Pregnancy Conception Calculator
Adjust the inputs below. Results update as you type.
How it works
Due date = LMP + 280 days, so conception ≈ LMP + 14 days. Working backward from due date: conception = due date − 266 days. Fertilization window is 3–5 days before ovulation through 1 day after. Ultrasound crown-rump length in the first trimester is more accurate than LMP for dating, especially with irregular cycles. Paternity testing is the only definitive way to determine biological paternity.
Input guidance
- Use measured values (recent weight, height, activity level) rather than guesses.
- Recalculate every few weeks as your routine and body metrics change.
- Pair screening metrics with context such as sleep, stress, and medical history.
The formula
Conception is estimated as roughly the LMP + 14 days (around ovulation), or due date − 266 days. Actual conception varies with cycle length.
Worked example
With a due date of October 8, estimated conception is about January 15 (266 days earlier), give or take a few days depending on cycle length.
More examples to test
- Maintenance case: keep activity constant and observe expected baseline ranges.
- Adjustment case: change only one major variable to understand sensitivity.
How to interpret results
Use results for awareness and planning, then validate with qualified medical or nutrition professionals for individualized decisions.
When this can be inaccurate
Population formulas cannot account for all biological factors such as medical conditions, medication effects, body composition differences, and genetics.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
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General wellness resource.
Frequently asked questions
Can I know the exact conception date?+
Rarely. It is estimated from the cycle or due date. Sperm can survive several days, so fertilization may not match intercourse timing exactly.
Why estimate conception?+
It helps approximate fetal age and a possible window of conception, useful for planning and curiosity, though dating from LMP is the clinical standard.
Does cycle length matter?+
Yes. Longer or shorter cycles shift ovulation and conception timing, so the estimate is most accurate for regular 28-day cycles.
Can this calculator diagnose a condition?+
No. It is a screening and education tool only, not a clinical diagnosis.
When should I seek professional advice?+
Seek professional advice when results are extreme, persistent, or conflict with symptoms, goals, or prior medical guidance.