# Situation: A 26-year-old woman comes to the Rural Health Unit (RHU) for her first prenatal visit on May 6, 2026. Her last menstrual period (LMP) began on January 28, 2026, and her menstrual cycles have always been regular at 28 days. She has no medical conditions and reports no danger signs. Using Naegele's rule, what is her expected date of delivery (EDD)?

> source: MyMerci (mymerci.kr)  
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> subject: Nursing Practice II — Maternal and Child Health Nursing

## 문제

Situation: A 26-year-old woman comes to the Rural Health Unit (RHU) for her first prenatal visit on May 6, 2026. Her last menstrual period (LMP) began on January 28, 2026, and her menstrual cycles have always been regular at 28 days. She has no medical conditions and reports no danger signs.

Using Naegele's rule, what is her expected date of delivery (EDD)?

## 보기

1. October 4, 2026
2. October 21, 2026
3. November 4, 2026 **✔ 정답**
4. November 4, 2027

**정답: 3**

## 해설

Naegele's rule is the first day of the LMP plus 7 days, minus 3 months, plus 1 year. January 28 plus 7 days is February 4 (January has 31 days); minus 3 months is November 4, 2025; plus 1 year gives November 4, 2026. Counting 280 days from January 28, 2026 gives the same date.

## 심화 해설

Understanding Naegele's Rule

Naegele's rule estimates the expected date of delivery (EDD) by taking the first day of the last menstrual period (LMP), adding 7 days, subtracting 3 months, and then adding 1 year. This calculation assumes a regular 28-day menstrual cycle with ovulation occurring around day 14. The underlying premise is that pregnancy lasts approximately 280 days (or 40 weeks) from the LMP, not from the date of conception. The rule is a calendar-based estimation tool, not a definitive biological predictor of the actual day of birth.

Step-by-Step Calculation for This Client

The client's LMP began on January 28, 2026. Applying Naegele's rule:

| Calculation Step | Result |
| --- | --- |
| LMP + 7 days | January 28 + 7 days = February 4, 2026 (January has 31 days) |
| Subtract 3 months | February 4, 2026 minus 3 months = November 4, 2025 |
| Add 1 year | November 4, 2025 + 1 year = November 4, 2026 |

The EDD is therefore November 4, 2026. Counting forward 280 days from January 28, 2026 yields the same date, which serves as a useful cross-check when a calendar is available.

Why the Other Options Are Incorrect

Watch out! A common error is forgetting that January has 31 days. If you incorrectly treat January as having 30 days, adding 7 days to January 28 gives February 5, and the final EDD shifts to November 5—which is not among the options. Another frequent mistake is subtracting only 2 months instead of 3, which would produce December 4, 2026. Option 1 (October 4, 2026) reflects subtracting 4 months instead of 3. Option 4 (November 4, 2027) results from adding 2 years instead of 1, or from misreading the year in the problem stem.

Clinical Context and Limitations of the Rule

Naegele's rule remains widely used in prenatal care because it is simple, requires no equipment, and provides a consistent starting point for scheduling antenatal visits and planning interventions. However, the evidence base highlights important limitations. A critical review found that Naegele's rule tends to predict the EDD approximately 3.3 days too early on average, even among women with regular cycles and a known LMP. [1] In contrast, ultrasound-based dating in the first trimester was found to predict the EDD about 2.0 days too late on average, with similar standard deviations between the two methods.  This means that neither method is perfectly accurate, and the actual date of spontaneous labor varies considerably around any estimated date.

Key point! For nursing licensure examinations, the calculation itself is what is tested. The formula must be applied precisely: LMP + 7 days − 3 months + 1 year. Do not adjust the result based on the research finding that Naegele's rule may be slightly early—the exam expects the unmodified rule. The clinical nuance about ultrasound accuracy is relevant for understanding why providers may revise the EDD after a first-trimester scan, but it does not change the arithmetic required here. [1]

Application to the Rural Health Unit Setting

In a primary care or rural health setting, the nurse often performs the initial dating assessment before an ultrasound is available. The LMP-based EDD is used to estimate gestational age at the first visit, determine the timing of subsequent prenatal visits, and identify whether the pregnancy is post-term (beyond 42 weeks). If the client's cycles are irregular or the LMP is uncertain, the calendar method becomes less reliable, and ultrasound dating takes priority. In this case, the client reports regular 28-day cycles and a clear LMP, so Naegele's rule is appropriate as the initial estimate. References (research sources)

- [1]Naegele's rule: a reappraisal.Research articleBaskett TF, Nagele F (2000) · DOI: 10.1111/j.1471-0528.2000.tb11661.x

## 임상 시나리오

Naegele's Rule in Prenatal CareAccurate EDD calculation for early pregnancy dating
Always use the first day of the LMP, not the last day or the date of conception. Add 7 days, subtract 3 months, then add 1 year. For LMP January 28, 2026, the EDD is November 4, 2026.

Double-check by counting 280 days from the LMP. This cross-check is especially useful when the LMP falls near the end of a month, where calendar day errors are common.

CautionDo not forget that January has 31 days. Treating it as 30 days shifts the EDD by one day. Also confirm the cycle is regular 28 days; irregular cycles make Naegele's rule less reliable.

## 핵심 개념

- **Naegele's rule** — A standard formula for estimating the expected date of delivery: first day of LMP + 7 days - 3 months + 1 year.
- **Last menstrual period (LMP)** — The first day of the woman's most recent menstrual period, used as the reference point for pregnancy dating.
- **Expected date of delivery (EDD)** — The estimated date when a pregnant woman is expected to give birth, typically calculated as 280 days from LMP.
- **Gestational age** — The age of the pregnancy measured from the first day of the LMP, expressed in weeks and days.
- **280-day rule** — An alternative method to Naegele's rule, counting 280 days forward from the LMP to estimate the EDD.

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