Pregnancy
Naegele's rule
Forty weeks counted from the first day of the last menstrual period. The most common alternative is to subtract three months, add a year and add seven days.
The Formula
due date = LMP + 280 days
Forty weeks counted from the first day of the last menstrual period. The most common alternative is to subtract three months, add a year and add seven days.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| LMP | Last menstrual period | date | The first day of bleeding, not the last. |
| 280 | Days | days | Forty weeks, the conventional length of a pregnancy from the LMP. |
| D | Due date | date | The estimated date of delivery. |
What Each Symbol Means
LMPdate
Last menstrual period
The first day of bleeding, not the last.
280days
Days
Forty weeks, the conventional length of a pregnancy from the LMP.
Ddate
Due date
The estimated date of delivery.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate a due date page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate A Due Date, filed under Health. Every number the guide produces comes from substituting into this one line.