Conception Date Calculator
Calculate your estimated conception date from your due date or LMP. Find your fertile window (5-day range) and current gestational age in weeks.
- American College of Obstetricians and Gynecologists data · June 2026
- Edited by Martín Rodríguez
- Private — runs on your device
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Once a week, the number that changed and the calculator to understand it. Two minutes, zero spam.
When to use this calculator
- Estimate conception date when the due date is known from an ultrasound
- Identify the fertile window around your likely ovulation date
- Confirm gestational age in weeks and days as of today
- Understand how a longer or shorter cycle shifts the conception estimate
- Cross-check conception timing for paternity or personal records
- Plan or document pregnancy milestones from a known LMP
Cycle Length Adjustment: Days from LMP to Ovulation
| Cycle length | Days from LMP to ovulation | Shift vs. 28-day cycle | Fertile window (days before due date) |
|---|---|---|---|
| 21 days | 7 days | −7 days | 273–277 days before EDD |
| 22 days | 8 days | −6 days | 272–276 days before EDD |
| 24 days | 10 days | −4 days | 270–274 days before EDD |
| 26 days | 12 days | −2 days | 268–272 days before EDD |
| 28 days | 14 days | 0 days | 266–270 days before EDD |
| 30 days | 16 days | +2 days | 264–268 days before EDD |
| 32 days | 18 days | +4 days | 262–266 days before EDD |
| 34 days | 20 days | +6 days | 260–264 days before EDD |
| 35 days | 21 days | +7 days | 259–263 days before EDD |
Fuente: ACOG Practice Bulletin – Methods for Estimating the Due Date (2017) y Wilcox AJ et al., NEJM (1995). Fórmula aplicada: ovulation_day = cycle_length − 14; fertile window = ovulation day −4 to ovulation day; days before EDD = 266 − shift (±4 days for sperm survival window). Assumes regular cycles; ultrasound dating takes precedence in clinical settings.
How it works
What is conception date?
Conception date is the day a sperm fertilizes an egg, typically occurring around 14 days after the last menstrual period in a standard 28-day cycle. It falls approximately 266 days before the due date. Conception timing varies by cycle length, shifting the fertile window by 1–10 days, making it difficult to pinpoint without medical imaging.
How It Works
The calculation uses the Naegele's Rule framework combined with cycle-length adjustment.
Key biological facts
Formula
ovulation_day_from_lmp = cycle_length - 14
conception_date = LMP + ovulation_day_from_lmp
fertile_window_start = conception_date - 4 (days)
fertile_window_end = conception_date
EDD (due date) = LMP + 280 - (cycle_length - 28)When you enter a due date, the calculator first derives LMP:
LMP = EDD - 280 + (cycle_length - 28)Then applies the ovulation formula above.
Cycle-length adjustment
| Cycle length | Days from LMP to ovulation | Shift vs. 28-day |
|---|---|---|
| 21 days | 7 days | −7 days |
| 28 days | 14 days | 0 |
| 35 days | 21 days | +7 days |
Worked example
Suppose EDD = July 15, 2026 and cycle = 30 days.
1. LMP = July 15 − 280 + (30 − 28) = July 15 − 278 = October 10, 2025
2. Ovulation offset = 30 − 14 = 16 days
3. Conception date = Oct 10 + 16 = October 26, 2025
4. Fertile window = Oct 22 – Oct 26, 2025
5. If today is April 28, 2026: gestational age = (Apr 28 − Oct 10) = 198 days = 28 weeks 2 days
Gestational age
Gestational age is counted from LMP, not from conception, which is the standard used by OB-GYNs and ultrasound dating.
Limitations
Example: Due date July 15, 2026 with a 30-day cycle
Frequently asked questions
How accurate is the estimated conception date?
Why is gestational age counted from LMP, not conception?
What if my cycles are irregular?
Can the fertile window extend beyond 5 days?
My due date changed after an ultrasound — which should I use?
Does cycle length affect the due date estimate?
What is Naegele's Rule?
Can I use this to determine paternity?
Why does the calculator show gestational age from LMP, not from the conception date?
Sources & references
- ACOG Practice Bulletin: Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists (2017)
- Wilcox AJ et al. — Timing of Sexual Intercourse in Relation to Ovulation (NEJM 1995) — New England Journal of Medicine (1995)
- CDC — Reproductive Health: Pregnancy — Centers for Disease Control and Prevention (2024)
- WHO — Preterm Birth Fact Sheet — World Health Organization (2023)
Methodology & trust
familia calculator reviewed by the Hacé Cuentas editorial team, checked against ACOG Practice Bulletin: Methods for Estimating the Due Date, following our editorial policy and methodology.
Last reviewed: June 2026. Parameters are verified periodically against the cited sources.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Conception Date Calculator. Hacé Cuentas. https://hacecuentas.com/en/conception-date-from-due-date-calculator
Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.