Women's Health
Conception Calculator
Work backwards from a due date or a scan to the most likely date of conception — and the range of days it could have been.
Conception Date Calculator
These dates are estimates from an average cycle. Real cycles vary month to month, so treat every result as approximate and confirm anything you are relying on with a midwife or doctor.
Looking forward instead of back? The Ovulation Calculator predicts your upcoming fertile window.
About the Conception Date Calculator
This conception calculator works like a reverse due date calculator. Given a due date, a dating scan, or the first day of your last period, it estimates when conception most likely happened — and, more usefully, the range of days across which it could have occurred.
The range is the important output. Conception cannot be pinned to a single day from a due date, because the due date was itself estimated from averages. What the arithmetic genuinely supports is a window of several days, and presenting one date would be false precision.
If you are looking forward rather than back — trying to work out when you will next be fertile — the ovulation calculator is the tool you want.
How to use the Conception Date Calculator
- Choose what you are working backwards from — a due date, a period, or a scan.
- Enter that date.
- For the last-period method, set your cycle length.
- For a scan, enter the gestational age measured at that appointment.
- Select Find the conception date and read the range, not just the single date.
The three methods
From a due date : conception = due date − 266 days
From your period: conception = LMP + 14 + (cycle length − 28)
From a scan : work back to the LMP, then as above
266 days is the average gestation from conception; 280 from the last period.
Worked example
A due date of 10 June.
Conception: 10 June − 266 days = 17 September.
Last period: 10 June − 280 days = 3 September.
Most likely 17 September — possible range roughly 12–18 September
The range comes from sperm surviving up to five days before ovulation and the egg lasting about a day after it.
Why there is a range rather than a date
Three separate uncertainties stack up, and each widens the window:
- Sperm survival. Intercourse up to five days before ovulation can result in conception, so the act and the conception may be days apart.
- Ovulation timing. The calculation assumes a 14-day luteal phase. Yours may differ, and ovulation can be delayed by stress, illness or travel.
- The due date itself is an estimate. A period-based date carries one to two weeks of uncertainty; even a first-trimester scan carries five to seven days.
Combine them and the realistic window is usually six days at best, and often ten to fourteen when the due date came from a period rather than an early scan.
How accurate is a conception date from an ultrasound?
Better than one from a remembered period, and still a range. An early scan measures the embryo and compares it against expected growth, which is consistent enough in the first trimester to date a pregnancy within about five to seven days.
| Dating source | Uncertainty in the due date | Resulting conception window |
|---|---|---|
| IVF transfer | About 1 day | Known almost exactly |
| Scan before 14 weeks | ±5 – 7 days | About 10 – 12 days |
| Scan at 14–20 weeks | ±7 – 14 days | About 2 – 4 weeks |
| Last period, regular cycles | ±1 – 2 weeks | About 2 – 4 weeks |
| Scan after 20 weeks | ±2 – 3 weeks | 4 weeks or more |
Later scans are much weaker for dating, because by then babies genuinely differ in size for reasons unrelated to age. A third-trimester scan tells you about growth, not about when conception happened.
Can this establish who the father is?
No, and it is worth being direct about why. The conception window this tool produces spans several days at minimum — often ten to fourteen. If intercourse with more than one partner occurred anywhere within or near that window, the arithmetic cannot separate them. Sperm surviving up to five days widens it further, so even precisely remembered dates do not resolve the question.
The only reliable answer is a DNA paternity test. Non-invasive prenatal paternity testing is available from around 7 to 9 weeks of pregnancy using a maternal blood sample, and post-birth testing is straightforward and highly accurate. Both are widely available, and a doctor or a clinic can explain the options without judgement.
Please do not use a calculated conception date to draw conclusions about paternity. The window is genuinely too wide, and people have made serious decisions on this arithmetic that a test would have contradicted.
Conception and implantation are different events
Conception is fertilisation — sperm meets egg, usually in the fallopian tube, within about 24 hours of ovulation. Implantation comes later, when the embryo embeds in the uterine lining, typically 6 to 12 days afterwards.
Pregnancy hormone (hCG) only becomes detectable after implantation, which is why a test taken very early can be negative even though conception has occurred. Some people notice light implantation spotting around this time; many notice nothing at all, and its absence means nothing.
To go the other way — from a conception date to a due date — use the due date calculator. To find out how far along you are now, try the pregnancy calculator.
This is an estimate, not medical advice. It works from average cycle figures, and real bodies vary a great deal month to month. Nothing here replaces a consultation. For anything you are relying on — a pregnancy, fertility treatment, a health concern, or a decision about contraception — speak to a midwife, doctor or sexual health service who can assess your own situation.
Conception month and due date chart
A baby is usually born about 38 weeks (266 days) after conception. That is the same as 40 weeks from the last period. Here is what that looks like for conception in the middle of each month:
| Conceived around | Due around | Born around → conceived around |
|---|---|---|
| 15 January | 8 October | 15 January → 24 April (year before) |
| 15 February | 8 November | 15 February → 25 May |
| 15 March | 6 December | 15 March → 22 June |
| 15 April | 6 January | 15 April → 23 July |
| 15 May | 5 February | 15 May → 22 August |
| 15 June | 8 March | 15 June → 22 September |
| 15 July | 7 April | 15 July → 22 October |
| 15 August | 8 May | 15 August → 22 November |
| 15 September | 8 June | 15 September → 23 December |
| 15 October | 8 July | 15 October → 22 January |
| 15 November | 8 August | 15 November → 22 February |
| 15 December | 7 September | 15 December → 24 March |
So if you were born in May, you were most likely conceived in August of the year before. Only about 1 in 20 babies arrives exactly on the due date, so every row is the centre of a window of a few weeks.
If I'm 4 or 5 weeks pregnant, when did I conceive?
Pregnancy weeks are counted from the first day of your last period, not from conception. Conception usually happens about two weeks later. So:
- 4 weeks pregnant — you likely conceived about 2 weeks ago.
- 5 weeks pregnant — about 3 weeks ago.
- 8 weeks pregnant — about 6 weeks ago.
This assumes a 28-day cycle. Longer cycles push conception later. Choose My last period above and enter your cycle length for a closer estimate.
Conception date meaning
The conception date is the day the egg was fertilised. It usually falls within a day of ovulation, about two weeks after the first day of your last period. It is not the day pregnancy weeks are counted from — doctors count from your last period, so at conception you are already “2 weeks pregnant” on paper.
Sex up to five days before ovulation can still lead to conception, because sperm survive that long. That is why any conception calculator, however accurate, gives a window rather than one certain day.
Other ways to use this calculator
- Date of conception from due date — choose “My due date” to count back 266 days.
- Conception calculator from ultrasound — enter the scan date and the age the scan gave.
- Due date by conception date — add 266 days to a known conception date, or use the due date calculator.
- Conception and birth month chart — see the chart above to match conception months with due months.
- Reverse conception calculator — this page works back from a due date, last period or scan to the likely conception date.
Frequently asked questions
Subtract 266 days from the due date — the average gestation from conception, as opposed to 280 days from your last period. A due date of 10 June gives a conception date around 17 September. Because both figures rest on averages, treat the result as the centre of a window of several days rather than an exact date.
Because three uncertainties stack up. Sperm can survive up to five days, so intercourse and conception may be days apart. Ovulation timing varies with stress, illness and travel. And the due date is itself an estimate — give or take a week or two from a period date, five to seven days even from an early scan. Realistically the window is six days at best, often ten to fourteen.
Reasonably good if the scan was early. Before 14 weeks, embryos grow consistently enough to date a pregnancy within about five to seven days, giving a conception window of roughly ten to twelve days. Accuracy falls off sharply later — by the third trimester, babies differ in size for reasons unrelated to age, so a late scan measures growth rather than dating the pregnancy.
No. The conception window spans several days at minimum and often ten to fourteen, so if more than one partner is possible within that period, the arithmetic cannot separate them. Only a DNA paternity test can answer it reliably — non-invasive prenatal testing is available from about 7 to 9 weeks using a maternal blood sample, and testing after birth is straightforward. A doctor or clinic can explain the options.
Add 266 days, or 38 weeks, to the conception date. Conception on 15 June gives a due date around 8 March. Your midwife or doctor may adjust it after a dating scan.
Most likely in August of the year before. Counting back 38 weeks from 15 May lands on about 22 August. Because babies arrive over a range of weeks, late July to early September is possible.
Count back 266 days (38 weeks) from your due date. If your due date is 1 June, the most likely conception date is around 8 September of the year before. Because sperm can live up to five days, intercourse in the few days before that date could also have led to the pregnancy.
Sources and further reading
- ACOG — Methods for estimating the due date (Committee Opinion 700) — dating accuracy by method and gestational stage
- NICHD — How does conception occur? — fertilisation, implantation and their timing
- NHS — Working out your due date — dating scans and how dates are set in the UK
- American Association of Blood Banks — Relationship testing standards — accredited DNA paternity testing