How Naegele's rule works
Naegele's rule is the formula every nursing and midwifery student learns for estimating the date of delivery (EDD) from the last menstrual period. It's arithmetic you can do at the bedside: add 7 days, subtract 3 months, add 1 year. The "subtract 3 months, add 1 year" pair is just a shortcut for adding 9 calendar months, and the 7 days nudges the total to roughly 280 days, the conventional 40-week length of pregnancy counted from the LMP. The rule is named for Franz Karl Naegele (1778 to 1851), a German obstetrician at Heidelberg who popularized it, building on the 280-day estimate attributed to Hermann Boerhaave a century earlier.
This calculator shows each intermediate date, because that's how the rule is graded on exams, and how errors get caught in practice.
The formula
LMP is the first day of the last menstrual period (a common exam trap is using the last day). The rule assumes a 28-day cycle with ovulation on day 14. For other cycle lengths, Parikh's rule adjusts it: EDD = LMP + 9 months + (cycle length − 21 days). Enter a cycle length above and the calculator shows that variant too. For a 28-day cycle the two rules agree exactly.
Worked example
LMP: January 10, 2026
Step 1 (add 7 days): January 17, 2026
Step 2 (subtract 3 months): October 17, 2025
Step 3 (add 1 year): EDD = Saturday, October 17, 2026
Here the plain 280-day count also lands on October 17, but it doesn't always. For an LMP of May 10, 2026, Naegele's rule gives February 17, 2027 while LMP + 280 days gives February 14, a 3-day gap, because the 9 months from mid-May to mid-February span 276 days instead of the usual 273.
Exam tips for nursing students
Three things trip students up. The year rollover: if the LMP is in January, February, or March, subtracting 3 months crosses into the previous year and adding 1 year brings the EDD back into the same calendar year as the LMP; for an April-through-December LMP, the EDD lands in the following year. Write the year at every step. Month lengths: when a subtracted month doesn't have the day you need, say the arithmetic asks for "February 31", the convention is to take the last day of that month instead. Exam questions usually avoid the ambiguity, but real dates do not: it comes up whenever adding 7 days lands you on the 29th, 30th or 31st. Clamping to the end of the month is also the reading that keeps Naegele's rule agreeing with the plain 280-day count, which is a useful way to remember which direction to go. Order of operations: textbooks state the rule as +7 days first, then −3 months; if the 7 days push you into a new month, doing the steps in the other order can produce a date one day off, so follow the stated order. And remember what the answer means: Naegele's rule assumes textbook ovulation on day 14, real patients vary, and first-trimester ultrasound overrides the calculated date clinically. On the NCLEX, though, the answer is Naegele's.
Naegele's rule is taught as 280 days. It gives 280 days less than half the time.
Every textbook glosses the rule as "add 280 days", and the two are treated as the same thing. They are not, and the gap is bigger than the wording suggests.
Running the rule across every possible start date in a four-year span, 1,461 of them, it lands on exactly 280 days of gestation 643 times. That is 44%. On the other 56% of start dates it produces a longer pregnancy than the plain count does:
| Gestation the rule actually gives | How often |
|---|---|
| 280 days | 44.0% |
| 281 days | 27.2% |
| 282 days | 21.1% |
| 283 days | 7.7% |
Two things are worth taking from that. First, the error only ever runs one way: the rule never produces fewer than 280 days, only more, by up to three. Second, the reason is purely the calendar. Stepping back three months and forward a year crosses a different number of days depending on which months you cross, and February is the culprit more often than not. Neither method is wrong; they are answering with different arithmetic.
Clinically, three days inside a 37-to-42-week window is noise. For an exam, use the method the question asks for, and if it says "Naegele's rule" then do the three steps rather than adding 280, because on most dates those give different answers.