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Browse all calculatorsEnter a due date, your last period or a scan date to see the most likely conception date, the fertile window and how far along you are today. An estimate only.
These dates are estimates. Only a midwife, a doctor or an ultrasound dating scan can confirm your dates. The results assume cycles of typical length. Sperm can survive for days and ovulation timing varies, so the real conception day can differ, and a dating scan is more accurate than a last-period calculation.
An estimate like this is not proof of when or whether anything happened, and it is not contraception or a way to find safe days.
This is a general estimate for educational use. It is not medical advice, a diagnosis or a treatment plan. Talk to a doctor, midwife or other qualified professional before you make any decision about your pregnancy or health care.
Private by design. Everything runs in your browser and is never uploaded.
A pregnancy conception calculator works backwards from a due date, the last period or an ultrasound dating scan to the most likely conception date. It also shows the fertile window, the estimated ovulation day and weeks pregnant today. The result is an estimate, and only a midwife, doctor or dating scan can confirm dates.
This page answers one question: when did I most likely conceive? For a week-by-week table with trimesters, use the pregnancy calculator. For a due date by method, including IVF, use the due date calculator. To plan ahead for the fertile window, use the ovulation calculator.
| Starting point | Due date | Likely conception | Fertile window |
|---|---|---|---|
| Due date Mar 10, 2027 | Mar 10, 2027 | Jun 17, 2026 | Jun 12 to Jun 17, 2026 |
| Last period Jun 3, 2026, 28-day cycle | Mar 10, 2027 | Jun 17, 2026 | Jun 12 to Jun 17, 2026 |
| Scan on Sep 2, 2026 at 13 weeks 0 days | Mar 10, 2027 | Jun 17, 2026 | Jun 12 to Jun 17, 2026 |
| Last period Jun 3, 2026, 35-day cycle | Mar 17, 2027 | Jun 24, 2026 | Jun 19 to Jun 24, 2026 |
| Last period Jun 3, 2026, 24-day cycle | Mar 6, 2027 | Jun 13, 2026 | Jun 8 to Jun 13, 2026 |
The first answer is already on screen, using a date set from today. Change any date and the results update at once, with no Calculate button.
Pick Due date, Last period or Scan. Switching keeps the same pregnancy on screen, so you can compare what each method says.
Use the date box or type the date as YYYY-MM-DD. For a last period, add your usual cycle length. For a scan, add the weeks and days it reported.
See the most likely conception date, the fertile window as a day-by-day strip, the estimated ovulation day and how far along you are today.
Open the working to see each subtraction, copy a summary, or share a link that opens with the same dates. Reset returns to the example.
Every date is a whole day, found by adding or subtracting whole days. Nothing is rounded. The sources behind each rule are listed further down.
| Starting point | Rule | Where it comes from |
|---|---|---|
| Due date | conception = due date - 266 days |
A due date is 280 days after the first day of the last period. Ovulation on day 14 of a 28-day cycle makes the ovulation-based length 266 days (Jukic and colleagues, 2013). |
| Last period and cycle length | due date = first day + 280 + (cycle - 28)ovulation = first day + cycle - 14 |
The 280-day convention assumes a 28-day cycle (ACOG). A longer or shorter cycle moves the due date by the difference. The NHS says ovulation most often falls 10 to 16 days before the next period, so the 14 can be changed. |
| Ultrasound scan | due date = scan date + 280 - days pregnant at the scanconception = due date - 266 days |
The scan gives the age of the pregnancy in weeks and days. ACOG dating guidance looks for an ultrasound before 22 0/7 weeks. |
| Fertile window | ovulation - 5 days to ovulation day |
NICHD says sperm can live up to 5 days in the body and the egg is fertilizable for only a short time, usually 8 to 12 hours. The 7-day option uses the NHS figure. |
| How far along today | today - (due date - 280 days) |
Weeks pregnant are counted from the start of the 40-week count, which is why conception falls at about week 2. |
Pregnancy length varies even when it is timed from ovulation. In a study of 125 pregnancies with ovulation found from daily urine hormone tests, the median time from ovulation to birth was 268 days, the mean 267 days with a standard deviation of 10 days, and the range was 37 days.
The same paper notes that only 4 percent of women deliver on day 280 and only 70 percent within 10 days of the estimated due date. A date worked back from a due date inherits that spread, so treat it as a best guess for the middle of a window, not a measured day.
Sperm can also survive for days. NICHD gives up to 5 days and the NHS up to 7, so intercourse several days before ovulation can lead to the same pregnancy as intercourse on ovulation day. Only a midwife, a doctor or an ultrasound dating scan can confirm dates.
| Fact | Figure | Source |
|---|---|---|
| Sperm survival | Up to 5 days | NICHD |
| Sperm survival | Up to 7 days in the fallopian tubes | NHS |
| Egg fertilizable | Usually 8 to 12 hours | NICHD |
| Ovulation timing | 10 to 16 days before the next period | NHS |
| Typical cycle length | 21 to 35 days for most women | NICHD |
| Due date convention | 280 days from the last period, 28-day cycle | ACOG |
| Ovulation to birth, observed | Median 268 days, mean 267 days, SD 10 days | Jukic and colleagues |
| Ovulation on day 14 of a 28-day count | 266 days (280 – 14) | Jukic and colleagues |
| Scan changes the date | More than 5 days before 9 weeks, more than 7 days before 14 weeks | ACOG |
Many pages work only forwards from a conception date to a due date, or show one date with no explanation. These are the differences that matter when you want to understand the number.
| Feature | Basic calculator | This calculator |
|---|---|---|
| Starting points | Often one method | Due date, last period with cycle length, or scan date with weeks and days; switching keeps the same pregnancy |
| What it shows | One date or a bare range | Conception date, fertile window as a day strip, ovulation day and weeks pregnant today |
| Sperm survival | A fixed assumption, rarely explained | 5 days (NICHD) or 7 days (NHS), your choice, with both sources named |
| Honesty about error | Dates shown as if exact | Labelled as an estimate, with the published spread of pregnancy length |
| Working | Answer only | Every subtraction written out |
| Results | Calculate button | Updates as you type, with an answer on first load |
| Sharing | Rare | Copy a summary or share a link that opens with the same dates |
See which conception date your due date implies, and why the weeks pregnant count starts about two weeks before it.
Enter the same pregnancy by last period and by scan. If the two conception dates are far apart, a midwife or doctor decides which due date is used.
Enter your usual cycle length to see how ovulation and the due date move. To predict future periods, use the period calculator.
Copy a summary or share a link so a midwife or doctor can see exactly which dates you used. They confirm the dates.
Note the estimated month of conception, with the fertile window to show how wide the real range is. After the birth, the age calculator works out age from a birth date.
Once the dates are settled, the pregnancy weight gain calculator shows published weight-gain ranges for each stage.
These points come from the pages listed under Sources. They explain why the answer is a window and not a single certain day.
Only a midwife, a doctor or an ultrasound dating scan can confirm dates. This calculator gives general estimates for learning. It is not medical advice, and it is not proof of when or whether anything happened.
Pregnancy weeks run from the first day of the last period. ACOG says the 280-day convention assumes a 28-day cycle with ovulation on day 14, so conception falls near week 2.
NICHD says up to 5 days and the NHS says up to 7, while the egg is fertilizable for only hours. That is why the fertile window runs before ovulation, not after it.
Only about 4 percent of births happen on day 280, and about 70 percent within 10 days of the due date (Jukic and colleagues). A due date is the middle of a range.
ACOG changes a last-period due date when an early scan differs by more than 5 days before 9 weeks or more than 7 days before 14 weeks. For other methods, see the due date calculator linked above.
The last-period method needs a usual cycle length. NICHD describes 21 to 35 days as the range for most women, so cycles outside it, or that vary a lot, give less certain dates.
When a transfer date is known, the pregnancy is dated from it. The due date calculator handles that case, and a midwife or doctor confirms the dates.
No calculator can know the day of conception. The true day can differ by days, and the result is not proof or evidence of anything. It also is not contraception and does not show safe days.
The due date and last-period rules assume ovulation about 14 days before the next period. Cycle lengths outside 21 to 45 days get a message instead of a result, and an irregular cycle is better dated by a scan.
Dates that make the pregnancy more than 44 weeks old today, or put conception after today, get a message instead of a result. Twins, complications and treatment are outside its scope.
The sperm and egg facts, the 280-day and 266-day rules, the scan thresholds and the spread of pregnancy length come from the pages below, each opened while building this page. ACOG blocked automated access to its own site, so the text of Committee Opinion No. 700 was read through an archived copy of the same page. Source figures differ slightly, for example sperm survival of 5 days against 7 days, which is why both are offered. The NICHD conception infographic describes conception as possible from 6 days before ovulation through ovulation day; this page uses 5 days before plus ovulation day by default.
Short answers about conception dates, the fertile window and the limits of an estimate.
Subtract 266 days, which is 38 weeks, from the due date. A due date of Mar 10, 2027 gives an estimated conception date of Jun 17, 2026. The calculator does the subtraction for you and shows the working.
The calculator adds the cycle length minus 14 days to the first day of the last period, because ovulation usually comes about 14 days before the next period starts. A last period starting Jun 3, 2026 with a 28-day cycle gives Jun 17, 2026. The NHS says ovulation most often falls 10 to 16 days before the next period, so the 14 can be changed.
Pregnancy weeks are counted from the first day of the last period, and the standard count assumes a 28-day cycle with ovulation on day 14 (ACOG). Conception can only happen around ovulation, so it falls about two weeks into the count.
NICHD says sperm can live up to 5 days in the female body and that the egg is fertilizable for only a short time, usually 8 to 12 hours. The NHS gives up to 7 days for sperm in the fallopian tubes. That is why intercourse before ovulation can lead to a pregnancy.
It is the 5 days before ovulation plus ovulation day, 6 days in all, based on the NICHD figure for sperm survival. Choose 7 days under Sperm survival to use the NHS figure, which makes the window 8 days. It shows when intercourse could have led to this pregnancy and is not a way to find safe days.
It is an estimate. In a study of 125 pregnancies timed from ovulation, the time from ovulation to birth averaged 267 days, with a range of 37 days from shortest to longest, and only about 4 percent of women delivered on day 280 from the last period. A dating scan is usually more accurate than a last-period calculation.
A due date confirmed by a scan is the firmest starting point. ACOG changes a last-period due date when an early scan differs by more than 5 days before 9 weeks or more than 7 days before 14 weeks. If you only know the last period, enter it with your usual cycle length.
No. Sperm can survive for days and ovulation timing varies, so the calculator gives one most likely day and a window. It is not proof of when or whether anything happened, and it is not evidence of anything.
Enter your usual cycle length, from 21 to 45 days. The due date moves by the number of days your cycle is longer or shorter than 28, and ovulation moves with it. NICHD describes 21 to 35 days as the range for most women, so dates are less certain outside it.
When the transfer date is known, the pregnancy is dated from it and the conception date is not an estimate. The due date calculator on this site covers IVF dating, and a midwife or doctor confirms the dates.
The calculator counts the days from the start of the 40-week count, which is the due date minus 280 days, and shows them as weeks and days. For a week-by-week table with trimesters, use the pregnancy calculator.
No. The calculation runs in your browser and nothing is sent to a server. The dates appear in the page address only after you change a value or tap Share link.