Already know your due date? Work backward to see your estimated conception date and the first day of your last period.
A due date is normally estimated as the first day of your last period (LMP) plus 280 days — Naegele's rule. Reversing that: LMP = due date − 280 days.
Conception typically happens around ovulation, roughly 14 days after the LMP, so it's estimated as due date minus 266 days. If you conceived via IVF, your IVF due date calculator will be more precise since it's based on your actual transfer date.
| Conceive in | Baby due around |
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Subtract about 266 days from the due date. This assumes conception happened close to ovulation, roughly two weeks after the start of the last period, which is the standard assumption behind most due-date estimates.
Subtract 280 days (40 weeks) from the due date. This reverses Naegele's rule, the standard formula that adds 280 days to the first day of the last period to estimate a due date.
It carries the same assumptions and margin of error as any due-date estimate — useful as a general guide, but actual conception can vary by several days depending on individual cycle length and ovulation timing.
Not precisely, since IVF due dates are based on a known transfer date rather than an assumed ovulation day. For an IVF pregnancy, use the IVF Due Date Calculator instead for a more accurate result.
Find your conception month in the left column of the table above and read the approximate due month across from it — useful for questions like "conceived in May, when is my baby due?" For an exact date rather than just a month, enter the specific date above.
Read the table above in reverse: find your due month in the right column to see the approximate conception month across from it — this answers questions like "baby due in July, when did I conceive?" For a precise date, enter your actual due date above instead.