NaegelePregnancy Due Date & Gestational Age
Menstrual History
Required
Select the first day of your last normal period.
Days
Default is 28 days. Adjust for irregular cycles (between 20–45 days).
Naegele's Rule assumes ovulation on Day 14. Cycle correction shifts EDD by +0 day(s) relative to standard 28-day baseline.
Pregnancy Due Date
Select your Last Menstrual Period date. The calculator will automatically display your estimated due date and current gestational age.
Guidelines & Evidence
Verified
Last Review: 2026
When to Use
What Is Gestational Dating?
Gestational dating establishes the estimated due date (EDD) and current gestational age (GA) — both essential for safe pregnancy management. The LMP-based method (Naegele's Rule) counts 280 days (40 weeks) from the first day of the last menstrual period, assuming a 28-day cycle with ovulation on Day 14. Cycle-length correction shifts this by (cycle − 28) days. Where possible, first-trimester ultrasound (crown-rump length, CRL) provides superior accuracy and supersedes the LMP date when there is a discrepancy beyond guideline thresholds.
Primary Clinical Indications
Establishing the EDD at the first prenatal visit — required for all subsequent gestational-age-dependent decisions.
Timing prenatal screening windows — nuchal translucency (11–13⁺⁶ weeks), anatomy scan (18–22 weeks), GDM screen (24–28 weeks), GBS swab (36–37 weeks).
Defining prematurity and post-term thresholds — preterm <37 weeks; post-term ≥42 weeks.
Guiding decisions about induction of labour (IOL) — elective IOL requires confirmed ≥39 weeks; post-dates IOL at 41 weeks (ACOG) or 41⁺⁰–41⁺⁶ (NICE).
Assessing fetal growth — small-for-gestational-age (SGA) and large-for-gestational-age (LGA) diagnoses depend on an accurate EDD.
ART pregnancies — embryo transfer date allows precise calculation; LMP-equivalent is derived from transfer date minus embryo age minus 14 days.
Reconciling LMP vs ultrasound discrepancies — ACOG re-dating thresholds define when ultrasound should replace LMP as the dating standard.
Limitations of LMP-Based Dating
Irregular cycles — women with oligomenorrhoea (e.g., PCOS) or recent OCP use may have unpredictable ovulation, making LMP unreliable.
Recall bias — up to 40% of women are uncertain of their LMP date; this is the leading source of dating error.
First-trimester bleeding — implantation bleeding or a threatened miscarriage may be confused with a period, causing the LMP to be recorded incorrectly.
Cycle-length variability — the standard formula assumes 28-day cycles; cycles outside 20–45 days require correction.
Late presentation — women presenting after 22 weeks for the first time have significantly less accurate LMP-based dating (±3–4 weeks).
ACOG Re-Dating Thresholds (Ultrasound vs LMP)
| Gestational Age at US | Biometric Parameter | Re-date if Discrepancy Exceeds | Notes |
|---|---|---|---|
| ≤ 8⁺⁶ weeks | Crown-Rump Length (CRL) | 5 days | CRL most accurate dating parameter; re-dating nearly always justified if available |
| 9⁰–15⁺⁶ weeks | CRL (or mean sac diameter if no heartbeat) | 7 days | CRL still preferred; BPD emerging |
| 16⁰–21⁺⁶ weeks | BPD, HC, AC, FL composite | 10 days | Composite biometry; anatomy scan window |
| 22⁰–27⁺⁶ weeks | Composite biometry | 14 days | Dating less precise; only re-date if LMP very uncertain |
| ≥ 28⁰ weeks | Composite biometry | 21 days | Third-trimester US unreliable for dating; rely on established EDD |
How it Works
Naegele's Rule — Formula
EDD = LMP + 280 days
Equivalent shortcut: EDD = LMP + 9 months + 7 days
Cycle-length correction:
EDD (corrected) = EDD (standard) + (Cycle length − 28 days)
Example (Cycle = 35 days):
LMP: 1 January 2025
Standard EDD (28-day): 8 October 2025
Correction: +7 days (35 − 28)
Corrected EDD: 15 October 2025
Example (Cycle = 21 days):
Correction: −7 days
Corrected EDD: 1 October 2025
ART-Specific Dating Rules
| ART Method | Dating Reference | LMP Equivalent Calculation | Notes |
|---|---|---|---|
| Day-3 embryo transfer (IVF) | Embryo transfer date | Transfer date − 3 days − 14 days = LMP equivalent | Subtract embryo age (3 days) then assumed ovulation offset (14 days) |
| Day-5 blastocyst transfer (IVF) | Embryo transfer date | Transfer date − 5 days − 14 days = LMP equivalent | Most common; subtract 19 days from transfer date |
| IUI (Intrauterine insemination) | Insemination date | Insemination date − 14 days = LMP equivalent | Assumes insemination at ovulation (Day 14) |
| Ovulation induction (timed intercourse) | Ovulation trigger date | Trigger date − 14 days = LMP equivalent | hCG trigger marks ovulation; subtract 14 days |
| Frozen embryo transfer (FET) | Transfer date | Same as fresh IVF (transfer date − embryo age − 14 days) | LMP equivalent is immutable; do not re-date with US |
Ultrasound Biometric Parameters for Gestational Age
| Gestational Age | Primary Parameter | Accuracy (95% CI) | Secondary Parameters | Comments |
|---|---|---|---|---|
| 5–6 weeks | Mean Gestational Sac Diameter (MSD) | ±1 week | Yolk sac (present from 5⁺⁵ weeks) | CRL not yet measurable; MSD less precise |
| 6–14 weeks | Crown-Rump Length (CRL) | ±3–5 days (gold standard) | None needed if CRL measurable | Most accurate of all parameters; use whenever available |
| 14–22 weeks | Composite: BPD + HC + AC + FL | ±7–10 days | Femur length alone: ±7 days | Hadlock formula (1984) most widely validated |
| 22–28 weeks | Composite biometry | ±14 days | Cerebellar diameter (TCD): ±1–2 weeks | TCD particularly useful if head shape is abnormal (dolichocephaly) |
| 28–40 weeks | Composite biometry | ±21–28 days | None reliably accurate | Third-trimester US should NOT be used for primary dating; use for growth assessment only |
Gestational Age: Definitions and Terminology
| Term | Definition | Clinical Significance |
|---|---|---|
| Gestational age (GA) | Weeks + days since LMP (or LMP equivalent) | Standard unit for all pregnancy management decisions |
| Embryonic age / menstrual age | Fertilisation age = GA − 2 weeks | Used in embryology; NOT in clinical obstetrics |
| Term | 37⁰–41⁺⁶ weeks GA | Full term: 39⁰–40⁺⁶ weeks (lowest neonatal risk) |
| Preterm | <37⁰ weeks GA | Late preterm 34–36⁺⁶; extreme preterm <28 weeks |
| Post-term | ≥42⁰ weeks GA | Associated with increased stillbirth and meconium aspiration risk |
| Late preterm | 34⁰–36⁺⁶ weeks | Higher NICU admission rate vs term; not "almost term" |
| Early term | 37⁰–38⁺⁶ weeks | Higher respiratory morbidity than full term; avoid elective delivery |
Clinical Pearls
Critical Pearl #1: The Earliest Ultrasound is the Gold Standard — Never Re-Date Late
Once an EDD has been established by a first-trimester CRL measurement, it should not be changed by any subsequent ultrasound. Third-trimester growth scans assess fetal size relative to the established dates — they do NOT reset the EDD. Re-dating in the third trimester can cause a growth-restricted fetus to be misclassified as appropriately grown (and vice versa), leading to missed diagnoses and dangerous management errors.
Rule: "First scan wins." If a 10-week CRL gives EDD of 15 October and a 32-week scan suggests EDD of 1 November, keep 15 October as the clinical EDD.
Critical Pearl #2: PCOS and Irregular Cycles — LMP Unreliable, Ultrasound Mandatory
Women with polycystic ovary syndrome (PCOS) have highly variable ovulation timing; LMP-based dating frequently overestimates gestational age by 2–4 weeks. The cycle correction in this calculator (adjusting for cycle length) partially addresses this, but a definitive first-trimester ultrasound is mandatory in these patients.
Common error: A woman with PCOS and a 60-day cycle presents at an apparent "10 weeks" by LMP. In reality she may be only 6–7 weeks (ovulated on Day 46, not Day 14). An ultrasound will reveal the true CRL. Without it, she may be falsely labelled as having a missed miscarriage.
Critical Pearl #3: Post-Term Diagnosis Requires Accurate Dating
Post-term pregnancy (≥42 weeks) is associated with 2–3× increased perinatal mortality. However, before labelling a pregnancy as post-term and initiating induction, ensure dating is accurate:
- If EDD was based on a third-trimester ultrasound alone (no early scan, uncertain LMP), the "post-term" label may be erroneous.
- ACOG recommends offering IOL at 41⁰–41⁺⁶ weeks with reliable dates; at 42⁰ weeks IOL is strongly recommended.
- An incorrect EDD of 2 weeks late means the fetus may actually be only 39–40 weeks — unnecessary induction increases Caesarean rate.
Critical Pearl #4: Twins — Each Method Has Specific Rules
In twin pregnancies:
- Dichorionic-diamniotic (DCDA): Date by the larger (more advanced) twin's CRL in the first trimester.
- Monochorionic (MCDA, MCMA): Date by the CRL of the larger twin at 10–14 weeks; note chorionicity is established by 14 weeks (lambda/T sign).
- LMP alone should not be used for twin dating; first-trimester US is mandatory.
- Twins are delivered at 38 weeks (DCDA), 36–37 weeks (MCDA), or 32–34 weeks (MCMA) — all gestational-age-dependent decisions.
Common Pitfalls in Gestational Dating
Using LMP in patients with irregular cycles without cycle correction — always adjust or defer to ultrasound.
Re-dating in the third trimester — never change an established EDD based on a growth scan.
Confusing implantation bleeding with a period — the "LMP" recorded may be 4 weeks earlier than actual conception, leading to a 4-week overestimate of GA.
Using mean gestational sac diameter (MSD) when CRL is measurable — CRL is always preferred once visible (≥6 weeks).
Forgetting to account for daylight-saving time or UTC offsets in ART transfer records from different time zones.
Applying singleton dating rules to twins — chorionicity must be established and the appropriate twin used for dating.
Not documenting the source of EDD — always record "EDD by LMP" vs "EDD by US (CRL, date)" in the medical record.
Assuming all IVF cycles use Day-5 blastocysts — Day-3 embryo transfers require a different offset; confirm with the fertility clinic.
Next Steps
Step-by-Step Actions After Establishing EDD
Pregnancy Milestone Timetable (by Gestational Age)
| Gestational Age | Milestone / Screening | Details |
|---|---|---|
| 6–10 weeks | Viability scan | Confirm heartbeat, CRL; rule out ectopic; establish EDD |
| 10–13⁺⁶ weeks | First-trimester combined screen | Nuchal translucency + PAPP-A + β-hCG (for aneuploidy risk) |
| 10–14 weeks | NIPT (Non-invasive prenatal testing) | Cell-free fetal DNA; screens T21, T18, T13, sex chromosomes |
| 11–13⁺⁶ weeks | CVS (if indicated) | Chorionic villus sampling for definitive karyotype |
| 15–20 weeks | Quadruple / AFP screen | If first-trimester screen not done; also neural tube defect screen |
| 15–18 weeks | Amniocentesis (if indicated) | Definitive chromosomal diagnosis; also fetal infection |
| 18–22 weeks | Anatomy ultrasound | Fetal anatomy survey; placental location; cervical length |
| 24–28 weeks | GDM screening (OGTT) | 1-hour 50g GCT (ACOG) or 75g 2-hour OGTT (WHO/IADPSG) |
| 26–28 weeks | Rhesus antibody screen + Rh immunoglobulin | Rh D-negative women receive anti-D at 28 weeks |
| 28–34 weeks | Growth scan (if indicated) | SGA/LGA assessment; Dopplers if growth restriction suspected |
| 35–36 weeks | Late preterm growth scan | Rule out IUGR; confirm presentation |
| 36⁰–37⁺⁶ weeks | GBS rectovaginal swab | Intrapartum prophylaxis if positive |
| 39⁰ weeks | Elective IOL / LSCS eligible from here | Not before unless medically indicated; ARRIVE trial |
| 41⁰ weeks | Post-dates surveillance begins | CTG + amniotic fluid index (AFI) twice weekly; offer IOL |
| 42⁰ weeks | Strongly recommend IOL | Perinatal mortality risk increases significantly; offer LSCS if cervix unfavourable |
The Evidence
Landmark Guideline: ACOG, AIUM & SMFM
Methods for Estimating the Due Date. Committee Opinion No. 700
ACOG Committee on Obstetric Practice et al. • Obstetrics & Gynecology. 2017;
Accuracy of Crown-Rump Length Dating
Determination of Gestational Age by Ultrasound
Butt K et al. • Journal of Obstetrics and Gynaecology Canada. 2014;
International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy
Papageorghiou AT et al. • Ultrasound in Obstetrics & Gynecology. 2014;
Post-Term Pregnancy and IOL Timing
Labor Induction versus Expectant Management in Low-Risk Nulliparous Women
Grobman WA et al. • New England Journal of Medicine. 2018;
Naegele's Rule — Historical Validation
The length of uncomplicated human gestation
Mittendorf R et al. • Obstetrics & Gynecology. 1990;
Origins & History
Franz Karl Naegele (1778–1851)
Franz Karl Naegele was a German obstetrician based in Heidelberg who published his eponymous rule in 1812 (Lehrbuch der Geburtshülfe). His rule built on earlier work by the Dutch physician Hermann Boerhaave (1668–1738), who first observed that human gestation lasted approximately 10 lunar months (280 days). Naegele operationalised this as: add 7 days and subtract 3 months from the LMP (equivalent to adding 280 days). The rule assumed a 28-day cycle with conception on Day 14 — a simplification that holds reasonably well for populations with regular cycles but breaks down in the era of irregular cycles, hormonal contraception, and ART. Despite its limitations, Naegele's Rule remains the global standard nearly 200 years later due to its simplicity, reproducibility, and reasonable accuracy in average populations.
Key Milestones in Gestational Dating
| Year | Contributor / Event | Contribution |
|---|---|---|
| 1812 | Franz Karl Naegele, Heidelberg | Published the LMP + 280-day rule (Naegele's Rule) |
| 1958 | Ian Donald, Glasgow | First diagnostic obstetric ultrasound; visualised fetal parts non-invasively |
| 1964 | Stuart Campbell, London | First clinical measurements of biparietal diameter (BPD) for gestational age estimation by ultrasound |
| 1984 | Hadlock FP et al., USA | Validated composite biometry formula (BPD+HC+AC+FL) — the "Hadlock equation," still standard today |
| 1992 | Robinson HP, Fleming JEE | Established CRL dating charts (Robinson-Fleming tables); CRL confirmed as most accurate early-pregnancy parameter |
| 2014 | INTERGROWTH-21st Consortium | New international CRL-to-GA standards from 10 countries; replaced older population-specific charts |
| 2017 | ACOG / AIUM / SMFM Joint Opinion | Committee Opinion No. 700: standardised re-dating thresholds; CRL established as definitive gold standard |
| 2018 | ARRIVE Trial (Grobman et al.) | Validated clinical importance of accurate EDD: accurate ≥39-week dating required for elective IOL benefit |
The Shift from LMP to Ultrasound Dating
Before routine ultrasound became available (pre-1980s), LMP-based dating was the only method, and post-term pregnancy rates of 10–15% were reported — most of these were likely dating errors, not true post-maturity. The introduction of first-trimester CRL measurements dramatically reduced unnecessary inductions and improved the diagnosis of genuine fetal growth restriction. Today, in countries with universal first-trimester ultrasound access (e.g., UK, Scandinavia, Australia), LMP dating is confirmed or replaced at the first-trimester dating scan, and the clinical EDD is locked at that point. In resource-limited settings where first-trimester ultrasound is not available, LMP with cycle correction remains the primary dating tool.
Last Comprehensive Review: 2026