Conception Date Estimator
Estimate the day conception most likely happened from your due date, last period or a dated ultrasound — with the fertile window it fell inside.
How it works
From a due date: conception ≈ due date − 266 days · from a last period: conception ≈ LMP + (cycle − 14) · from a scan: conception ≈ (scan date − gestational age) + 14 days.
Conception occurs at or just after ovulation, which is roughly two weeks after the first day of the last period and about 266 days before the due date. Because sperm can survive up to five days and the egg about one, the actual day sits somewhere in a fertile window rather than on a single certainty. Fetal (post-conception) age therefore runs about two weeks behind the gestational age clinicians quote, which is counted from the last period.
Worked example
Working back from an October 8, 2026 due date, conception is estimated around January 15, 2026 (266 days earlier), inside a fertile window of January 10–16.
Frequently asked questions
Why is the fetal age two weeks less than my “weeks pregnant”?
Weeks pregnant (gestational age) is counted from the first day of your last period, about two weeks before you conceived. Age measured from conception itself is roughly two weeks lower — both describe the same pregnancy.
Can this pinpoint the exact day I conceived?
No. Sperm can live several days, so conception may happen days after intercourse. It gives the most likely date and the window around it, not a certainty — and it should never be used to establish legal facts such as paternity.
Which method is most reliable?
A dated early ultrasound is generally the most accurate, because it measures the embryo directly rather than assuming an average cycle. Use your provider’s dating whenever you have it.
Related calculators
Educational estimates from standard formulas and population averages — every pregnancy and baby is different. This is not medical advice; your doctor, midwife or pediatrician, with an examination or ultrasound, gives the accurate guidance.