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)