Conception Date Calculator
Enter your due date, the first day of your last period, or an ultrasound result to estimate when conception most likely happened.
Your calculation is performed in your browser.This result is an estimate for informational and educational purposes. It isn’t a diagnosis and isn’t a substitute for advice from a doctor, registered dietitian, or other qualified professional. See our methodology and sources for the formula used and its limitations.
How it works
Pregnancy is dated as 40 weeks (280 days) from the first day of the last period, but conception happens roughly two weeks in, about 266 days before the due date. The calculator works backward from your due date, forward from your last period using your cycle length, or from the gestational age on your ultrasound. It then shows the most likely conception date and a window of three days either side.
Formula & methodology
The calculator uses the standard 266-day interval between conception and birth. For last-period dating, ovulation is assumed 14 days before the next period, so a longer or shorter cycle moves the conception date later or earlier. For an ultrasound, your measured gestational age on the scan date is subtracted from 280 days to find the due date, and conception is then placed 266 days before it.
Conception date = Due date − 266 daysConception date = LMP + (cycle length − 14 days)Due date = Scan date + (280 days − gestational age at scan in days); Conception date = Due date − 266 daysWindow = Conception date ± 3 daysWhat each term means
- LMP
- The first day of your last menstrual period.
- Cycle length
- Days from the first day of one period to the first day of the next. Defaults to 28.
- Gestational age at scan
- The weeks and days of pregnancy measured on the ultrasound report.
Conception date by cycle length (last period method)
| Cycle length | Conception date = LMP + |
|---|---|
| 21 days | 7 days |
| 28 days | 14 days |
| 35 days | 21 days |
| 45 days | 31 days |
Worked example
If your due date is Thursday, October 8, 2026, conception is estimated 266 days earlier, on Thursday, January 15, 2026, with a likely window of January 12 to January 18. If the first day of your last period was January 1, 2026 and your cycle is 28 days, you get the same date. If an ultrasound on February 12, 2026 measured 8 weeks and 3 days, the due date is September 21, 2026 and conception is estimated on Monday, December 29, 2025.
Common uses
- —Working out roughly when conception happened after a positive pregnancy test.
- —Converting a due date or ultrasound report into a conception date.
- —Checking which cycle a pregnancy most likely began in.
- —Comparing your own record of ovulation with the estimate from your dates.
Important notes
- —The exact moment of conception is rarely known. Treat the result as the most likely date, not a certain one.
- —Sperm can survive for up to five days, and ovulation timing varies from cycle to cycle, so conception can fall several days either side of the estimate.
- —A due date is itself an estimate, so a conception date worked back from it carries that uncertainty too. An early ultrasound or IVF transfer date gives a more precise answer.
- —This calculator is for information only. It cannot tell you who the biological parent is, and it is not a substitute for advice from your doctor or midwife.