How to use the conception calculator
- Choose what you know. Your due date is the most common starting point. You can also use the first day of your last period, the date and pregnancy age from an ultrasound scan, or — for a baby already born — the date of birth and how many weeks pregnant you were.
- Enter the date and, depending on the method, your cycle length or the weeks and days.
- Read the results: the most likely conception date, the window of days when sex most likely led to the pregnancy, a wider range that allows for dating error, and the estimated due date.
How the conception date is calculated
Doctors count a pregnancy from the first day of the last menstrual period (LMP), about two weeks before conception actually happens. A full-term pregnancy is 280 days (40 weeks) from the LMP, so conception — which happens around ovulation, roughly day 14 of a 28-day cycle — falls about 266 days (38 weeks) before the due date.
| Starting point | Most likely conception date |
|---|---|
| Due date | Due date − 266 days |
| Last period | LMP + (cycle length − 14) days |
| Ultrasound scan | Scan date − pregnancy age at the scan + 14 days |
| Date of birth | Birth date − pregnancy age at birth + 14 days |
The cycle adjustment matters: the time from ovulation to the next period is fairly constant at about 14 days, so a 32-day cycle usually means ovulation around day 18, and a 25-day cycle around day 11. The birth-date method needs the pregnancy age at birth because very few babies arrive exactly on their due date — a baby born at 37 weeks was conceived about three weeks later than one born at 40 weeks on the same day.
Worked example
Your due date is October 8, 2026. Counting back 266 days gives a most likely conception date of January 15, 2026, and 280 days gives an estimated last period of January 1. The intercourse window is January 10 to January 16. Allowing ±7 days for the usual dating error, conception could have happened anywhere from January 8 to January 22. Starting from a last period of January 1 with a 32-day cycle instead, conception moves to about January 19 and the due date to October 12.
The conception window and the intercourse window
Conception — the moment a sperm fertilizes the egg — can only happen in the 12 to 24 hours after ovulation, while the egg survives. Sperm last much longer: usually up to about five days in the reproductive tract, and the NHS says up to seven. A landmark 1995 study by Allen Wilcox and colleagues in the New England Journal of Medicine found that pregnancies came from intercourse in a six-day window ending on the day of ovulation. That is why sex up to five days before conception can be the cause, and why the calculator's intercourse window runs from five days before the most likely conception date to one day after.
How accurate is it?
The estimate is only as good as the date you start from. Ovulation does not happen on the same cycle day every month, even in regular cycles, and it shifts with stress, illness, breastfeeding or recently stopped birth control. According to the American College of Obstetricians and Gynecologists (ACOG), measuring the embryo by ultrasound in the first trimester is the most accurate way to date a pregnancy, and the accuracy falls as pregnancy goes on. The calculator uses ACOG's redating thresholds as the margin for the scan method:
| Pregnancy age at the scan | Margin |
|---|---|
| Up to 8 weeks 6 days | ± 5 days |
| 9 weeks 0 days to 15 weeks 6 days | ± 7 days |
| 16 weeks 0 days to 21 weeks 6 days | ± 10 days |
| 22 weeks 0 days to 27 weeks 6 days | ± 14 days |
| 28 weeks 0 days or later | ± 21 days |
For a due date, last period or birth date, the calculator uses ±7 days — the first-trimester margin, because most due dates are set or confirmed by an early scan. After IVF the conception date is known exactly: it is the date the egg was fertilized in the lab, not a calendar estimate.
Can a conception date tell who the father is?
No. A calculated conception date cannot establish paternity. If there were two possible partners within a week or two of each other, the dates cannot reliably tell them apart: the window already spans about a week, ovulation varies from cycle to cycle, and the dating margin adds several days on each side. Even intercourse that seems to fall outside the window cannot be ruled out. The only reliable answer is a DNA paternity test, which is available after birth and, from some laboratories, during pregnancy from a blood sample. A doctor, midwife or genetic counselor can explain the options.
When to talk to a professional
If you are pregnant, your provider will set your due date, usually after an early ultrasound, and that date should take precedence over any calculator. Start prenatal care as soon as you know you are pregnant, and seek care at once for heavy bleeding or severe pain. To work forward from these dates, use the due date calculator; if you are trying to conceive, the ovulation calculator shows your next fertile window.
Important: results are estimates for general information only and are not medical advice. They cannot replace an assessment by a doctor, midwife, registered dietitian or other qualified health professional.
Sources
- American College of Obstetricians and Gynecologists, Methods for Estimating the Due Date (Committee Opinion 700, reaffirmed 2025).
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med, 1995.
- NHS, Periods and fertility in the menstrual cycle and Pregnancy due date calculator.
Frequently asked questions
How do I calculate when I conceived from my due date?
Subtract 266 days (38 weeks) from your due date. For a due date of October 8, 2026, that gives January 15, 2026. Allow about a week either way, because both ovulation and due dates are estimates.
Is the conception date the day we had sex?
Not necessarily. Sperm can survive for several days, so the sex that led to the pregnancy may have happened up to about five days before conception. The intercourse window in the results shows those days.
Can this calculator tell who the father is?
No. The dates are too uncertain to tell apart partners whose intercourse dates were within a week or two of each other, and no calendar method can prove paternity. Only a DNA paternity test can.
How accurate is a conception date from an ultrasound?
A first-trimester scan is the most accurate dating method — within about 5 to 7 days according to ACOG. Later scans are less precise: about ±10 days at 16 to 21 weeks and up to ±21 days in the third trimester.
How do I find out when I was conceived from my birthday?
Choose “Baby’s date of birth”, enter your birthday and, if you know it, how many weeks pregnant your mother was when you were born. If you were born around the due date, about 266 days before your birthday is the best estimate.
What if my cycles are irregular?
The last-period method assumes ovulation about 14 days before your next period. With irregular cycles that guess can be off by a week or more, so the due date from an early ultrasound gives a better answer.
Last updated: October 19, 2026