Conception Calculator

Start from a due date, a birthday or your last period — get the most likely conception date and the window around it.

What do you know?

Most likely conception date

—

Enter a date on the left to begin.

Likely conception window—
Intercourse window—
Equivalent last period (LMP)—
Due date implied—

Conception can only ever be bracketed, not pinpointed: eggs are fertilisable for ~24 hours, but sperm survive up to five days — so the responsible act may be days before the biological moment.

Working backwards to conception

Conception happens at ovulation — on average 266 days (38 weeks) before the due date. So given a due date, the most likely conception day is simple subtraction; given a birth date, we first rebuild the due-date arithmetic using how many weeks the pregnancy had reached at delivery; given a last period, we add your cycle length minus 14 to estimate ovulation directly.

Why it's a window, not a day

Two kinds of uncertainty stack up. Ovulation itself varies a few days around any calendar estimate — hence the ±3-day conception window. And because sperm survive up to five days, the intercourse that led to conception may sit anywhere in the five days before the biological moment. The calculator reports both windows separately because they answer different questions.

Starting from a birth date

A birthday alone can't reveal conception — babies arrive anywhere from weeks early to weeks late. If you know the gestational age at delivery (it's on most birth records, e.g. "born at 38+2"), enter the weeks and the estimate tightens considerably. Left at 40, the result assumes a textbook-length pregnancy.

Common uses

People reach for this tool to satisfy curiosity about an anniversary-of-sorts, to reconcile dates with a partner or clinician, or while assembling a personal or family timeline. For anything with legal weight — paternity in particular — calendar arithmetic isn't evidence; DNA testing is the only reliable route.

ListCalc health tools are educational estimates, not medical advice. Dates and ranges here are population averages — your midwife, OB-GYN or doctor confirms what applies to you. See our disclaimer.

Frequently asked questions

Can this tell me exactly which day I conceived?
No tool can. Ovulation timing varies and sperm survive for days, so an honest answer is always a window \u2014 here, about a week wide at best.
Does conception date equal the day we had sex?
Not necessarily. Sperm can wait up to five days for the egg, so intercourse up to five days before ovulation can be the cause. That's why the intercourse window is shown separately.
I only know the birthday. How good is the estimate?
It depends on how close to 40 weeks the pregnancy ran. If you know the gestational age at birth (\u201cborn at 38+2\u201d), enter it and the window narrows a lot; without it, allow a couple of weeks of slack either way.
Why 266 days from conception to due date?
The standard 280-day (40-week) due-date convention counts from the last period, which precedes ovulation by roughly 14 days. Subtract those and you get 266 days from conception \u2014 38 weeks.
Can this settle a paternity question?
No. Windows overlap easily and calendar estimates carry real uncertainty. Only DNA testing can answer paternity reliably \u2014 treat these dates as informational.
My scan-based due date changed. Which one do I use here?
Use the most recent clinical due date \u2014 usually the one set by an early ultrasound. It's the most accurate anchor, and every window here inherits its accuracy from the date you enter.

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Pregnancy Due Date Pregnancy Weight Gain Ovulation Period BMI