Working backwards from a due date
A due date sits at exactly 40 weeks of pregnancy, so every other date can be recovered from it by subtraction:
| Date | Calculation |
|---|---|
| First day of last period (equivalent) | due date − 280 days |
| Estimated conception / ovulation | due date − 266 days |
| Fertile window | due date − 271 to − 265 days |
| Weeks pregnant today | (today − (due date − 280)) ÷ 7 |
This is useful when your due date comes from an ultrasound or an IVF clinic rather than your period: the period date you remember may not match the “equivalent LMP” that your care team now uses, and the reverse calculation gives you the dates that line up with your official due date.
Reverse due date examples
| Due date | Estimated conception (−266 days) | Equivalent last period (−280 days) |
|---|---|---|
| March 15, 2027 | June 22, 2026 | June 8, 2026 |
| July 4, 2027 | October 11, 2026 | September 27, 2026 |
| December 25, 2027 | April 3, 2027 | March 20, 2027 |
Counting back by hand is easy to get wrong across month ends and leap years, which is why the calculator counts exact days. Because conception can only be estimated to within several days, the result also shows your likely fertile window rather than just one date.
How many weeks pregnant am I, from my due date?
Once you know your due date, today’s week of pregnancy follows directly: subtract the number of days left until the due date from 280, then divide by 7. With 100 days to go, you are at day 180 — 25 weeks and 5 days. The calculator does this automatically and also shows your trimester and progress. If you only need that number, the Pregnancy Week Calculator accepts a due date too.
When people use a reverse due date calculator
- Your due date was changed after a dating scan and you want to know your updated week of pregnancy.
- You want to estimate roughly when conception happened.
- You are curious when you yourself were conceived — in that case, the Birth Date Calculator works from a real birthday.
Keep in mind that a conception date worked back from a due date carries the same uncertainty as the due date itself — usually several days. For details on the fertile window, read the Conception Date Calculator; for today’s week only, use the Pregnancy Week Calculator.