Naegele's Rule Calculator

Enter the first day of the last menstrual period (LMP) and see Naegele's rule applied step by step — add 7 days, subtract 3 months, add 1 year — with the resulting estimated date of delivery (EDD).

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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 (1778–1851), 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

EDD = LMP + 7 days − 3 months + 1 year

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 you land on "February 30"), the date rolls forward into early March — this calculator handles that the same way charting software does, but exam questions almost always avoid the ambiguity. 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 — but on the NCLEX, the answer is Naegele's.

Frequently asked questions

What is Naegele's rule?

Naegele's rule is the classic bedside formula for estimating a due date: take the first day of the last menstrual period, add 7 days, subtract 3 months, and add 1 year. It approximates a 280-day (40-week) pregnancy and is the standard method taught in nursing and midwifery programs.

How do I remember Naegele's rule for the NCLEX?

Memorize it as "+7, −3, +1": plus 7 days, minus 3 months, plus 1 year. Do the steps in that order, one at a time, and write each intermediate date down. Most exam errors come from doing the month math in your head and forgetting the year rollover when the LMP falls in October, November, or December.

Does Naegele's rule work for irregular or long cycles?

Not well — it assumes a 28-day cycle with ovulation on day 14. For other cycle lengths, Parikh's rule adjusts the formula: LMP + 9 months + (cycle length − 21 days). For a 28-day cycle the two rules agree exactly; for a 35-day cycle Parikh's rule lands 7 days later.

Why does Naegele's rule give a slightly different date than adding 280 days?

Because "subtract 3 months, add 1 year" is really "add 9 calendar months," and 9 calendar months span 273 to 276 days depending on which months are included. Adding 7 days on top gives 280 to 283 days, so the two methods can disagree by up to about 3 days. Clinically the difference is irrelevant; on an exam, use Naegele's.

Who was Naegele?

Franz Karl Naegele (1778-1851) was a German obstetrician and professor at Heidelberg who popularized the rule in the early 1800s, building on the 280-day pregnancy estimate attributed to the Dutch professor Hermann Boerhaave. His name also survives in "Naegele's pelvis," an obliquely contracted pelvis he described.

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