Planning & time calculators
Conception date calculator
Estimate a conception date from a due date, last period, ultrasound, or birth date, with a likely ±5 day window.
Runs in your browser with no account and no stored data
How it works
Pick a calculation mode based on what you know (due date, first day of last period, ultrasound pregnancy age, or birth date) and enter the date. The calculator returns an estimated conception date and a likely ±5 day window.
Due date = first day of last period + 280 days. Conception ≈ due date − 266 days. From last period: conception ≈ last period + cycle length − 14 days. From ultrasound: gestational start = scan date − gestational age in days, conception = gestational start + 14 days. From birth date: conception ≈ birth date − ((pregnancy weeks × 7) − 14) days.
Last period 1 January, 28-day cycle → ovulation 15 January, conception around 15 January, due date about 8 October. Window: 10–20 January.
Curious parents-to-be wondering when conception likely occurred, lining up a pregnancy timeline with a scan or due date, or sense-checking dates after a birth.
What do you want to do next?
Want to plan further? Use the time calculator for date differences or explore baby names for inspiration.
FAQ
Is this medical advice?+
No. This calculator gives a date estimate based on standard pregnancy dating rules. Talk to a healthcare professional for medical guidance, an accurate due date, or anything related to fertility or pregnancy.
How is the conception date estimated?+
Conception is assumed to happen around ovulation, roughly 14 days before the next expected period. Depending on the mode the calculator works backwards from a due date, last period, ultrasound or birth date using standard formulas.
Why a ±5 day window instead of one date?+
Sperm can survive several days and ovulation timing varies between cycles, so a window is more realistic than a single date. The window is centred on the estimated conception date.
Can this confirm paternity?+
No. Conception dating is an estimate and cannot prove paternity. Only a DNA test can do that.
Why does ultrasound mode use 14 days?+
Pregnancy is dated from the first day of the last period, but conception happens about 14 days later. So the calculator subtracts the gestational age from the scan date to find the dating start, then adds 14 days to estimate conception.