Pregnancy Conception Calculator: Conception Date
Work out when a pregnancy was conceived from the last period, usually about 14 days after it, or from the due date, 266 days before it, with the weeks pregnant today.
Enter a date to see your estimated due date.
About the Pregnancy Conception Calculator
The Pregnancy Conception Calculator works out when a pregnancy was most likely conceived. It opens on the Last period method: enter the first day of your last menstrual period and your usual cycle length, and the calculator shows the estimated conception date together with the due date, how many weeks pregnant you are today, the trimester and the main milestones. If you already know your due date, choose Due date instead and the calculator works back from it.
All dates are whole calendar days, so the answer is the same wherever you are.
This is an estimate, not medical advice. The day of conception cannot be known exactly from dates alone, because ovulation varies from cycle to cycle and sperm can live for up to five days. An early ultrasound scan dates a pregnancy more accurately than any calendar.
Why conception is about 2 weeks after the last period
How pregnancy is dated
Doctors and midwives count pregnancy from the first day of the last period, because most people know that date. Conception usually happens around ovulation, about 14 days later in a 28-day cycle. So at the moment of conception a pregnancy is already described as two weeks along, and the due date is 280 days after the last period but only 266 days after conception.
Longer and shorter cycles
The time from ovulation to the next period is fairly fixed at about 14 days, so in a longer cycle ovulation comes later. With a 32-day cycle, conception is estimated about 18 days after the period started, and the due date moves four days later as well. Enter any whole cycle length from 20 to 45 days.
The formula
For a 28-day cycle this simplifies to conception = LMP + 14 days.
Worked example
With a last period on 1 January 2026 and a 28-day cycle, the estimated conception date is Thursday, 15 January 2026 and the due date Thursday, 8 October 2026. With a 32-day cycle, both move four days later, to 19 January and 12 October.
How to use it
- Keep Last period selected, or choose conception, IVF transfer or due date.
- Enter the date and, for a last period, your cycle length.
- Read the estimated conception date and the rest of the results.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. Your dates are never sent to a server and are not stored.
Related tools
See the full milestone list on the Due Date Calculator, go straight to the due date method on the Conception Calculator, or check how many weeks along you are on the Pregnancy Calculator.
Sources
- American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion 700, 2017.
- Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. Br J Obstet Gynaecol 1984;91:685-689.
Open-source credits
The calculations are implemented in-house in TypeScript, with no third-party libraries. They follow Naegele's rule with the cycle-length correction, and place conception 266 days before the due date.
Frequently asked questions
When did I conceive if I know my last period?
In a 28-day cycle conception is estimated at 14 days after the first day of the last period. In a longer cycle it moves later by the same number of days, because the time from ovulation to the next period stays at about 14 days.
Why is the due date 266 days after conception but 280 days after the last period?
Pregnancy is dated from the last period, about two weeks before conception. Naegele's rule puts the due date 280 days after the last period, which leaves 266 days from conception.
How does cycle length change the conception date?
Each day your cycle is longer than 28 days moves ovulation, the conception date and the due date one day later. Each day shorter moves them one day earlier. Enter a whole number from 20 to 45 days.
Is the estimated conception date exact?
No. It is an estimate, not medical advice. Ovulation varies from cycle to cycle and sperm can survive for several days, so conception may have happened a few days either side. A first-trimester scan dates a pregnancy most accurately.
Are my dates stored?
No. Every calculation runs in your browser, nothing is sent to a server, and the dates are cleared when you close the page.