C − 14). The difference from the 28-day reference is (C − 28) days. The adjusted EDD is therefore: EDD = LMP + 280 days − (C − 28). For example, a 32-day cycle adds 4 days to the standard 280-day count; a 24-day cycle subtracts 4 days. This page implements the 21–45 day adjustment range.
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The 280-day (40-week) reference interval from the last menstrual period to the estimated due date is a longstanding population-level convention. It corresponds to approximately 266 days (38 weeks) from presumed ovulation/conception under the 28-day cycle model. The 280-day value derives from historical obstetric dating conventions summarized by Naegele in the 19th century and subsequently adopted by ACOG, WHO, and other bodies as the standard LMP-based reference constant. Population studies confirm that actual spontaneous delivery dates show a distribution centered near this reference, with substantial individual variability.
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LMP-based gestational dating counts elapsed time from the first day of the last menstrual period, yielding the familiar 40-week (280-day) EDD reference. Conception-based dating counts from the presumed fertilization/ovulation date, which is approximately 266 days (38 weeks) before the EDD under the 28-day cycle model, i.e., about 14 days after the LMP. Conversion between the two conventions: Gestational Age (LMP) = Gestational Age (Conception) + 14 days. LMP dating is the standard convention used in clinical guidelines, prenatal visit scheduling, and viability/term classification thresholds unless an earlier ultrasound dating is available.
GENERAL CALCULATION REFERENCE – NOT MEDICAL ADVICE
This page performs purely calendar and arithmetic date calculations based on the last menstrual period (LMP) method, 280-day mean gestational duration reference constant, and cycle-length adjustment constants commonly cited in public-health guidelines. Outputs are mathematical reference values only, not personalized estimates of fetal development, delivery probability, or any health-related outcome. Gestational age assignment, dating accuracy, viability classification, and prenatal timing decisions should be discussed with a qualified prenatal care clinician. Values presented do not replace obstetric dating via ultrasonography. No guidance, recommendation, or prescriptive statement is made on this page regarding prenatal testing, prenatal care utilization, pregnancy management, or any clinical intervention.
SITE-WIDE YMYL DISCLAIMER
VivMetric is a calculation and public-reference website, not a healthcare provider, clinician, or medical organization. All calculators, timelines, tables, and articles present standard public formulas, demographic constants, and population-level reference ranges extracted from publicly available government and peer-reviewed publications. No content is personalized. No content is diagnostic, prognostic, prescriptive, therapeutic, or treatment-oriented. All health, pregnancy, nutrition, sleep, fitness, and lifestyle decisions should be made in consultation with appropriately licensed qualified professionals in the relevant jurisdiction.