Pregnancy Conception Calculator
Work backwards from a pregnancy you already know about to the days conception most likely happened, and the days intercourse could have led to it. Start from a due date, a last period, or a dating scan — whichever number you actually have.
Conception dates
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Dates of intercourse that led to the pregnancy
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Conception dates
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Dates of intercourse that led to the pregnancy
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These are estimates only. Conception is not directly observed, and the ranges above are wide because the biology genuinely is.
All three routes end at the same place: a notional first day of the last period, from which ovulation and then conception are estimated.
| Range | Spans | Why |
|---|---|---|
| Most probable conception | 5 days | The fertile window either side of estimated ovulation |
| Most probable intercourse | 8 days | The same window, opened 3 days earlier for sperm survival |
| Possible conception | 11 days | Allows for ovulation landing earlier or later than average |
| Possible intercourse | 16 days | The wider window, opened 5 days earlier for sperm survival |
Running the clock backwards
Most pregnancy tools point forwards: you know where you are and you want a date ahead of you. This one runs the other way. It starts from something already established — a due date on a maternity note, a remembered period, a scan measurement — and reconstructs the days conception is likely to have happened.
That reversal matters because reversing a calculation does not reverse its uncertainty. It adds to it. A due date is itself an estimate, so working backwards from it inherits everything already approximate about it and then layers on the further guess of when ovulation occurred. Nothing here tightens as it goes; it widens.
Which is why this page returns ranges rather than a day. That is not caution for its own sake. Conception is not observed by anybody, it leaves no timestamp, and there is no test that can recover the date after the fact. What can be reconstructed is a window, and the honest output is the window.
The two weeks that were never pregnancy
Here is the thing that catches almost everyone out. Pregnancy is counted from the first day of the last menstrual period, but nobody is pregnant on that day — conception is still roughly two weeks away. The clock starts before the event it is measuring.
due date → last period = due date − 280 days
last period → used as entered
ultrasound → last period = scan date − (weeks × 7 + days)
Then, from that one date
ovulation = last period + (cycle length − 14)
most probable = ovulation − 2 … ovulation + 2
possible = ovulation − 3 … ovulation + 7
Worked through for a due date of 7 December 2026
7 Dec − 280 = 2 Mar → 2 Mar + 14 = 16 Mar
most probable conception 14 Mar – 18 Mar
So a pregnancy described as eight weeks along involves an embryo around six weeks old. The two-week gap is a convention, not an error: the last period is the last thing that can be dated reliably from the outside, so obstetric counting uses it as the anchor and everyone accepts the offset.
The practical upshot for this page is that the notional last period it works out is a bookkeeping date, not necessarily a day you bled. If you came in through a due date or a scan, that date is reconstructed, and it may not match what you remember. That is expected.
Three routes in, and which to trust
The three options are not equally good, and if you have more than one number available it is worth knowing which to enter.
An early ultrasound is the strongest. A scan in the first trimester measures the embryo directly and dates the pregnancy from its size, which sidesteps the assumption about when you ovulated entirely. Early scans are the most precise, and dating drifts as pregnancy progresses because babies grow at increasingly different rates. If your scan was late, the number it gave is a weaker anchor than an early one.
A last period is next, if you are sure of it. Its weakness is not the date but the assumption stacked on top: that you ovulated on a predictable schedule. It also lets you enter your real cycle length, which the other two routes cannot — they both assume twenty-eight days. If your cycles run notably longer or shorter, this route will fit you better than a due date will.
A due date is the weakest anchor of the three, because it is already a derived figure. Whoever produced it worked forwards from a period or a scan, so entering it here just runs the same arithmetic in reverse and reintroduces every assumption made along the way. Use it when it is the only number you have.
Why four ranges instead of one date
The results split along two axes, and the difference between them is worth reading properly.
Conception versus intercourse. The conception ranges cover when the egg was likely fertilised. The intercourse ranges are wider at the front because sperm survive: intercourse several days before an egg appears can still result in pregnancy, so the window of relevant days opens earlier than the window of conception itself. That difference is three days for the most probable range and five for the possible one.
Most probable versus possible. The narrow pair assumes ovulation landed near the expected day. The wide pair allows for it landing early or late, which happens often enough that the wider range is the more realistic answer for most people. Note that the possible range is asymmetric — it stretches further forwards than backwards, because ovulation is more prone to running late than early.
If you are trying to answer a specific question, the wide ranges are usually the ones to use. The narrow ones describe the most likely scenario, not the only one, and a date sitting outside them is not thereby excluded.
If you are here about paternity
A large share of people running this calculation are trying to work out which of two possible dates a pregnancy came from. It is worth saying plainly what this page can and cannot do about that.
It cannot settle it. The possible intercourse range is sixteen days wide, and that width is not a flaw in the tool — it reflects genuine uncertainty about when ovulation happened. If the dates you are weighing both fall inside that window, the arithmetic offers no way to separate them. Even when one date sits outside the narrow range and the other inside, that is a difference in likelihood, not proof, and ovulation timing varies enough that likelihood is a weak thing to build a conclusion on.
What does settle it is a DNA test. Non-invasive prenatal paternity testing can be done from a maternal blood sample from around the eighth or ninth week, and after birth a standard test is straightforward and conclusive. If the answer genuinely matters — emotionally, legally, or medically — that is the route to a real answer, and no amount of counting backwards from a due date will substitute for it.
An early dating scan will also narrow the window more than any of these three routes can, so if the pregnancy is early, asking about one is worth doing.
Fertilised, conceived, pregnant
These three words get used as though they mean the same thing, and in ordinary conversation that rarely causes trouble. In a context where dates are being argued over, the distinctions start to matter.
Fertilisation is the moment sperm and egg combine, in the fallopian tube, usually within a day of the egg being released. Implantation is when the resulting embryo embeds in the uterine wall, roughly six to ten days later. In medical use, pregnancy is generally taken to begin at implantation rather than at fertilisation, which is why a home test cannot detect anything until after it has happened — the hormone it looks for is not produced before then.
The dates on this page estimate fertilisation, since that is what follows from ovulation. If you need the date pregnancy began under a definition that starts at implantation, add roughly a week. Which definition applies is not a scientific question but a legal and policy one, and it differs between jurisdictions.
What it cannot settle
Every route through this page assumes ovulation happened a predictable interval before the next period would have been due. That assumption is reasonable on average and unreliable in any individual cycle, and it is the reason the ranges are as wide as they are.
It is weaker still if your cycles are irregular, if you conceived shortly after stopping hormonal contraception, or if you were breastfeeding — in all of those, ovulation timing is unusually unpredictable. It does not apply to IVF pregnancies at all, where the transfer date is known exactly and the clinic's dating is the correct one. Nor does it account for twins, which are dated the same way but grow differently.
None of these numbers is a medical record, and none of them should be presented as establishing a date. If a precise figure matters for antenatal care, your midwife or doctor has the scan measurements, which beat anything reconstructed from arithmetic.
Frequently asked questions
How do I work out when I conceived?
Start from the most reliable date you have. From a due date, subtract 280 days to recover the notional last period; from a scan, subtract the gestational age you were given. Then add your cycle length minus fourteen to reach estimated ovulation, and take two days either side. For a due date of 7 December 2026 that lands on 14 to 18 March.
Can this calculator tell me exactly what day I conceived?
No, and nothing can. Conception is not observed and leaves no record that can be dated afterwards. Ovulation moves between cycles, so even a perfectly remembered period only narrows things to a window. The ranges here are the honest form of the answer.
Which option gives the most accurate result?
An early ultrasound, because it measures the embryo directly rather than assuming when you ovulated. A last period is next best if you are confident of the date, and it has the advantage of letting you enter your real cycle length. A due date is the weakest, since it was itself derived from one of the other two.
Can I use this to work out who the father is?
No. The possible intercourse range spans sixteen days precisely because ovulation timing is uncertain, so two dates falling inside it cannot be separated by arithmetic. Even a date outside the narrow range is only less likely, not excluded. A DNA test answers this properly — prenatally from around the eighth or ninth week, or straightforwardly after birth.
Why is the conception date about two weeks after my last period?
Because pregnancy is counted from the last period, which is roughly a fortnight before conception actually occurs. The last period is simply the last event that can be dated reliably from outside, so obstetric counting anchors to it. It means a pregnancy at eight weeks contains an embryo of about six.
What is the difference between the conception range and the intercourse range?
Conception is when the egg was fertilised. The intercourse range starts earlier because sperm can survive for several days, so sex that happened before the egg was released can still have caused the pregnancy. The gap is three days on the narrow range and five on the wide one.
Does this work for IVF?
No. With IVF the fertilisation date is known exactly, so there is nothing to estimate, and the dating your clinic uses is based on the transfer rather than on a menstrual cycle. Working backwards from an IVF due date with this page will produce a date that is wrong.
These are estimates reconstructed by arithmetic and are not a record of what happened. They cannot establish a date of conception for any legal, immigration or paternity purpose, and they should not be relied on for one. Ovulation timing varies between cycles and is less predictable after hormonal contraception, while breastfeeding, and where cycles are irregular. Questions about paternity are answered by DNA testing; questions about how far along a pregnancy is are answered by a dating scan. For anything concerning your pregnancy or your care, please speak to a doctor or midwife. Learn more about how we build our calculators and read our privacy policy.