Due Date Calculator — Naegele's Rule, Ultrasound CRL & IVF Transfer Dating
Compute your estimated due date (EDD) three ways: from last menstrual period (LMP + 280 days), from conception date (+266 days), or from a first-trimester ultrasound crown-rump length (Robinson-Fleming 1975). ACOG Committee Opinion 700 endorses first-trimester ultrasound as the most accurate dating method when LMP and CRL disagree by >7 days. Live trimester arc, gestational age in weeks + days, and an EDD-window breakdown.
Quick Conversion
Formula: weeks = days / 7
Pregnancy Timeline (40-Week Arc)
Dating Method Accuracy (ACOG CO 700)
| Window | Measurement | Accuracy (+/- days) | Note |
|---|---|---|---|
| 7w0d - 13w6d | CRL (crown-rump) | +/- 5-7 | Robinson-Fleming 1975 - gold standard |
| 14w - 20w | BPD / HC / AC / FL | +/- 7-10 | Hadlock 1991 composite |
| 20w - 30w | BPD / HC / AC / FL | +/- 14 | Lower accuracy - biological variation |
| > 30w | FL / AC composite | +/- 21-30 | Do NOT redate from late scans |
| LMP (regular 28-day) | Naegele's Rule | +/- 14-21 | Use if no early scan available |
| IVF transfer date | Day-3 + 263 / Day-5 + 261 | +/- 0 | Exact - no estimation needed |
LMP Presets - Real Antenatal Scenarios
Naegele's Rule + Robinson-Fleming CRL
EDD = LMP + 280 daysEDD_adj = LMP + 280 + (cycle_length - 28)GA_days(CRL) = 8.052 × √CRL_mm + 23.73EDD(IVF day-5) = transfer + 261 daysWorked: LMP 2025-12-20, 28-day cycle. EDD = 2025-12-20 + 280 days = 2026-09-26. Today is 2026-05-27 = day 158 = 22w4d gestational age. Trimester 2. 122 days remaining to EDD.
Gestational Age vs. Pregnancy Milestone
| GA (weeks) | Days from LMP | % complete | Milestone |
|---|---|---|---|
| 4 w | 28 | 10.0% | Implantation - positive home test |
| 6 w | 42 | 15.0% | Heartbeat visible on TVS |
| 10 w | 70 | 25.0% | End of organogenesis |
| 12 w | 84 | 30.0% | End of T1 - dating ultrasound |
| 13 w | 91 | 32.5% | NT scan window (11w-13w6d) |
| 16 w | 112 | 40.0% | Sex visible on ultrasound |
| 20 w | 140 | 50.0% | Anatomy scan (anomaly scan) |
| 24 w | 168 | 60.0% | Viability threshold (NICU) |
| 28 w | 196 | 70.0% | Start T3 - GTT, anti-D |
| 32 w | 224 | 80.0% | Growth scan, fetal movement count |
| 36 w | 252 | 90.0% | GBS swab, head-engaged check |
| 37 w | 259 | 92.5% | Early term begins |
| 40 w | 280 | 100.0% | EDD - full term midpoint |
| 41 w | 287 | 102.5% | Late term - induction discussion |
| 42 w | 294 | 105.0% | Post-dates - induction recommended |
Need a week-by-week development view? Pregnancy Week Calculator. Want to reverse-compute conception from an EDD? Conception Date Calculator.
Saved Lookups
How To Compute Your Due Date — 5 Steps
- Step 1. Pick a dating method. LMP works if your cycles are regular; conception works for IVF patients; CRL is best if you have a first-trimester ultrasound.
- Step 2. Enter the relevant date. For CRL mode also enter the crown-rump length in millimetres from the report.
- Step 3. If your cycle is not 28 days, enter the actual length so EDD is shifted by (cycle - 28) days.
- Step 4. Read the EDD, your gestational age (weeks + days), trimester, and percentage of the 280-day term completed.
- Step 5. Click Save lookup to store the result in your browser's local storage for future visits.
A Brief History of Pregnancy Dating
In 2026, an OB-GYN at Kokilaben Hospital in Mumbai books a new antenatal patient who arrives without an EDD on her referral. She gives an LMP of December 20 2025 and reports a 30-day cycle. Naegele's Rule plus a two-day cycle adjustment puts her EDD at September 28 2026 - and today, May 27 2026, places her at 22w4d, mid second trimester, well inside the anomaly-scan window. That entire workflow rests on three discoveries spanning 200 years of obstetric science.
Franz Karl Naegele, a German obstetrician at Heidelberg, published his eponymous rule in 1812 in his textbook Lehrbuch der Geburtshilfe. It built on Hermann Boerhaave's 1744 observation that pregnancies last roughly nine calendar months and ten lunar months from the last period. Naegele formalised the count to 280 days = 40 weeks. The rule predated the discovery of the ovum (Karl Ernst von Baer, 1827) and the understanding of ovulation - both of which would later show that Naegele's 280-day count actually includes ~14 days before conception even occurs.
Ian Donald, professor of obstetrics at Glasgow, pioneered medical ultrasound in 1958, scanning a patient with a borrowed Kelvin Hughes flaw-detector originally built for inspecting ship hulls. By 1971 his lab had published the first standardised CRL nomograms. T G Robinson and J E E Fleming's 1975 paper A critical evaluation of sonar crown-rump length measurements gave the formula still used in 2026: GA_days = 8.052 × √CRL_mm + 23.73 - accurate to within 5-7 days when applied between 7w0d and 13w6d.
The American College of Obstetricians and Gynecologists (ACOG) Committee Opinion 700, first published in May 2017 and reaffirmed in 2022, established the modern dating algorithm: use first-trimester CRL whenever LMP and ultrasound disagree by more than 5-7 days, and never redate a pregnancy from a third-trimester scan. The companion Committee Opinion 611 (2014) set the term-definitions used today: early term 37w0d-38w6d, full term 39w0d-40w6d, late term 41w0d-41w6d, post-term 42w+.
In-vitro fertilization, first successful with Louise Brown's birth on July 25 1978 (Patrick Steptoe and Robert Edwards, Oldham General Hospital), introduced a new dating method: the embryo's exact age is known. By 2026, day-5 blastocyst transfer is standard and dating is exact: EDD = transfer date + 261 days (266 days from conception minus 5 days of in-vitro development).
Doppler-shift ultrasound (Christian Doppler 1842, applied to medicine by Dean Franklin 1961) enabled fetal heartbeat detection from ~5w6d via transvaginal probe in modern practice. The four-chamber view at the anomaly scan (Lindsey Allan, 1980s) and the nuchal-translucency aneuploidy screen (Kypros Nicolaides 1992, Fetal Medicine Foundation) are the diagnostic landmarks that the EDD anchors. Without an accurate EDD, the NT scan window (11w-13w6d) and quad-test window (15w-22w) become guesswork.
Pair this with Pregnancy Week for week-by-week milestones, or Conception Date Calculator to reverse-compute from a known EDD.
Trusted by OB-GYNs, midwives, family physicians and fertility specialists
“I use this in clinic when patients arrive without an EDD on their referral. The trimester arc SVG is the cleanest visual I've seen - patients immediately understand where they are. The CRL-input mode mirrors what we read off the Voluson E10 verbatim.”
“Naegele's Rule with cycle-length adjustment is exactly what midwives teach in antenatal class. I send patients to this page after every booking visit. Far better than the generic EDD wheels on display.”
“I see >40 antenatal patients a week. The fact this tool computes Naegele AND ultrasound CRL Robinson-Fleming dating, and tells the patient which to trust per ACOG CO 700, is gold. My patient education time dropped 30%.”
“For my IVF patients the conception-date input mode (day-3 / day-5 transfer +263/+261 days) matches what I write on their FET discharge. The split between menstrual and embryologic age is finally explained correctly.”
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