Naegele Due Date Calculator
Last reviewed: by Jayson Minagawa, BSN, RN
Free Naegele's rule pregnancy due date calculator. Enter the first day of the last menstrual period (LMP) — returns the estimated date of delivery (EDD). Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. Clinical reference only — ultrasound dating is more accurate, especially for irregular cycles.
Every L&D nurse knows Naegele's rule by heart, and every nursing student gets it wrong on the first NCLEX practice question. The math is straightforward: add 7 days, subtract 3 months, add 1 year. The TRICK is that most patients can't remember their actual LMP, so this is a starting estimate that gets refined by ultrasound at the dating scan.
— Jayson Minagawa, BSN, RNCalculate
Naegele's rule
EDD = LMP + 7 days − 3 months + 1 year. Named after Franz Naegele (German obstetrician, 1812). Assumes a 28-day menstrual cycle with ovulation on day 14, fertilization within 24 hours, and a 280-day gestation from LMP (266 days from conception). For a regular 28-day cycle, this rule is accurate within ±5 days for ~80% of pregnancies.
Worked example
LMP = January 1, 2026. Add 7 days = January 8, 2026. Subtract 3 months = October 8, 2025. Add 1 year = October 8, 2026. EDD = October 8, 2026.
When Naegele's rule is wrong
Naegele's rule fails for irregular cycles (PCOS, perimenopause), recent contraceptive use (post-pill or post-IUD irregular cycles), breastfeeding patients, and any patient who cannot reliably recall the LMP date. First-trimester ultrasound (crown-rump length at 7-13 weeks) is the most accurate dating method — accurate within ±3-5 days. ACOG recommends ultrasound dating supersede LMP-based dating if they disagree by more than 7 days in the first trimester.
Trimester boundaries
First trimester: conception through 13 weeks 6 days. Second trimester: 14 weeks 0 days through 27 weeks 6 days. Third trimester: 28 weeks 0 days through delivery. Term: 37-41 weeks (37-38 6/7 early term, 39-40 6/7 full term, 41-41 6/7 late term, ≥42 postterm). Most patients deliver between 38 and 41 weeks; only ~5% deliver on the EDD itself.
Why nurses calculate gestational age at every prenatal encounter
Gestational age drives the entire prenatal-care calendar: when to draw quad screen labs, when to schedule the anatomy ultrasound, when GBS swabs are due, when to start antenatal testing for high-risk pregnancies, when to give Tdap and influenza vaccines, and when to begin counseling on labor signs. A single-day error in dating can shift the timing of every downstream decision. In a busy OB clinic, the nurse calculates GA at every encounter from the EDD established at the dating ultrasound, and reconfirms with the provider if there is any discrepancy with patient-reported LMP. The calculation should match what the EHR shows; if it doesn't, raise it before the patient leaves.
For Labor & Delivery nurses, GA is the first thing documented when a patient arrives in triage. A patient at 37 weeks 0 days with contractions is term and is admitted for active labor; the same patient at 36 weeks 5 days is preterm and gets a different workup including fetal fibronectin, betamethasone consideration, and tocolytic decision-making. The single calendar day matters legally and clinically. Always confirm GA against the EDD on the prenatal record, not against patient self-report.
Ultrasound dating supersedes LMP — when and why
ACOG, ACNM, and SMFM all recommend that first-trimester ultrasound dating supersede LMP-based dating when the two disagree by more than the trimester-specific threshold. In the first trimester (≤13 weeks 6 days), if LMP-based EDD differs from ultrasound by more than 7 days, use the ultrasound date. In the second trimester (14–21 weeks 6 days), the threshold is 10 days; in 22–27 weeks 6 days, 14 days; and after 28 weeks, 21 days, but late-trimester re-dating is generally avoided because biometric measurements become less precise. Crown-rump length (CRL) measured at 7–13 weeks is the gold standard with a margin of error of ±3–5 days. After 14 weeks, biparietal diameter, head circumference, abdominal circumference, and femur length combine to estimate GA, and the error widens with advancing gestation.
Documenting gestational age and EDD in the chart
The standardized format is "GA weeks + days," for example "32+4" or "32 4/7," meaning 32 weeks and 4 days. The EDD is documented in MM/DD/YYYY format with the dating method noted: "EDD 09/14/2026 by 8w0d CRL ultrasound" or "EDD 10/02/2026 by LMP." If the dating method changes mid-pregnancy (rare but possible — for instance, a discordant late-second-trimester ultrasound that prompts re-dating), document the change explicitly with the rationale. Never silently overwrite a prior EDD; chart the original, the new, and the reason. This matters legally for any case involving preterm birth, post-term induction, or fetal growth concerns where the dating window is contested.
Frequently asked
Why subtract 3 months and add 1 year instead of just adding 9 months?
Both methods produce the same result, but Naegele's classical formula uses '−3 months + 1 year' because pregnancy crosses a calendar year for most patients. It's also easier to do mentally — most months don't have to be added in sequence.
What if my cycle is 35 days?
For longer cycles, ovulation is delayed. A common adjustment: add the difference between your cycle and 28 days. For a 35-day cycle, add 7 more days to the Naegele EDD. But ultrasound dating is more reliable than this manual adjustment.
Is EDD the same as 'due date'?
Yes — EDD (Estimated Date of Delivery) and 'due date' are interchangeable terms. The phrase 'estimated date of confinement' (EDC) is the older term, still used in some textbooks but largely obsolete.