Two subtractions
Every due date is built on the same convention: 40 weeks from the last period. Reverse it and you get the last period at due date minus 280 days. Conception sits about two weeks after that, so it is due date minus 266 days. The calculator adds the six-day window in which sex could have led to that conception, because sperm survive up to five days.
What the other numbers mean
Once day zero is known, everything else is a count from it: today's weeks and days, the trimester boundaries at 14 and 28 weeks, and the start of term at 37 weeks. For a fuller week-by-week view use the pregnancy calendar, and for the conception estimate with a last-period option see the conception date calculator.
Frequently asked questions
How does a reverse due date calculator work?
A due date is defined as 40 weeks (280 days) after the first day of the last period, and conception happens about 14 days after that. So the calculator subtracts 280 days from the due date to find the last period and 266 days to find the most likely conception date, then counts forward to today.
How accurate is the conception date it gives?
As accurate as the due date. A due date from an early ultrasound is usually within five days of the truth, so the conception estimate is too. A due date from period dates assumes ovulation on day 14 and can be a week or more out for long or short cycles. Either way, sex that caused the pregnancy could have been up to five days before conception.
Can I use this to find out who the father is?
Only to rule people out, and only roughly. The conception window is about a week wide and the due date itself has error, so two people within a couple of weeks of each other cannot be separated by dates. A DNA test is the only reliable answer.
What if my due date changed after a scan?
Use the newest one. Clinics update the due date when an early scan differs from period dates by more than about five to seven days, and the scan date is the better estimate of when the pregnancy really started.
Why is the last period date before I actually got pregnant?
Because pregnancy is counted from the last period, which is about two weeks before ovulation and conception. The calculator shows both dates so you can see the two-week gap that all pregnancy dating includes.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.