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Learn more →Conception Date Calculator — from due date or birth date
Enter your due date or your baby's date of birth and the calculator estimates when conception most likely happened, the surrounding window, the days intercourse could have led to it, and the matching last menstrual period.
Enter a due date or birth date to estimate conception.
How it works
How conception dating works
A due date is conventionally 280 days (40 weeks) after the first day of the last menstrual period, and conception happens about 14 days after that — so conception is roughly 266 days before the due date. This calculator simply runs that math in reverse: it subtracts 266 days from your due date (or your baby's birth date) to land on the most likely day of conception.
Conception is the moment a sperm fertilises the egg, which can only happen for about 24 hours after ovulation. Because sperm survive up to five days in the body, the intercourse that led to conception may have happened a few days earlier — which is why the calculator also shows a short intercourse window before the conception date.
Why it's a window, not a single day
Two things blur the exact date. First, ovulation doesn't always fall on cycle day 14 — it shifts with cycle length and from month to month. Second, pregnancies are not all exactly 40 weeks: a full-term birth is anywhere from 37 to 42 weeks, so working back from an actual birth date spreads the possible conception date across a couple of weeks.
That's why the result is shown as a window around the most likely day. If you tracked ovulation, a positive ovulation test, or had an early dating ultrasound, those give a tighter estimate than this reverse calculation.
Due date vs birth date
Starting from a due date gives a narrower window, because the due date is defined as the 40-week point — so conception is pinned close to 266 days before it. Starting from an actual birth date gives a wider window, because the baby could have arrived any time in the full-term range, and you don't know in advance how long that particular pregnancy ran.
If you only know the birth date and want a single best guess, use the most likely conception date; if you need to narrow it, an early-pregnancy ultrasound report (which records gestational age at the scan) is the most reliable anchor.
Frequently asked questions
›How is the conception date calculated?
It subtracts 266 days (38 weeks) from your due date, or from your baby's birth date. That's the standard interval from conception to delivery, the same convention clinicians use.
›Is the conception date the same as the day I had sex?
Not necessarily. Sperm can survive up to five days, so conception can happen several days after intercourse. The calculator shows a short intercourse window before the conception date for this reason.
›How accurate is it?
It's a good estimate but not exact. Ovulation timing varies and pregnancies range from 37 to 42 weeks, so treat the result as a window. Ovulation tracking or an early ultrasound is more precise.
›Why is the window wider for a birth date?
A due date is fixed at 40 weeks, so conception is pinned near it. An actual birth could have come anywhere from 37 to 42 weeks, which spreads the possible conception date across about two weeks.
›Can this tell me the baby's father?
No. The calculator gives an estimated date range, not certainty about a specific day. For paternity questions, only DNA testing gives a definitive answer.
›What's the difference between conception and gestational age?
Gestational age is counted from the last menstrual period (about two weeks before conception), while conception age is counted from fertilisation. Gestational age is roughly two weeks larger.
›I had IVF — does this apply?
For IVF, the conception (fertilisation) date is known precisely from the egg retrieval, so you don't need to estimate it. This tool is most useful for natural conception.
›Is my data sent anywhere?
No. The calculation runs entirely in your browser; nothing is uploaded.
Sources & methodology
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DisclosureRelated tools
This tool is for general information only and is not medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about your health.
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