When Did Conception Happen?
Because ovulation timing (not the exact moment of conception) is what's clinically trackable, this conception date calculator estimates your likely ovulation and conception window from your last menstrual period (LMP) and typical cycle length — the same logic behind Naegele's rule, used to estimate due dates.
💡 Quick Summary
Ovulation typically occurs 14 days before your next period, regardless of cycle length — so a longer cycle means ovulation (and likely conception) happened later after your LMP.
How the Calculation Works
The luteal phase (from ovulation to the next period) is usually a fairly constant ~14 days, even when total cycle length varies. So:
- Estimated ovulation day = LMP + (cycle length − 14 days)
- Fertile window = about 2 days before to 1 day after ovulation (accounting for sperm survival time)
- Estimated due date = LMP + 280 days, adjusted for cycles longer or shorter than the standard 28 days
Why Cycle Length Matters
A standard due-date estimate assumes a 28-day cycle. If your cycles run longer or shorter, ovulation shifts accordingly, which is why this calculator adjusts both the conception estimate and the due date based on your actual average cycle length.
Important Notes
These are statistical estimates based on average ovulation timing, not a certainty — actual ovulation can vary cycle to cycle, especially with irregular periods, stress, or underlying conditions. An early ultrasound (typically most accurate between 8-13 weeks) provides a more precise due date estimate. Always confirm dates and pregnancy details with your healthcare provider.