Conception is the moment a sperm fertilises an egg, which happens within about a day of ovulation. Working backwards from a due date or a last period gives a good estimate of when that was — but conception and the sex that caused it are not the same date. Sperm survive for up to five days, so the useful answer is a window rather than a day.
- What it estimates
- The date of fertilisation
- From due date
- Due date − 266 days
- From last period
- LMP + (cycle length − 14) days
- Conception window
- 5 days before to 1 day after ovulation
- Gestational age offset
- Conception is 2 weeks after "week 0"
Formula
Reverse Naegele's ruleConception = due date − 266 days Conception = LMP + (cycle length − 14) Window = conception − 5 days to conception + 1 day
- LMP
- — First day of the last menstrual period
- 266 days
- — 38 weeks — the 280 of Naegele's rule minus 14
- cycle length
- — Day 1 of one period to day 1 of the next
The window is asymmetrical because the two cells involved have very different lifespans. Sperm can survive up to five days in fertile cervical mucus, so intercourse well before ovulation can still cause a pregnancy; the egg lasts only 12 to 24 hours, so the window closes almost immediately after ovulation.
Conception Date Calculator examples
| Input | Result | Category |
|---|---|---|
| Due date Oct 15, 2026 | Jan 22, 2026 | 9w pregnant today |
| Due date Jan 1, 2027 | Apr 10, 2026 | 9w 3d pregnant today |
| Due date Jun 30, 2026 | Oct 7, 2025 | 17w pregnant today |
| LMP Jan 15, 2026 · 28-day cycle | Jan 29, 2026 | 8w pregnant today |
| LMP Jan 15, 2026 · 34-day cycle | Feb 4, 2026 | 7w 1d pregnant today |
| LMP Apr 2, 2026 · 24-day cycle | Apr 12, 2026 | 9w 1d pregnant today |
How do you calculate your conception date?
From a due date, subtract 266 days — 38 weeks. A due date of 15 October 2026 puts conception around 22 January 2026. The 266 is 40 weeks minus the two weeks that a pregnancy is counted for before conception happens.
From a last menstrual period, add your cycle length and subtract 14, because ovulation sits about a fortnight before the next period whatever the cycle length. A 28-day cycle conceives around 14 days after the period starts; a 34-day cycle around day 20.
Why is the conception window a week wide?
Because the date of conception and the date of intercourse can be nearly a week apart. Fertilisation itself is a narrow event — within roughly 24 hours of ovulation — but the sperm responsible may have been waiting for days.
- Sperm survive up to five days in fertile cervical mucus, so intercourse five days before ovulation can still result in pregnancy.
- The egg survives 12 to 24 hours, so the window shuts roughly a day after ovulation.
- The two days before ovulation and the day itself carry the highest probability of conception.
- On top of that, the estimated ovulation date is itself uncertain by several days unless it was tracked.
How does the conception date differ from gestational age?
These are two different clocks and they are always 14 days apart. Gestational age counts from the first day of the last period; embryonic or fetal age counts from conception. Every clinical measure — scan reports, screening windows, viability thresholds — uses gestational age.
| Gestational age | Embryonic age | What is happening |
|---|---|---|
| 4 weeks | 2 weeks | Missed period; pregnancy tests turn positive |
| 6 weeks | 4 weeks | Cardiac activity usually visible on ultrasound |
| 8 weeks | 6 weeks | All major organ systems have begun forming |
| 12 weeks | 10 weeks | End of the embryonic period |
| 40 weeks | 38 weeks | Due date |
The offset is why someone told they are "8 weeks pregnant" at a first scan can be certain they were not pregnant eight weeks earlier. Both statements are true; they are measuring from different starting lines.
How accurate is a conception date estimate?
It inherits the accuracy of whatever it was calculated from. A due date set by IVF transfer gives an exact conception date. One set by a first-trimester ultrasound is good to about five days. One derived from a remembered last period, in someone whose cycles vary, can easily be out by a fortnight.
| Source of the due date | Conception date accuracy |
|---|---|
| IVF transfer | Exact |
| Ultrasound before 14 weeks | ± 5 to 7 days |
| Tracked ovulation (LH kit or temperature) | ± 1 to 2 days |
| Last menstrual period, regular cycles | ± 7 to 10 days |
| Last menstrual period, irregular cycles | ± 2 weeks or more |
Frequently asked questions
How do I know exactly when I conceived?
Only tracked ovulation or IVF gives an exact date. Everything else is an estimate: subtract 266 days from your due date for the best single guess, then allow a window of five days before to one day after, because sperm survive that long.
Can a conception date be used to prove paternity?
No. Even the best dating method carries about a week of uncertainty either side, which is wide enough to include more than one possible encounter. Non-invasive prenatal paternity testing, available from around 8 weeks of pregnancy, is the only reliable answer.
Why is my conception date two weeks after my "week 0"?
Pregnancy is dated from the first day of your last period, which precedes ovulation by about two weeks. So at the moment of conception you are already counted as two weeks pregnant. The convention exists because the period date is knowable and the conception date usually is not.
Does a long cycle change the conception date?
Yes. Ovulation sits about 14 days before your next period, so a 34-day cycle ovulates around day 20 rather than day 14. Conception is six days later than the standard assumption, and the due date shifts by the same six days.
Is the conception date the day I had sex?
Not necessarily. Fertilisation happens within a day of ovulation, but the sperm involved may have arrived up to five days earlier. Intercourse on any of the five days before ovulation, or on the day itself, could be responsible for the pregnancy.
Can I get pregnant on the day of ovulation?
Yes, though it is not the peak day. Conception probability is highest from intercourse on the two days before ovulation, because sperm need time to reach the fallopian tube. Intercourse on ovulation day still works; the day after almost never does.
Sources
- Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, 2017
- How to get pregnant — Mayo Clinic
- Pregnancy — Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Fetal development — MedlinePlus, U.S. National Library of Medicine