Conception Date Calculator
Estimate when conception occurred based on your gestational age.
Last updated: June 13, 2026
Calculator
Results
Approximately 2 weeks after LMP
Age since conception (not gestational)
Measured from last menstrual period
Sperm can survive 3-5 days
How to Use This Calculator
- 1Enter your current gestational age in weeks.
- 2Add extra days if applicable (e.g., 12 weeks 3 days).
- 3View your estimated conception timing and fetal age.
- 4See the fertile window when conception likely occurred.
Formula Explanation
Conception typically occurs about 14 days after the first day of the last menstrual period (LMP). Gestational age counts from LMP, while fetal age counts from conception — fetal age is about 2 weeks less than gestational age.
Variables:
Worked Examples
12 weeks 0 days pregnant
- 1. Gestational days = 84
- 2. Conception ≈ 84 - 14 = 70 days ago
- 3. Fetal age = 70 days = 10 weeks 0 days
- 4. Fertile window: 67-71 days ago
20 weeks 3 days pregnant
- 1. Gestational days = 143
- 2. Conception ≈ 143 - 14 = 129 days ago
- 3. Fetal age = 18 weeks 3 days
Why Use This Calculator
- Understand the difference between gestational and fetal age.
- Estimate the window when conception likely occurred.
- Accounts for sperm survival time in the fertile window.
- Helpful for personal records and medical discussions.
Tips & Best Practices
- Gestational age (from LMP) is always about 2 weeks more than fetal age (from conception).
- Sperm can survive 3-5 days, so intercourse several days before ovulation can result in conception.
- The exact moment of conception is nearly impossible to determine without IVF.
- Your provider uses gestational age for all milestones and due date calculations.
Common Mistakes to Avoid
❌ Confusing gestational age with fetal age.
✅ Gestational age starts from LMP and is ~2 weeks longer. A "12-week" pregnancy has a fetus that is actually about 10 weeks old.
❌ Assuming conception happened on a specific date of intercourse.
✅ Sperm survive 3-5 days. Conception could have resulted from intercourse up to 5 days before ovulation.
Frequently Asked Questions
What is the difference between gestational age and fetal age?
Can I determine the exact date of conception?
Why is gestational age 2 weeks more than fetal age?
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<iframe src="https://redicalc.com/embed/health/conception-date-calculator" width="100%" height="600" style="border:none;border-radius:12px;box-shadow:0 1px 3px rgba(0,0,0,0.1);" title="Conception Date Calculator — RediCalc.com" loading="lazy" allowtransparency="true"></iframe>Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.
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