The short answer: conception happens about 266 days before the due date — that is, 38 weeks, not 40. Counting from a last menstrual period instead, conception is the day of ovulation, roughly 14 days before the next period would have been due. Because ovulation moves by several days between cycles, a conception date is always a window of about four days either side rather than a single day, and the date of conception is not necessarily the date of intercourse: sperm can wait up to five days for the egg.
The three ways to work it out, and which one to trust
| Starting point | Formula | Typical accuracy |
|---|---|---|
| Due date | Due date − 266 days | As good as the due date it came from — which itself came from a scan or an LMP |
| Last period | LMP + (cycle length − 14 days) | ±4–5 days if cycles are regular; much wider if they are not |
| First-trimester scan | Scan date − (gestational age − 14 days) | Best available — an 8–13 week scan dates a pregnancy to within about 5–7 days |
If you have had a dating scan, use it. ACOG is explicit that an early ultrasound is the most reliable method of establishing gestational age, and everything else — including the conception date — follows from that.
Why you were "two weeks pregnant" before conception
This is the part that confuses almost everyone, and it is not a technicality that gets corrected later. Every date in your pregnancy will be counted this way.
Gestational age runs from the first day of your last menstrual period, because that is the one date most women can state with confidence. The day of conception almost never can be. Ovulation follows roughly two weeks later, so:
- At conception, you are already dated as 2 weeks pregnant.
- At implantation, 6 to 12 days after conception, you are 3 weeks.
- When a home test first turns positive, usually around the missed period, you are 4 weeks.
- A 40-week pregnancy therefore contains about 38 weeks of actual embryo and fetus.
The week 1 page covers this in detail — week 1 of pregnancy is the week of your period, before there is an embryo at all.
Conception is not the same as the day you had sex
Fertilisation happens when a sperm reaches the egg, which is fertilisable for only about 12 to 24 hours after ovulation. But sperm can survive in cervical mucus and the reproductive tract for up to five days, waiting for that release.
So intercourse on a Monday can produce a conception on the following Friday. This is the mechanism behind the six-day fertile window, and it is why the calculator above reports a range of possible dates for intercourse alongside the single most likely date of fertilisation.
On paternity questions
A large share of the people who look for a conception date calculator are trying to answer a paternity question. It is worth being direct about what a calculator can and cannot do here.
It cannot establish paternity. Even at its most precise — a first-trimester ultrasound — dating carries an error of roughly five to seven days, and an LMP-based estimate is wider still. Add the five-day sperm survival window on top and the range of possible intercourse dates spans well over a week. That is not a basis for a conclusion about anybody.
What does answer the question is a DNA test: a non-invasive prenatal paternity test, which uses fetal DNA circulating in the mother's blood and can be done from around 8 weeks, or a standard test after birth. Both are definitive in a way no calculation ever is.
What changes your conception date later
- The dating ultrasound. If the scan estimate differs from the LMP-based one by more than about 5 to 7 days, the scan date is adopted — and your conception date shifts with it.
- Late ovulation. Common after stopping hormonal contraception, during illness, or with long cycles. It pushes conception later than the day-14 assumption.
- An uncertain LMP. Spotting mistaken for a period, or simply not having noted the date, both widen the estimate considerably.
- Recent pregnancy loss or a recent birth. The first cycle afterwards is often unpredictable, which makes LMP-based dating unreliable.
After the dating scan, the due date is fixed for the rest of the pregnancy even if later scans measure the baby big or small — growth varies, but the first-trimester measurement of when the pregnancy started does not get revisited. The early ultrasound guide explains what each early scan can and cannot show.
What to do with the date once you have it. Put it into the conception tab of the due date calculator to confirm your due date, then into the pregnancy calendar to see every screening window against your own calendar dates. If you are earlier than 8 weeks and have had serial blood tests, the hCG doubling calculator is the more relevant tool right now.
When to get checked rather than calculate
Contact your doctor or midwife the same day if you have:
- Severe pain on one side of the lower abdomen with a positive pregnancy test
- Shoulder-tip pain, faintness or collapse
- Vaginal bleeding that is heavy, or bleeding with pain
- Uncertainty about how far along you are combined with any of the above
Those are the symptoms of an ectopic pregnancy, which is a medical emergency and cannot be ruled out by any calculation.
Related tools and guides
FAQ — Conception dates
How is the conception date calculated?
From a due date, conception = due date − 266 days (38 weeks). From a last menstrual period, conception is the day of ovulation, which is the LMP plus your cycle length minus 14 days. From a scan, conception is the scan date minus the gestational age, plus 14 days — because gestational age already counts from the last period rather than from conception.
Why does pregnancy count two weeks before conception?
Because the last menstrual period is the only date that can usually be stated reliably, and the day of conception almost never can. Gestational age is counted from the first day of the LMP, which is roughly two weeks before ovulation. That is why you are already "4 weeks pregnant" when a test first turns positive, and why 40 weeks of pregnancy contains about 38 weeks of actual embryo and fetus.
How accurate is a conception date?
It is an estimate with a window of several days on either side. Ovulation moves between cycles in the same woman, and the 14-day assumption behind the LMP method is only an average. An early dating ultrasound narrows things considerably — between 8 and 13 weeks it dates a pregnancy to within about 5 to 7 days — but even that is a range, not a day.
Is the conception date the same as the day I had sex?
Not necessarily. Sperm can survive in the reproductive tract for up to five days, so fertilisation may happen days after intercourse. Conception happens when the egg is fertilised, which is within roughly 12 to 24 hours of ovulation — regardless of when the sperm arrived.
Can a conception date calculator establish paternity?
No, and it should not be used to try. The window this tool produces is several days wide, and an early scan narrows it to about five to seven days at best. If paternity is genuinely in question, a non-invasive prenatal paternity test from around 8 weeks, or a post-birth DNA test, is the only thing that answers it. A calculator gives a probability of timing, not an identification.
What if I had IVF?
Then you already know the date exactly, and this calculator is unnecessary. Conception is the day of fertilisation in the lab. Dating runs from the transfer: a day-5 blastocyst transfer means the pregnancy is already 2 weeks and 5 days along on transfer day. The IVF tab on the due date calculator handles the arithmetic.
My scan gave a different conception date from my period. Which is right?
The scan. An early ultrasound measures the embryo directly, while an LMP-based date assumes ovulation on day 14 of a 28-day cycle. If the two differ by more than about 5 to 7 days, the ultrasound date is normally adopted — and once it is, it is not changed again for the rest of the pregnancy.
Can I calculate the conception date if I do not know my last period?
Yes — use the ultrasound tab. Any scan report that states a gestational age in weeks and days, along with the date it was performed, is enough. That is also the most accurate of the three routes if the scan was done in the first trimester.
Sources
- ACOG Committee Opinion 700 — Methods for Estimating the Due Date
- ACOG — Trying to Get Pregnant? Here’s When to Have Sex
- NHS — Due date calculator
The conception date calculator — at a glance
Conception is dated 266 days before the due date, or equivalently at ovulation, which falls about 14 days before the next period would have been due rather than 14 days after the last one ended. Because gestational age is counted from the last menstrual period rather than from fertilisation, a pregnancy is already dated at two weeks on the day conception happens, three weeks at implantation, and four weeks when a home test first turns positive — so 40 weeks of pregnancy contains roughly 38 weeks of embryo and fetus. Conception is not the same event as intercourse: sperm survive up to five days, so fertilisation can occur days after sex, which is why this calculator reports both a most likely date and the intercourse window that could have produced it. A first-trimester ultrasound is the most accurate route of the three and dates a pregnancy to within about five to seven days; where it disagrees with the period-based estimate by more than that, the scan is adopted and the due date is then fixed for the rest of pregnancy. No calculation is precise enough to settle a paternity question — only a DNA test does that. Once you have the date, carry it into the due date calculator and the week-by-week pregnancy calendar.