Conception calculator: when did I conceive?

Pregnancy is dated from the first day of your last period, and conception usually happens about two weeks after that. A due date is 280 days after your last period, so conception falls about 266 days (38 weeks) before the due date. The calculator works backward from whichever date you know:

You enter	How conception is estimated
Due date	Due date − 266 days
First day of last period + cycle length	LMP + cycle length − 14 days (ovulation is about 14 days before your next period)
Birth date + weeks at birth	Birth date − gestational age at birth + 14 days

For the birth-date option, enter how many weeks and days pregnant you were when your baby was born (22 to 44 weeks). If you don't know, leave it at 40 weeks.

From a due date: your due date is 09/15/2026. 09/15/2026 − 266 days = 12/23/2025. The likely window shown runs from 12/18/2025 to 12/24/2025.

From your last period: your last period started on 01/05/2026 and your cycles are 32 days. 01/05/2026 + 32 − 14 = day 18 after your period started, so the estimated conception date is 01/23/2026. With a 28-day cycle it would be 01/19/2026.

From a birth date: your baby was born on 08/20/2026 at 39 weeks 2 days. 08/20/2026 − 275 days + 14 days = 12/02/2025.

The birth-date method is the most uncertain of the three, because it relies on the gestational age at birth, which itself came from an estimate.

• Most likely conception date with the weekday. This is the estimated day of ovulation.
• Likely window: five days before the estimated ovulation through the day after it. Sperm can survive for several days, so sex in the days before ovulation can lead to pregnancy.
• Intercourse that could have led to the pregnancy: the five days before ovulation through ovulation day.
• Implantation range: roughly 6 to 12 days after conception, shown for orientation.
• Gestational age at conception: always 2 weeks 0 days, because the count starts at your last period.

The calculator runs entirely in your browser. Your dates aren't uploaded or stored.

Every conception date is an estimate built on other estimates:

• Ovulation moves. Even with regular cycles, it can come a few days earlier or later from month to month.
• Sperm survive for days. Sex on Monday can lead to conception on Thursday, so the date of sex and the date of conception aren't the same.
• Due dates are adjusted. If an early ultrasound changed your due date, use the updated date. ACOG considers a first-trimester ultrasound the most accurate way to date a pregnancy, which also makes it the best base for working backward.
• Births are spread out. Most babies arrive between 37 and 42 weeks, so a birth date alone points to a range, not a day.

If you conceived through IVF, your transfer date gives a much more precise answer; use the IVF due date calculator.

Because ovulation shifts and sperm live for several days, a calculator can't tell which of two encounters a few days apart led to the pregnancy. Treat the result as a window, not proof. Only a DNA test can settle a paternity question.

If you're still pregnant, the due date calculator shows how far along you are and your key dates, and the ovulation calculator helps if you're planning a future pregnancy. This calculator is informational and is not a substitute for medical advice.