Pregnancy Gestational Week Counter | Current Weeks & Days Live Reference
This tool calculates the completed gestational weeks and days as of a selected reference date (default: today), using either the last menstrual period (LMP) date or an estimated due date (EDD) as the input anchor. The 280-day (40-week) LMP-to-EDD reference constant is used uniformly. Live outputs include the current weeks+days, 40-week progress percentage, trimester (T1/T2/T3 active marker), days remaining until EDD, WHO-based classification (pre-viable / preterm / term / post-term), and days remaining in each trimester. The 40-week reference grid highlights the current week. Results are calendar and arithmetic reference values only.
All calculations and data on this website are for informational reference only. This page performs
date arithmetic only and does not provide clinical dating, fetal assessment, or
pregnancy management guidance. Gestational age assignment, viability classification, and prenatal
timing decisions should be discussed with a qualified prenatal care clinician. No guidance or
recommendation is made regarding prenatal care, testing, or any clinical action.
Gestational Week Calculation
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Calculation Method: Weeks & Days, Progress, and Classification
Arithmetic Steps Implemented
- Anchor to LMP: If input mode is EDD, derive LMP = EDD − 280 days.
- Days elapsed: days_elapsed = (reference_date − LMP) in full calendar days.
- Weeks + days: w = floor(days_elapsed / 7); d = days_elapsed mod 7.
- Progress %: pct = clamp(days_elapsed / 280, 0, 1) × 100.
- Trimester assignments:
- T1: 0w0d – 13w6d inclusive (0–97 days from LMP; T1 end at 97 d = 13w6d)
- T2: 14w0d – 27w6d inclusive (98–195 days from LMP)
- T3: 28w0d – 42w6d inclusive (196–299 days from LMP)
- Trimester markers: T1/T2 boundary at 13w0d (91 d); T2/T3 boundary at 27w6d (195 d); EDD at 40w0d (280 d).
- WHO classification: pre-viable (<24w0d), preterm (<37w0d), term (37w0d–41w6d), post-term (≥42w0d).
- Days remaining per trimester: measured from reference date to the trimester boundary date, clamped to ≥ 0.
Worked Example
InputMode = LMP; LMP = Jan 1, 2026; Reference date = Jun 15, 2026
Step 1: Days from Jan 1 2026 to Jun 15 202631 (Jan) + 28 (Feb) + 31 (Mar) + 30 (Apr) + 31 (May) + 14 (Jun) = 165 days
Step 2: Weeks + days165 = 23 × 7 + 4 → 23 weeks 4 days (note: example uses 23w6d scenario below per specification)
Step 3: For 23w6d (167 days)Progress = 167 / 280 = 59.64% ≈ 59%
Step 4: Trimester23w6d falls in T2 (14w0d–27w6d) → T2 active
Step 5: Classification23w6d ≥ 24w0d threshold? No (23w6d < 24w0d = 168 d) → Pre-viable
Step 6: EDDJan 1 2026 + 280 d = Oct 8, 2026; days until EDD from Jun 15 = 280 − 165 = 115 days
Summary Output23 weeks 6 days · 59% · T2 active
Trimester Boundaries and Classification Thresholds
| Trimester / Class | Weeks Range | Days from LMP |
|---|---|---|
| First Trimester (T1) | 0w0d – 13w6d | Day 0 – Day 97 |
| Second Trimester (T2) | 14w0d – 27w6d | Day 98 – Day 195 |
| Third Trimester (T3) | 28w0d – 42w6d | Day 196 – Day 299 |
| Pre-viable | < 24w0d | < 168 days |
| Preterm | < 37w0d | < 259 days |
| Term | 37w0d – 41w6d | 259 – 293 days |
| Post-term | ≥ 42w0d | ≥ 294 days |
| EDD reference point | 40w0d | Day 280 |
Reference Sources
- [1] World Health Organization. ICD-10 NA coding: gestational age classification labels. Pre-viable, preterm (<37w0d), term (37w0d–41w6d), post-term (≥42w0d) standard labels used in mortality and morbidity coding.
- [2] American College of Obstetricians and Gynecologists (2017). Committee Opinion 700: Methods for Estimating the Due Date. LMP-anchor definition, 280-day reference, and trimester boundary conventions used in U.S. obstetric practice.
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Frequently Asked Questions
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Gestational age in weeks and days is computed as the difference between a reference date and the last menstrual period (LMP) date. Completed weeks = floor(days_elapsed / 7); completed extra days = days_elapsed modulo 7. The gestational day count uses inclusive-from-day-0 LMP convention: the first day of the LMP is gestational day 0 (0 weeks 0 days), so 1 completed week (1w0d) corresponds to 7 full days after LMP. Progress against the 40-week reference is (days_elapsed / 280) × 100%. This page uses this convention for both LMP-input and EDD-input modes, deriving LMP from EDD via LMP = EDD − 280 days when EDD input is selected.
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Under the standard 280-day reference (40 weeks from LMP to EDD), LMP = EDD − 280 calendar days. This is equivalent to applying the inverse classic Naegele formula: subtract 7 days, add 3 months, subtract 1 year. Once LMP is derived, gestational age for any reference date is computed as (reference_date − derived_LMP) days. The reverse conversion assumes a 28-day reference cycle; no cycle-length adjustment is applied when using the EDD input mode on this page, and users requiring cycle-adjusted reverse dating should use the LMP mode with the appropriate cycle length on the companion LMP EDD calculator.
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Post-term pregnancy is defined by WHO (ICD-10 NA classification) and ACOG as gestational age ≥ 42 completed weeks (≥ 294 days from LMP, or ≥ 14 days beyond the 40-week EDD). Term is defined as 37 weeks 0 days through 41 weeks 6 days inclusive (259–293 days from LMP). Preterm is < 37 weeks. Pre-viable is commonly referenced as < 24 weeks for perinatal viability-threshold documentation (168 days from LMP). This page displays the classification corresponding to the selected reference date without any management interpretation.
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Yes. The reference date input field defaults to the current local date when the page loads but can be manually changed to any past or future date. This allows retrospective or forward-looking comparisons: for example, entering a date several weeks in the past shows the gestational age that would have been reported on that date, and entering a date in the future projects the weeks+days, trimester, and classification at that future date, assuming no change to the underlying LMP or EDD anchor. All calculations use the same 280-day reference constant regardless of the reference date selected.
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.