Calcul Date De Conception
Enter one date you already know — your due date, the first day of your last period, or your ovulation date — and see the estimated conception date, the fertile window and your whole pregnancy timeline right away.
Instant, private and free. Estimates are for reference only — this is not medical advice.
Conception date calculator
Pick the date you know. The result updates as you type — no page reload, nothing sent to a server.
Your dates never leave your device. Estimate only — not medical advice.
How the conception date is calculated
A pregnancy is dated from the first day of your last period: LMP + 280 days (40 weeks) gives the due date. Subtracting 14 days lands on the most likely ovulation and conception day.
Ovulation happens about 14 days before the next period, not 14 days after the last one. With a 35-day cycle the estimate shifts about a week later; with a 24-day cycle, about 4 days earlier.
Sperm survive up to 5 days and the egg about 24 hours, so the realistic conception window is conception −5 days to +1 day, shown as a ± 5-day uncertainty band.
What the dates mean
The two are about two weeks apart. If you were given a last-period date, use "First day of last period (LMP)" — entering it as a conception date shifts the answer by roughly 14 days.
Pregnancy week 1 and 2 happen before conception. So at 4 weeks pregnant you are only about 2 weeks past conception — that is normal, not an error.
All arithmetic runs inside this page on your phone. No account, no upload, no storage — close the tab and nothing remains.
A dating ultrasound (usually between 8 and 14 weeks) measures the baby directly. If it differs from this estimate by more than a few days, the scan is the more reliable date.
If you know the ovulation, insemination or embryo-transfer day, choose "Known conception / ovulation date" — cycle length then only positions the LMP and the due date.
Outside that range the calculator clamps the value and tells you, because irregular or very long cycles make a single conception date unreliable.
FAQ
How accurate is the conception date?
It is a good estimate, not a fact. Based on a due date it is typically within about ± 5 days, and it cannot tell you which act of intercourse caused the pregnancy.
Why is my LMP two weeks before conception?
Because pregnancy is counted from the last period, and ovulation happens about two weeks later. This convention lets every pregnancy be measured on the same 40-week scale.
My doctor's due date differs from yours — why?
Doctors often adjust the due date after a dating ultrasound, and they assume a 28-day cycle by default. Change the cycle length here, or trust the ultrasound — it measures the baby, not the calendar.
Can I use this for a 2-day or 45-day cycle?
Cycle lengths are accepted between 20 and 45 days. Anything outside is clamped to the nearest bound, with a note explaining the ovulation offset used.
Is my data stored?
No. The calculation happens in your browser, the page makes no network request, and nothing is saved on any server.
Estimate, not medical advice
Every date on this page is an arithmetic estimate built from Naegele's rule and your cycle length. Conception dates, fertile windows, gestational age and due dates can only be confirmed by a healthcare professional, usually with a dating ultrasound. Never use this tool to make a medical decision — talk to your midwife or doctor instead.