Planning & time calculators
Conception date calculator
Estimate ovulation and possible conception from a due date, last period, ultrasound, or birth date, with a calculated fertile 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 estimated ovulation and possible conception timing plus a calculated fertile window from five days before through one day after.
Due date = first day of last period + 280 days, adjusted by (cycle length − 28) days. Possible conception ≈ due date − 266 days. From last period: estimated ovulation ≈ last period + cycle length − 14 days. From ultrasound: gestational start = scan date − gestational age in days, estimated ovulation = gestational start + 14 days. The calculated fertile window runs from five days before through one day after estimated ovulation.
Last period 1 January, 28-day cycle → estimated ovulation and possible conception around 15 January, due date about 8 October. Calculated fertile window: 10–16 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.
A calendar cannot establish the actual day of conception. Ovulation varies even with regular cycles, and dates inferred from a due date, scan or birth inherit uncertainty. Do not use this estimate to determine parentage or make medical decisions.
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 show a fertile window instead of one date?+
Ovulation timing varies and sperm can survive for several days. The calculator therefore shows the six-day interval from five days before through one day after estimated ovulation, following ACOG guidance, rather than claiming one exact conception day.
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.