Calculator Pregnancy
Conception date calculator
Conception typically happens around ovulation — roughly two weeks after your last period began. Work it out from your last period or backward from your due date, with the likely window.
Estimated conception date
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By the numbers Context
How a conception date is estimated
Conception — when a sperm fertilises an egg — happens around ovulation, which for most people is about two weeks after the first day of their last period. That two-week gap is the source of a lot of confusion: because doctors count pregnancy from the last period rather than from conception, you are considered “two weeks pregnant” at the moment you actually conceive. This calculator can estimate conception from your last period or backward from a known due date.
It is a window, not a precise day. Sperm can survive up to about five days in the body, so fertilisation may happen several days after intercourse, and the exact moment is rarely knowable. For that reason a conception estimate cannot establish paternity or serve as a legal date — a first-trimester ultrasound, which measures the baby directly, is the accurate way to date a pregnancy.
~2 weeks
gap between the start of your last period and conception
Clinical
~266 days
from conception to the estimated due date
Clinical
Sources: standard obstetric dating; American College of Obstetricians and Gynecologists.
Frequently asked questions
How can I know the exact date I conceived?
You usually cannot know the exact day. Conception happens around ovulation, and because sperm survive several days, fertilisation can lag intercourse. This calculator estimates the most likely window from your last period or due date.
Does 4 weeks pregnant mean I conceived 4 weeks ago?
No. Pregnancy is counted from the first day of your last period, which is about two weeks before conception. So at “4 weeks pregnant”, conception was roughly two weeks earlier.
How do I estimate conception without my last period?
You can work backward from your due date — conception is about 266 days before it — or from a dating ultrasound. This calculator includes a due-date method for exactly this case.
Can a conception calculator establish paternity or a legal date?
No. It gives an estimated window based on average timing, not a precise or legally reliable date. Questions of paternity or legal dating need medical testing, not a calendar estimate.
How accurate is a conception date estimate?
It is an approximation. The most accurate way to date a pregnancy is a first-trimester ultrasound, which your clinician will use in preference to any calculated date.