Conception Calculator
Estimate the conception date and fertile window from a due date or birth date.
Key takeaways
- Conception is estimated as due date − 266 days, because a pregnancy averages 266 days from conception and 280 from the last period.1
- It is a window, not a day. Sperm survive up to five days, so conception can occur well after the intercourse that caused it.2
- Conception and implantation are different events. Implantation follows six to twelve days later, and that is when a test can turn positive.3
- Working backwards from a birth date is less reliable than from a due date, because only about 4% of babies arrive on schedule.
- Natural variation in pregnancy length is around five weeks, so any back-calculated date carries that uncertainty with it.4
How to use this calculator
Enter your due date (or a birth date for a retrospective estimate).
The calculator estimates conception and LMP, with a ±5 day fertile window.
Understanding your result
Conception is estimated as the due date minus 266 days, with LMP at due date minus 280 days.
Because ovulation varies, we show a fertile window rather than a single false-precision day.
How the calculation works
Pregnancy is dated in two units that differ by two weeks. Gestational age counts from the first day of the last menstrual period and averages 280 days to delivery. Fetal age, or conceptional age, counts from conception and averages 266 days. The fourteen-day difference is the approximate gap between a period starting and ovulation happening.
So estimating conception is a subtraction: due date minus 266 days. If you are working from a birth date instead, the same arithmetic applies, with an important extra caveat — the baby arrived when it arrived, not necessarily when it was due.
| Measure | Counts from | Average length |
|---|---|---|
| Gestational age | First day of last menstrual period | 280 days (40 weeks) |
| Fetal / conceptional age | Conception | 266 days (38 weeks) |
The 266 figure assumes an average cycle
It builds in a 14-day gap between the last period and ovulation. If your cycles run long or short, your own gap is different, and the estimate shifts by however much your ovulation differs from day 14.
Conception is a window, not a moment
This is the part that most often surprises people, and it occasionally matters a great deal.
Sperm survive up to five days in fertile cervical mucus. Wilcox and colleagues showed that all conceptions in their cohort resulted from intercourse in a six-day window ending on the day of ovulation, and that none resulted from intercourse afterwards.2 Fertilisation itself happens within about 24 hours of ovulation — so conception can occur as much as five days after the act that caused it.
A common source of confusion
If the estimated conception date does not line up with a particular date you remember, that is expected rather than surprising. The estimate identifies roughly when fertilisation occurred, which can be several days after intercourse — and it is derived from a due date that is itself an estimate.
For the same reason, this calculator should not be used to determine paternity or to resolve questions where a specific date carries weight. The uncertainty is measured in days and sometimes more; the appropriate tool for those questions is a DNA test, not arithmetic.
Conception, implantation, and a positive test
Three distinct events happen over roughly two weeks, and they are routinely conflated.
- Fertilisation (conception)
- Sperm meets egg in the fallopian tube, within about 24 hours of ovulation. Nothing is detectable at this point — no hormone change, no symptom, no test.
- Implantation
- The blastocyst embeds in the uterine lining, most often 8 to 10 days after ovulation. Wilcox found that later implantation carried a markedly higher risk of early loss: after day 11, the risk rose sharply.3 Only now does hCG production begin.
- Positive test
- hCG has to accumulate before a test can detect it, which takes a few more days. This is why testing before a missed period so often produces a negative result in a pregnancy that is genuinely progressing.
- Fertilisation (0–1)
- Travel and cell division (1–6)
- Implantation window (6–12)
- Test may turn positive (12–16)
| Category | Range |
|---|---|
| Fertilisation | 0 to 1 days after ovulation |
| Travel and cell division | 1 to 6 days after ovulation |
| Implantation window | 6 to 12 days after ovulation |
| Test may turn positive | 12 to 16 days after ovulation |
How much to trust the date
The estimate inherits every uncertainty in whatever you fed it, and pregnancy length is genuinely variable. Jukic and colleagues measured pregnancies from the day of implantation — removing ovulation-timing noise entirely — and still found a 37-day spread between the shortest and longest normal pregnancies.4
- From a first-trimester ultrasound due date: the best case. Dating is accurate to about ±5 days, so the conception estimate is too.1
- From an LMP-based due date: carries the assumption of a 14-day follicular phase. Accurate to a week or two if your cycles are regular, less if they are not.
- From an IVF transfer: the only situation where conception is genuinely known rather than estimated.
- From an actual birth date: the weakest, because only about 4% of babies are born on their due date and the natural spread is five weeks wide.
Where to go next
For the pregnancy timeline itself, the Due Date Calculator gives a dated week-by-week breakdown. If you are trying to conceive, the Ovulation Calculator works forward to the fertile window instead.
Terms used on this page
- Conception
- Fertilisation of the egg by sperm, within roughly 24 hours of ovulation. Distinct from implantation, which is when pregnancy becomes detectable.
- Implantation
- The embedding of the blastocyst in the uterine lining, typically 8–10 days after ovulation. Later implantation is associated with higher early loss.
- hCG
- Human chorionic gonadotropin — the hormone produced after implantation and detected by pregnancy tests. It cannot be present before implantation, however early you test.
- Fetal age
- Age counted from conception, running about two weeks behind gestational age. The source of most confusion between what a scan reports and what an app says.
The formula
- Due date Oct 8, 2026
- Conception = Oct 8 − 266 days
Source: due date − 266 days. Published clinical/scientific formula.
Limitations
- An estimate — early ultrasound dating is more precise.
- Assumes a 14-day luteal phase; individual cycles vary.
References
- 1.Committee Opinion No. 700: Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists, Obstetrics & Gynecology 129(5), 2017.
- 2.Timing of sexual intercourse in relation to ovulation. Wilcox, Weinberg & Baird, New England Journal of Medicine 333(23):1517–1521, 1995.
- 3.Time of implantation of the conceptus and loss of pregnancy. Wilcox, Baird & Weinberg, New England Journal of Medicine 340(23):1796–1799, 1999.
- 4.Length of human pregnancy and contributors to its natural variation. Jukic et al., Human Reproduction 28(10):2848–2855, 2013.
Frequently asked questions
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Open calculatorRelated articles
How Your Due Date Is Calculated (and Why It's an Estimate)
Four methods, one number, and a wide range of normal. What Naegele's rule assumes, why ultrasound usually wins, and what the date actually means.
Read articleOvulation and the Fertile Window: How to Time Conception
Why the fertile window is six days rather than one, where the day-14 assumption goes wrong, and what tracking methods actually tell you.
Read articleDue Date Accuracy by Method: LMP vs. Ultrasound vs. IVF
Four ways to date a pregnancy, accurate to anywhere from one day to two weeks. Which one your date came from tells you how much to trust it — and what happens when two of them disagree.
Read articleNot medical advice. This calculator is for general informational purposes only and is not a substitute for professional medical guidance. Always consult a qualified healthcare provider before making decisions about your health. Read the full medical disclaimer.
Printed from mystatura.com — for general information only, not medical advice.