Nursing care
Nagele Rule: the method, the errors, and the exam
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
Nagele's Rule estimates the due date by taking the first day of the last menstrual period, subtracting three months, and adding seven days and one year. It assumes a regular 28-day cycle with ovulation on day 14, so it is a starting estimate to confirm with ultrasound, not a fixed answer.
What the skill is for
Nagele's Rule gives a quick estimated date of delivery, or EDD, from a single data point: the first day of the patient's last menstrual period, abbreviated LMP. It does not require an ultrasound, a pelvic exam, or any lab value, which is exactly why it remains the first calculation done at nearly every initial prenatal visit.
It exists because clinicians need an early working due date to schedule prenatal visits, order gestational-age-dependent testing, and counsel the patient on trimester milestones, long before an ultrasound can confirm dating more precisely. It is a starting point, not a diagnosis, and every subsequent visit refines it against fetal growth measurements.
The method, step by step
Take the first day of the last menstrual period. Subtract three months from that date. Add seven days. Add one year. That single sequence, minus three months, plus seven days and a year, is the calculation that appears every time this skill is tested, whether in a classroom or on the floor.
For example, an LMP of June 10 becomes March 10 after subtracting three months, then March 17 after adding seven days, then March 17 of the following year after adding one year. The order of operations does not change the result mathematically, but working through the months-then-days sequence keeps the calculation error-free under pressure.
Where it goes wrong
The rule assumes a regular 28-day cycle with ovulation occurring on day 14. Any patient with irregular cycles, longer or shorter cycles, or recent hormonal contraceptive use will get a due date that is meaningfully off, sometimes by a week or more, because their actual ovulation did not happen on day 14.
It also depends entirely on the patient accurately recalling the first day of her last period, which is unreliable for patients with irregular bleeding, breakthrough spotting, or simple difficulty remembering the exact date. This is why an early first-trimester ultrasound, most accurate before 13 weeks 6 days, takes precedence over Nagele's Rule when the two disagree by more than about five to seven days.
Practising it deliberately
Drill the calculation with dates that force a year rollover and a month rollover, since those are where arithmetic slips happen. An LMP in October or November means subtracting three months crosses into the previous year, and a student who has only practised with mid-year dates will hesitate exactly when accuracy matters most.
Practise the reverse direction too: given an EDD, calculate the implied LMP. This checks genuine understanding of the mechanism rather than memorised muscle memory for one direction, and it mirrors how some exam questions are framed, working backward from a stated due date to test whether the cycle assumption is understood.
Applying it on the exam
NCLEX-style questions typically give an LMP date and ask for the EDD, or give inconsistent LMP and ultrasound dating and ask which should guide clinical decisions. When dating conflicts and the discrepancy is small, ultrasound performed earlier in pregnancy is generally trusted over Nagele's Rule, particularly a first-trimester scan.
Watch for scenario questions where the cycle history contradicts the assumption underlying the rule, an irregular cycle, recent hormonal contraception, or breastfeeding-related amenorrhea before conception. The correct answer in those cases is usually to note that Nagele's Rule is less reliable for this patient, not to apply the formula blindly and treat the resulting date as certain.
A worked example
A patient reports her LMP as January 20. Subtracting three months gives October 20 of the prior year. Adding seven days gives October 27. Adding one year gives October 27 of the current year, which is the estimated due date.
Now suppose the same patient has a first-trimester ultrasound at 9 weeks that estimates a due date of November 3, a week later than the Nagele's Rule calculation. Because the discrepancy falls within the range where dating uncertainty is expected and the ultrasound was performed early, the ultrasound date generally takes precedence for scheduling subsequent care.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our maternity and newborn practice questions are the closest set to what this page covers.
Common questions
What if a patient doesn't know the exact date of her last period?
Nagele's Rule cannot be applied reliably without a known LMP date, so an early ultrasound becomes the primary method for establishing gestational age. Document the uncertainty rather than estimating an LMP to force the calculation.
Does Nagele's Rule work for patients with irregular cycles?
Not accurately. The rule assumes ovulation on day 14 of a 28-day cycle, so patients with longer, shorter, or irregular cycles will get a due date that does not reflect actual conception timing. Ultrasound dating is preferred for these patients when available.
How is Nagele's Rule usually tested on NCLEX?
Most commonly as a direct calculation from a given LMP, or as a judgment question asking whether Nagele's Rule or ultrasound dating should be trusted when the two disagree. Both formats expect the same underlying method: minus three months, plus seven days and a year.
Is Nagele's Rule the same as a due date confirmed by ultrasound?
No. Nagele's Rule is an estimate based on menstrual history and an assumed cycle pattern, while ultrasound measures actual fetal size to infer gestational age. Early ultrasound, particularly before 14 weeks, is generally more accurate and takes precedence when dates conflict.
How accurate is Nagele's Rule in practice?
Studies generally show only a minority of patients deliver exactly on the Nagele's Rule due date, with most deliveries occurring within a window of roughly one to two weeks around it. It functions as a planning estimate rather than a precise prediction, which is consistent with how it should be used clinically.