Conception Date Calculator
38-Week Method
Conception to Due Date
Reverse: Due Date to Conception
How It Works
The Math Behind It
The forward calculation adds 266 days (38 weeks) to your estimated conception date — the time from fertilization to birth. The reverse calculation subtracts 266 days from your due date to find your conception window, then expands it by ±3 days to account for sperm survival and egg viability overlap. We also estimate your LMP by subtracting 280 days from the due date.
How to Use
Use the left panel if you know or can estimate your conception date. Use the right panel if you only know your due date and want to find your conception window. Both panels work independently — try both to cross-check. Save results to compare forward and reverse estimates.
Pro Tips
- Sperm can survive up to 5 days, so conception may have occurred days before the actual fertilization event.
- The reverse window is ±3 days around the estimated date — use this range, not a single day.
- If you conceived via IVF, use the IVF calculator instead — it accounts for exact embryo age.
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