About the Conception Calculator
If you know your due date or the first day of your last period, you can work out roughly when you conceived. This conception calculator uses the standard rule that a pregnancy lasts about 280 days from the first day of the last period and that conception happens around two weeks after it, on the day of ovulation for a 28-day cycle. Enter a date, adjust the usual cycle length if yours is longer or shorter, and the calculator shows the likely conception date, the days around it when it was most likely, the expected implantation date, the earliest day a home test might turn positive, the start of each trimester and the due date.
Updated October 2026
How to use the Conception Calculator
- 1Choose whether you know your due date or the first day of your last period.
- 2Pick that date.
- 3Enter your usual cycle length if it differs from 28 days.
- 4Read the likely conception date and the days around it.
- 5Check the implantation, test, trimester and due dates.
Reading the dates
- Fertile window: the five days before conception and the day after
- Implantation: about nine days after conception
- Earliest positive test: about 14 days after conception
- Second trimester: from week 14
- Third trimester: from week 28
Why the dates are estimates
Only about four in a hundred babies arrive on their due date, and normal pregnancies last anywhere from about 37 to 42 weeks. The last period is also easier to remember than the day of conception, but ovulation does not always follow the textbook pattern. Treat the calculated date as the middle of a window, not a single day, and rely on your healthcare team for official dating.
Related tools
The Pregnancy Due Date Calculator starts from a period date, the Ovulation Calculator and Period Calculator predict cycles, the Pregnancy Weight Gain Calculator gives weight guidance and the Date Calculator adds or subtracts days. These tools give estimates only and do not replace medical advice.
Features
- Conception date from a due date
- Conception date from the last period
- Cycle length adjustment
- Fertile window around conception
- Implantation and earliest test dates
- Start of second and third trimesters
- Runs in your browser
Frequently asked questions
How is the conception date estimated?
A due date is counted as 280 days from the first day of the last period, and conception usually happens about 14 days after that day, which is 266 days before the due date. For a due date of 1 March 2027, the estimated conception date is 8 June 2026.
How accurate is the estimate?
It is a good guide but not exact. Sperm can survive for up to five days and an egg lives about a day, so conception may have happened on any of several days. Ovulation also varies with cycle length and from month to month, so the window of about a week matters more than the single date.
What if my cycle isn't 28 days?
Ovulation happens about 14 days before your next period, so a longer cycle pushes conception later and a shorter one earlier. Enter your usual cycle length and the calculator moves the dates by the difference from 28 days, including the due date.
What is implantation?
It is when the fertilised egg attaches to the lining of the womb, usually 6 to 12 days after conception. Some women notice light spotting at this time. The calculator uses nine days after conception as a typical value.
When can I take a pregnancy test?
Hormone levels rise after implantation, and sensitive tests can detect them from around 10 to 14 days after conception. Testing from the day your period is due is more reliable. The tool shows the earliest day as 14 days after conception.
Can an ultrasound change the dates?
Yes. A dating scan measures the baby, and if it differs from your dates by more than about a week, your doctor may change your due date. Use the due date your doctor gives you in this tool for the best estimate.