Working backwards from your due date
Usually a due date is calculated forwards from the last period. But often you already have a due date — from your midwife, a dating scan or an IVF clinic — and want everything else to match it. This calculator turns the process around: enter your due date and it works back to:
- the equivalent first day of your last period (due date − 280 days);
- your estimated conception date (due date − 266 days) and fertile window;
- how many weeks and days pregnant you are today;
- when each trimester starts and ends, and every key milestone up to late term.
Why dates from a scan should drive everything
If your due date was adjusted after an ultrasound, the LMP-based dates no longer apply. Working back from the confirmed due date gives you an “LMP equivalent” that keeps your week count, trimester dates and test windows in line with what your care team uses. That is why the LMP shown here may not match the day your period actually started — and that is expected.
The formulas
LMP equivalent = due date − 280 days
Conception ≈ due date − 266 days
Weeks pregnant today = (today − LMP equivalent) ÷ 7
Related tools
To see the same information as a visual month-by-month calendar, use the Pregnancy Calendar by Due Date. If you mainly want to know when you conceived, the Conception Date Calculator adds more detail about the fertile window.