How the conception date is estimated
Pregnancies are dated from the first day of the last period, and a due date is 280 days after it. In a textbook 28-day cycle, ovulation happens around day 14, and conception within about a day of ovulation. So the calculator puts conception 14 days after the start of your last period, which is 266 days before your due date.
| If you know | Estimated conception |
|---|---|
| Your due date | 266 days before it |
| Your last period (28-day cycle) | 14 days after it started |
| Your last period (longer or shorter cycle) | About 14 days before your next period would have started |
| Weeks pregnant on a scan date | That age minus 2 weeks, counted back from the scan |
With a cycle longer or shorter than 28 days, the calculator moves ovulation by the difference. That's a common estimate rather than a clinical rule: ACOG notes that the 280-day convention assumes a 28-day cycle and doesn't account for irregular cycles.
The six-day fertile window
Sperm can survive in the body for several days. In a landmark study, Allen Wilcox and colleagues tracked ovulation in 221 women with daily urine hormone tests. Conception only followed sex in the six days ending on the day of ovulation, with the chance of conceiving ranging from about 10% for sex five days before ovulation to about 33% on the day itself. That's the window this calculator shows: it's when the sex that led to your pregnancy most likely happened.
Why the date is an estimate
Counting from a period assumes ovulation exactly 14 days before the next one. That's a convention, not a rule. Cleveland Clinic describes the luteal phase, the time from ovulation to your next period, as about 14 days, with anywhere from 10 to 17 days normal. An analysis of more than 600,000 cycles tracked in an app found an average of 12.4 days. So even with regular periods, the real day of conception can be several days either side of the estimate.
A first-trimester ultrasound measures the baby directly and is the most accurate way to date a pregnancy. If you've had one, enter the due date from the scan.
What dating can't tell you
A conception date estimate can't identify which day, or which time you had sex, led to the pregnancy, and it can't tell two possible partners apart when the dates are close together. For questions about paternity, only a DNA test is reliable. For questions about your dates, your doctor or midwife can use your history and scans to give the best estimate. To see where you are now, use the pregnancy week calculator.