Reverse Due Date Calculator
Work backwards from your due date or birth date to estimate the likely conception date and fertile window. Conception typically occurs 266 days before delivery (38 weeks gestational age).
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Key values and context for this tool: • Conception Window: ±5 Days • Gestation: 266 Days • Fertile Window: 6 Days
Ready to run the numbers?
Why: This page explains what the Reverse Due Date calculator does, what to enter, and how to read the results—without repeating the overview above.
How: Enter your values in the calculator fields below, keep units consistent, then run the calculation to see results and any step-by-step work shown on this page.
Run the calculator when you are ready.
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For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
Work backwards from your due date or birth date to estimate the likely conception date and fertile window. Conception typically occurs 266 days before delivery (38 weeks gestational age). Sperm survives 3-5 days; the fertile window is ~6 days.
Sources: ACOG, WHO, NIH, CDC.
Key Takeaways
- • Conception = Due Date - 266 days (adjusted for cycle)
- • Fertile window: 5 days before to 1 day after ovulation
- • Sperm lives 3-5 days; egg 12-24 hours
- • IVF provides most accurate conception date
Did You Know?
How Does Reverse Due Date Work?
From Due Date
LMP = Due - 280 days. Ovulation = LMP + (Cycle - Luteal Phase). Conception ≈ Ovulation.
From Birth Date
Adjust for gestational age at birth. Preterm: fewer days from conception. Post-term: more days.
Fertile Window
Sperm survives 3-5 days. Intercourse 5 days before ovulation can result in pregnancy.
Expert Tips
Accuracy by Scenario
| Scenario | Accuracy | Window |
|---|---|---|
| Regular cycles | ~85% | ±5 days |
| Irregular cycles | ~70% | ±7-10 days |
| IVF | 98%+ | ±1-2 days |
| Due date (vs birth) | Higher | Use early US date |
Frequently Asked Questions
How accurate is the conception date estimate?
The estimated conception date is typically accurate within a ±5 day window for women with regular cycles. The fertile window provides the most likely range. IVF pregnancies have the highest accuracy (98%+) since the transfer date is known.
Why is the fertile window several days long?
Sperm can survive 3-5 days in the female reproductive tract, while the egg is viable 12-24 hours after ovulation. This creates a fertile window of ~6 days. Intercourse days before ovulation can still result in pregnancy.
Does cycle length affect conception date?
Yes. Longer cycles typically mean later ovulation. The calculator adjusts for cycle length. Ovulation = LMP + (Cycle Length - Luteal Phase). Irregular cycles reduce accuracy by 10-20%.
Should I use due date or birth date?
If you have both, the original due date (from early ultrasound) is often more accurate. If using birth date, enter the actual gestational age at delivery. First-trimester due dates are accurate to ±5 days.
What is the luteal phase?
The luteal phase is the time between ovulation and your next period, typically 12-16 days. It is more consistent than the follicular phase. If you know it from BBT charting, entering it improves accuracy.
Can I use this for IVF pregnancies?
Yes. Select IVF calculation method and enter embryo transfer date and embryo age. IVF provides the most accurate conception date since the transfer date is known exactly.
Key Statistics
Official Data Sources
⚠️ Disclaimer: This calculator is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. For precise conception dating (e.g., legal, paternity), consult a qualified healthcare provider.
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