Naegele's rule: how to calculate a due date

Naegele's rule is the quick paper-and-pencil way to estimate a due date, and it is a staple of maternity nursing questions. The rule has three steps and one trap. The trap is the year: it is easy to subtract and add and forget to move to the next calendar year. This guide walks through the rule, four worked examples, the assumptions behind it and the mistakes that cost points.

You can check any example with the Naegele's rule calculator once you have done it by hand. This is a study aid for the exam, not clinical guidance; your program, facility policy and the provider decide how dates are set in practice.

The rule in three steps

Naegele's rule starts from the first day of the last menstrual period, written LMP. From there:

1. Subtract 3 months from the LMP.
2. Add 7 days.
3. Adjust the year.

The result is the estimated date of delivery, abbreviated EDD. Step 3 means adding one year, and that is what most people forget. For an LMP from January to March, step 1 slips back into the previous year, and the year step brings the date forward to the same year as the LMP. For an LMP from April to December, the date lands in the following year.

The rule assumes two things. The client has a 28-day cycle, and the first day of the LMP is certain. If either assumption fails, the rule gives a less reliable date.

Worked example 1: LMP 10 March 2026

• Subtract 3 months: 10 March 2026 becomes 10 December 2025.
• Add 7 days: 17 December 2025.
• Adjust the year: add one year, giving 17 December 2026.

EDD: 17 December 2026. A March LMP produces a December due date in the same calendar year. If you wrote 17 December 2025, the due date would fall before the LMP itself. This is the single most common slip, and a quick sanity check catches it: a due date is always roughly nine months after the LMP, never before it.

Worked example 2: LMP 25 August 2026

• Subtract 3 months: 25 May 2026.
• Add 7 days: 1 June 2026 (25 plus 7 is 32, and May has 31 days).
• Adjust the year: 1 June 2027.

EDD: 1 June 2027. The days crossed into a new month here, so you carry the extra days over. Count the days out loud: 26, 27, 28, 29, 30, 31 of May, then 1 June.

Worked example 3: LMP 28 November 2026

• Subtract 3 months: 28 August 2026.
• Add 7 days: 35 would be past the end of August, which has 31 days, so the date becomes 4 September 2026.
• Adjust the year: 4 September 2027.

EDD: 4 September 2027.

Worked example 4: the plus or minus 2 weeks

The EDD is not a promise. It is the expected birth date plus or minus 2 weeks, which allows for normal variation in cycles. Take example 1:

• EDD: 17 December 2026.
• Two weeks earlier: 3 December 2026.
• Two weeks later: 31 December 2026.

So a birth anywhere from 3 to 31 December 2026 fits the estimate. For example 2, the window is 18 May to 15 June 2027. If an exam question asks which date range is consistent with a particular EDD, this is the arithmetic.

Common mistakes

• Skipping the year step. Always run a sanity check. The EDD should be about nine months after the LMP.
• Using the wrong first day. The rule starts from the first day of the last menstrual period, not the day the bleeding ended and not the day of conception.
• Forgetting the assumptions. The rule assumes a 28-day cycle and a sure LMP. With irregular cycles or an uncertain LMP, expect a less reliable result.
• Treating the EDD as exact. It is plus or minus 2 weeks.
• Carrying days wrong. When the 7 days push past the end of a month, the leftover days go into the next month. Check how many days the month has.
• Confusing the rule with other obstetric math. Gravidity and parity are counts, not dates. Gravidity is the total number of pregnancies including the current one. Parity is the number of pregnancies that reached 20 weeks or more, live birth or not.

Where it shows up on the NCLEX-RN

Naegele's rule is part of prenatal care and fetal development, a chapter in unit 28 of the free NCLEX-RN flashcard course. It falls under Health Promotion and Maintenance, which is 6-12% of items under the 2026 test plan; the eight client needs categories explain how that weight is used. It is a small slice, but it is predictable content you can get right every time.

The same chapter covers facts that pair naturally with the due date:

• Prenatal visits in a normal pregnancy are every 4 weeks until 28 weeks, every 2 weeks from 28 to 36 weeks, then weekly until labor.
• From 20 to 36 weeks, fundal height in centimeters equals the weeks of gestation plus or minus 2. At 28 weeks, 27 cm is within the expected 26 to 30 cm. At 30 weeks, 25 cm is more than 2 cm below the expected range, so the provider is notified and an ultrasound for growth is expected.
• Daily fetal movement counts start in the third trimester, and fewer than 10 movements within 2 hours needs same-day provider contact.

On the exam, an item is rarely just "calculate the date." It is more often a scenario: a client gives an LMP, and the nurse must work out the gestation and decide what teaching or follow-up fits. That is application, which is why practice with scenarios matters; how the NCLEX-RN CAT exam works explains why most items are written that way.

How to memorize it

The rule is only three steps, which makes it a good candidate for retrieval practice. Cover the page and write the steps from memory, then run one date through them. The testing effect explains why that beats reading the rule again, and how to memorize a list, a table or a formula sheet covers the technique for short procedures. Make your own date practice: pick five random LMPs, calculate each EDD, and check them with the calculator. Include at least one that crosses a month end and one that is a winter LMP.

If you are building a full study calendar, how to study for the NCLEX-RN lays out an 8-12 week plan, and the study schedule generator turns it into dates. Other short calculation topics follow the same pattern; the Apgar score chart is another newborn-side topic that is mostly a lookup you can drill.

Calculation skills transfer across health exams. The NREMT tests its own formulas, such as IV flow rate and drip rate and the rule of nines for burns, and the habit is identical: learn the steps, do worked examples, then check them with a tool. Whatever exam you are taking, confirm current rules and policies with your program and your state board of nursing, and use the NCLEX-RN exam prep course to keep practicing.