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Due Date Calculator
Estimate pregnancy due date using Naegeles Rule with cycle length adjustment, trimester tracking, and progress bar.
Due Date Calculator
Free online pregnancy due date calculator using Naegele's Rule. Estimate your due date, current week, trimester, and conception date with AI-powered insights.
Average is 28 days. Typical range: 21–35 days.
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Your pregnancy due date is estimated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP), adjusted for cycle length. Only about 5% of babies are born on their exact due date; roughly 80% arrive within two weeks before or after. First-trimester ultrasound (±5–7 days) is more accurate than LMP dating alone. A full-term pregnancy spans 39 0/7 – 40 6/7 weeks (ACOG); 37–38 weeks is early term, and ≥42 weeks is post-term.
🤰 Due Date Calculator — Complete Guide
Reviewed by CalculatorApp.me Health Editorial Team · Updated June 2026 · 10 min read
How Due Dates Are Calculated
Naegele's Rule (1812): Add 280 days (40 weeks) to the first day of your last menstrual period (LMP), adjusted for cycle length. For a 28-day cycle, this equals subtracting 3 months and adding 7 days from your LMP.
Gestational age is counted from the LMP, not conception. This means you are considered "2 weeks pregnant" at the time of ovulation/conception (for a standard 28-day cycle). The full 40 weeks includes the ~2 weeks before fertilization.
For irregular cycles, the estimated due date is adjusted: if your cycle is 35 days (7 days longer than 28), add 7 days to the Naegele estimate. If 21 days, subtract 7 days. First-trimester ultrasound at 8–12 weeks is more accurate than LMP dating by ±5–7 days.
Pregnancy Term Definitions
- ▸Early Term: 37 0/7 – 38 6/7 weeks
- ▸Full Term: 39 0/7 – 40 6/7 weeks
- ▸Late Term: 41 0/7 – 41 6/7 weeks
- ▸Post-Term: ≥ 42 0/7 weeks
- ▸Preterm: < 37 weeks
- ▸Very Preterm: < 32 weeks
- ▸Extremely Preterm: < 28 weeks
- ▸Viable Gestational Age: ~22–24 weeks
Pregnancy Dating Methods Compared
| Method | When Available | Accuracy | Notes |
|---|---|---|---|
| LMP (Naegele's Rule) | Immediately | ±2 weeks | Assumes regular 28-day cycle; most widely used first estimate |
| 1st Trimester Ultrasound (8–12 wks) | Weeks 8–12 | ±5–7 days | Crown-rump length (CRL) is most accurate method; ACOG gold standard |
| 2nd Trimester Ultrasound (13–28 wks) | Weeks 13–28 | ±2 weeks | Uses biometric measurements; less accurate than 1st trimester US |
| IVF / ART Conception Date | From transfer date | ±1–2 days | Embryo age is precisely known; add 2 weeks (LMP equivalent) to transfer date |
| Fundal Height | After 20 weeks | ±3–4 weeks | Clinical measurement of uterine height; rough confirmation only |
Due Date Myths vs Facts
Frequently Asked Questions
What is Naegele's Rule?›
Naegele's Rule (1812) calculates the estimated due date by adding 280 days (40 weeks) to the first day of the last menstrual period. For a standard 28-day cycle, this equals: LMP + 1 year − 3 months + 7 days. It is the most widely used dating method globally.
My cycle isn't 28 days — does that change my due date?›
Yes. Standard due date calculation assumes a 28-day cycle with ovulation on day 14. For longer cycles (e.g., 35 days), add the extra days to the due date; for shorter cycles, subtract. For example, a 35-day cycle shifts the due date ~7 days later.
When is the best time for a dating ultrasound?›
Between 8–12 weeks (first trimester) using crown-rump length (CRL). This period provides the most accurate gestational age measurement (±5–7 days). After 14 weeks, dating accuracy decreases due to individual growth variation.
What if I don't know my last period date?›
Your doctor will use a first-trimester ultrasound measurement as the primary dating tool. If you conceived via IVF, the embryo transfer date is used. Serum hCG levels alone cannot reliably date a pregnancy.
What happens if I go past 42 weeks?›
Post-term pregnancy (≥42 weeks) carries increased risks including placental insufficiency and stillbirth. Most OBs recommend induction between 41–42 weeks. ACOG guidelines suggest offering induction at 41 0/7 weeks. Management is individualized based on maternal and fetal status.
Can I calculate my due date from conception date?›
Yes, if you know your conception date (e.g., from ovulation tracking or IVF), add 266 days (38 weeks) to get the estimated due date. This equals the standard Naegele calculation for a 28-day cycle (LMP + 280 days = conception + 266 days).
What is the difference between gestational age and fetal age?›
Gestational age counts from the LMP (includes ~2 weeks before conception). Fetal age (embryonic age) counts from conception. A pregnancy described as "8 weeks gestational age" is actually ~6 weeks of embryonic development. Medical dating always uses gestational age.
What are the signs that labor is approaching?›
Common signs: lightening (baby dropping into the pelvis, usually 2–4 weeks before labor in first-time mothers), Braxton Hicks contractions increasing, bloody show (mucus plug), cervical dilation detected on exam, and water breaking. However, labor timing varies widely.
Is it safe to calculate due date for twins or multiples?›
Yes, the calculation is the same. However, twin pregnancies have higher rates of preterm delivery (average delivery ~37 weeks for dichorionic twins, ~35 weeks for monochorionic twins). Planned delivery timing for multiples is individually managed by the care team.
References & Further Reading
- • ACOG — Methods for Estimating the Due Date, Committee Opinion No. 700 (2017)
- • ACOG — Medically Indicated Late-Preterm and Early-Term Deliveries, ACOG Practice Bulletin No. 764
- • Naegele FC — Lehrbuch der Geburtshilfe (1812; due date rule attributed)
- • Mongelli M — Evaluation of gestation assessment methods, J Ultrasound Med (2019)
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Reviewed by CalculatorApp.me Health Team
Due Date Calculator — Complete Guide
Estimated delivery date, Naegele's rule, pregnancy trimesters, fetal development milestones, and dating methods.
280 days
Standard gestation
40 weeks
Full-term pregnancy
5%
Babies born on due date
37-42 wk
Term range
How Due Date Calculation Works
The estimated due date (EDD) predicts when a pregnancy will reach 40 weeks (280 days) from the first day of the last menstrual period (LMP). This is a medical convention — actual conception typically occurs around day 14 of a 28-day cycle, meaning true gestational age is approximately 266 days from conception.
Only about 5% of babies are born on their exact due date. Most births occur within a 5-week window (37-42 weeks). A "term" pregnancy is now classified as: early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42+ weeks). First-time mothers (nulliparas) tend to deliver about 1-2 days past their due date on average.
Ultrasound dating in the first trimester (crown-rump length measurement) is the most accurate method — accurate to ±5-7 days. After 14 weeks, accuracy decreases. If LMP date and ultrasound differ by more than 7 days in the first trimester, the ultrasound date is preferred.
Due Date Formulas & Methods
EDD = LMP + 1 year − 3 months + 7 days Example: LMP = March 15, 2025 + 1 year = March 15, 2026 − 3 months = December 15, 2025 + 7 days = December 22, 2025 Simplified: EDD = LMP + 280 days Assumptions: • 28-day menstrual cycle • Ovulation on day 14 • Fertilization within 24 hours Accuracy: ±2 weeks for ~80% of pregnancies Exact date: only ~5% of births
Naegele's Rule was published in 1812 but was based on a calculation by Boerhaave from the 1740s. It assumes a 28-day cycle — adjust for longer or shorter cycles.
For non-28-day cycles: EDD = LMP + 280 + (Cycle length − 28) Example: 32-day cycle, LMP = March 15 EDD = March 15 + 280 + 4 = Dec 26 Example: 24-day cycle, LMP = March 15 EDD = March 15 + 280 − 4 = Dec 18 Rationale: Longer cycles → later ovulation Shorter cycles → earlier ovulation Luteal phase is ~constant (14 days) Variation is in follicular phase Irregular cycles (>7 day variation): LMP dating unreliable → Use first-trimester ultrasound
The luteal phase (ovulation to period) is remarkably consistent at 12-14 days. Cycle length variation comes from the follicular phase (period to ovulation).
First Trimester (most accurate): Crown-Rump Length (CRL) 6-14 weeks gestational age Accuracy: ±5-7 days Second Trimester: Biparietal Diameter (BPD) Head Circumference (HC) Femur Length (FL) Accuracy: ±10-14 days Third Trimester: Multiple measurements combined Accuracy: ±21-30 days (Too inaccurate for redating) ACOG Redating Guidelines: Change EDD if discrepancy between LMP and ultrasound exceeds: <9 weeks: >5 days 9-14 weeks: >7 days 14-16 weeks: >7 days 16-22 weeks: >10 days 22-28 weeks: >14 days >28 weeks: >21 days
First-trimester ultrasound is considered the gold standard for pregnancy dating. If LMP is uncertain or cycles are irregular, ultrasound dating is essential.
IVF dating is the most precise: Day-3 embryo transfer: EDD = Transfer date + 263 days (or: Transfer date − 3 + 266) Day-5 blastocyst transfer: EDD = Transfer date + 261 days (or: Transfer date − 5 + 266) LMP equivalent for IVF: Subtract 14 days from ovulation (egg retrieval date) Frozen embryo transfer (FET): Same formula as fresh transfer Based on embryo age at transfer IVF pregnancies have known fertilization dates, making EDD calculation most accurate: Accuracy: ±1-2 days
IVF pregnancies have the most accurate due dates because the exact fertilization date is known, eliminating the uncertainty of LMP-based calculations.
Pregnancy Trimester Breakdown
| Trimester | Weeks | Key Developments | Common Symptoms | Important Tests |
|---|---|---|---|---|
| First | 1-12 | Heart beats (6 wk), brain forming, all organs begin | Nausea, fatigue, breast tenderness | Dating US, NT scan, NIPT |
| Second | 13-27 | Gender visible (18-20), quickening (16-20), viability (24) | Energy returns, appetite increases | Anatomy scan (20 wk), GD screen |
| Third | 28-40 | Lungs mature (34), head engages, weight gain peaks | Back pain, Braxton Hicks, insomnia | GBS swab (36), non-stress test |
| Early Term | 37-38 | Full development; slightly higher respiratory risk | Nesting instinct, pelvic pressure | Bishop score, fetal monitoring |
| Full Term | 39-40 | Optimal for delivery — lowest complication risk | Labor signs: contractions, show | Routine monitoring |
| Late/Post | 41-42+ | Placenta aging, amniotic fluid decreases | Induction may be recommended | Biophysical profile, NST |
History of Due Date Science
Hippocrates — First Gestation Estimates
Hippocrates described pregnancy lasting about 280 days (10 lunar months). Ancient physicians recognized that delivery could occur weeks before or after the estimated date, establishing the concept of term variability early in medical history.
Boerhaave Develops the Calculation
Dutch physician Hermann Boerhaave, professor at Leiden, published the calculation method of adding 7 days and 9 months to the LMP — the mathematical basis later attributed to Naegele. This was one of the first systematic approaches to EDD prediction.
Naegele's Rule Published
German obstetrician Franz Karl Naegele formally published the rule: LMP + 1 year − 3 months + 7 days. Despite being based on Boerhaave's earlier work and assuming a 28-day cycle, it became the global standard — and remains the most commonly used EDD formula today.
Ultrasound in Obstetrics Begins
Ian Donald published his landmark paper using ultrasound for obstetric diagnosis. By the 1970s, crown-rump length measurement in the first trimester was established as the most accurate method for dating pregnancies, supplementing and often replacing LMP-based dating.
Mittendorf Study Challenges 280 Days
A study of 427 healthy Caucasian women found that average gestation was 288 days from LMP for first-time mothers — 8 days longer than Naegele's prediction. The study showed significant individual variation and questioned the universal 280-day assumption.
ACOG Redefines 'Term' Pregnancy
ACOG and SMFM replaced the single 'term' label with four categories: early term (37-38), full term (39-40), late term (41), and post-term (42+). This change discouraged elective deliveries before 39 weeks, reducing neonatal complications.
Key Research & Data
BJOG — Jukic et al. (2013)
Natural Variability in Human Gestation
Measured ovulation-to-delivery in 125 naturally conceived pregnancies. Median gestation was 268 days from ovulation (not 266). Range varied by 37 days. Older mothers and those who were heavier at birth themselves had longer pregnancies. Even after excluding preterm births, variability was 5 weeks.
ACOG Practice Bulletin #700
Methods for Estimating Due Date (2014)
ACOG recommends: first-trimester ultrasound is the most accurate method for dating (±5-7 days). LMP with confirmatory first-trimester US is standard practice. IVF pregnancies should be dated from transfer date. US dating should supersede LMP if discrepancy exceeds threshold.
BMJ — Olsen et al.
Cross-National Gestational Length Study
Analysis of 44,000+ pregnancies across multiple countries found that average gestational length varied by maternal origin: European-ancestry women averaged 281 days, while South Asian and African-ancestry women averaged 273-275 days — suggesting standard EDD may overestimate for some populations.
NEJM — Spong (2013)
Defining 'Term' Pregnancy
Led the ACOG/SMFM workgroup that redefined term pregnancy into four categories (early, full, late, post-term). Data showed that neonatal outcomes improve progressively up to 39-40 weeks. Elective delivery before 39 weeks increases NICU admission, respiratory distress, and feeding difficulties.
Pregnancy Myths vs. Facts
Your due date is when your baby will be born.
Only about 5% of babies are born on their due date. It's an estimate — a midpoint in a range. Most babies arrive within 2 weeks before or after. Think of it as a 'due month' rather than a specific date for more realistic expectations.
First babies always come late.
First-time mothers do tend to have slightly longer pregnancies (by 1-2 days on average), but 'always late' is a myth. About 50% of first babies arrive before their due date. Individual variation is far greater than the first-vs-subsequent difference.
Spicy food or walking induces labor.
No foods have been scientifically proven to induce labor. Walking may help labor progress once contractions have started, but it doesn't reliably start labor. The only evidence-based methods for labor induction are medical (prostaglandins, oxytocin, membrane sweep, amniotomy).
A bigger baby means an earlier delivery.
Baby size does not determine when labor starts. Labor onset is triggered by complex hormonal signals involving the fetal lungs, placenta, and maternal hormones — not fetal size. Large babies (macrosomia >4 kg) may actually have slightly longer gestations.
Frequently Asked Questions
How accurate is a due date calculator?▼
Can my due date change?▼
What if I don't know my last period date?▼
How is gestational age different from fetal age?▼
When is the first trimester ultrasound done?▼
What is considered premature?▼
Is there a difference in due date for twins?▼
What happens if I go past my due date?▼
Does sex of the baby affect due date?▼
How do conception date calculators work?▼
What is the ARRIVE trial?▼
Can stress affect my due date?▼
References
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