Conception Date Calculator
Work out the most likely date your baby was conceived, and the fertile window when intercourse could have led to the pregnancy, from your due date, first day of your last period or an ultrasound scan. Everything is calculated in your browser.
Calendar around conception
An estimate, not a medical or legal determination. Ovulation timing varies, so the real date can differ by several days; it cannot establish paternity.
How to use the Conception Date Calculator
- Choose a method: Due date, Last period (LMP) or Ultrasound. A dating scan from the first trimester is the most accurate.
- Enter the date and, for LMP, your usual cycle length; for an ultrasound, the scan date and the gestational age the sonographer gave you.
- Press Calculate conception date to see the estimated conception date, fertile window and calendar.
- Read the calendar strip: dark green is the estimated conception day, green the most likely intercourse days, light green the rest of the fertile window.
- Copy the summary if you want to keep the dates.
How the conception date is calculated
Pregnancy is conventionally dated from the first day of the last menstrual period (LMP), and the standard due date (Naegele's rule) is LMP + 280 days. Fertilization happens around ovulation, which in a 28-day cycle is about day 14. The calculator therefore uses these rules:
| You enter | Estimated conception (ovulation) date |
|---|---|
| Due date | Due date − 266 days (38 weeks) |
| Last period + cycle length | LMP + (cycle length − 14) days |
| Ultrasound date + gestational age | Scan date − gestational age + 14 days |
Worked example: a due date of 25 December 2026 minus 266 days gives an estimated conception date of 3 April 2026. The fertile window is 29 March to 4 April 2026, and the most likely intercourse days are 1 to 3 April.
Why the answer is a window, not a single day
Sperm can survive in the female reproductive tract for up to about five days, while the egg can be fertilized for only about 12 to 24 hours after ovulation. In the classic study by Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995), all pregnancies came from intercourse during a six-day window ending on the day of ovulation, and the chance was highest in the two days before ovulation and on ovulation day itself. That is why the calendar highlights ovulation −5 to +1 days as the fertile window and the last three days as the most likely.
- Ovulation varies. Even in regular cycles it can shift by several days from month to month, so an LMP-based estimate can be off by a few days or more.
- Ultrasound is the most reliable. The American College of Obstetricians and Gynecologists (ACOG Committee Opinion 700, 2017) recommends first-trimester crown-rump length measurement as the most accurate way to date a pregnancy; it is accurate to within about 5–7 days up to 13+6 weeks.
- IVF dates are exact. After IVF, conception is known from the transfer date and embryo age, so use those clinic dates rather than this estimate.
Conception date versus due date
If you only know how pregnant you are, count back: at 6 weeks of gestational age, conception was roughly 4 weeks ago; at 12 weeks, roughly 10 weeks ago. To go the other way, from a period or conception date to a due date, use the due date calculator; to plan a future pregnancy, the ovulation calculator predicts your coming fertile days.
Dates from any calculator are estimates. They cannot establish paternity when intercourse with different partners happened within about a week or two; a DNA test is the only reliable way.
Frequently asked questions
How do you calculate the conception date from the due date?
A due date is set 280 days (40 weeks) after the first day of the last period, and ovulation in a textbook 28-day cycle happens about 14 days after that. So conception is estimated as the due date minus 266 days (38 weeks). This calculator uses exactly that rule.
Can a conception date tell me exactly which day I got pregnant?
No. It gives the most likely ovulation and fertilization date, but ovulation varies from cycle to cycle and sperm can survive in the reproductive tract for up to 5 days. Intercourse anywhere from about 5 days before to 1 day after ovulation could have led to the pregnancy, which is why a window is shown.
Which method is the most accurate?
A first-trimester ultrasound dating scan (crown-rump length measured up to about 13+6 weeks) is the most accurate, typically within about 5 to 7 days. A due date from LMP is reliable when cycles are regular and the date of the last period is known; irregular cycles make LMP-based estimates less precise.
Why is the conception date about two weeks after my last period?
Pregnancy is counted from the first day of the last period, but conception happens around ovulation, which in a 28-day cycle is about day 14. The first two weeks of the 40-week count happen before you are actually pregnant.
Can this calculator prove paternity?
No. Because the fertile window spans about six days and ovulation timing varies, dates alone cannot reliably decide between possible fathers when intercourse dates are close together. Only a DNA paternity test can do that.
My cycle is longer than 28 days. Does that change the result?
Yes, in the LMP method. Ovulation is estimated at cycle length minus 14 days (the luteal phase is fairly constant), so a 35-day cycle moves ovulation to about day 21. The due date and ultrasound methods already account for this.