Due Date Calculator

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Due Date Calculator

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Free online pregnancy due date calculator using Naegele's Rule. Estimate your due date, current week, trimester, and conception date with AI-powered insights.

First Day of Last Menstrual Period

Average Cycle Length (days)

Average is 28 days. Typical range: 21–35 days.

Enter values above to see results.

About This Calculator

🤰 Due Date Calculator — Complete Guide

Reviewed by CalculatorApp.me Health Editorial Team · Updated June 2026 · 10 min read

🔬 ACOG Guidelines

280 days

Standard gestational period (40 weeks)

5%

Of babies born on their exact due date

80%

Born within 2 weeks of due date

39–41 wks

Full-term delivery window (ACOG)

Naegele's RuleDating MethodsTrimester GuideDue Date AccuracyMyths vs FactsFAQs

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

✗MYTH: The due date predicts exactly when birth will occur

✓FACT: Only 5% of babies are born on their exact due date. Births within ±2 weeks of the due date account for ~80%. The due date is a statistical estimate, not a biological certainty.

✗MYTH: Going past your due date means you need a C-section

✓FACT: Most overdue pregnancies (41–42 weeks) deliver vaginally. Induction may be offered at 41–42 weeks due to placental aging risks, but cesarean section is not automatically indicated. Management depends on individual clinical assessment.

✗MYTH: Ultrasound due dates are always more accurate than LMP

✓FACT: First-trimester ultrasound (8–12 weeks) is more accurate than LMP, but only if the ultrasound date differs from LMP by >7 days. ACOG recommends changing the due date if the discrepancy exceeds these thresholds.

✗MYTH: Your due date changes during pregnancy

✓FACT: The "estimated due date" (EDD) is typically set by the most accurate early measurement and does not change unless there is a compelling clinical reason. The date may be revised if early dating was uncertain.

History of Due Date Estimation

The standard method of estimating due dates — counting 280 days (40 weeks) from the first day of the last menstrual period — is known as Naegele's Rule, named after German obstetrician Franz Naegele, who described it in his 1812 textbook. The rule assumes a textbook 28-day cycle with ovulation on day 14, which is why it's treated as an estimate rather than a precise prediction.

Ultrasound-based dating, which measures fetal crown-rump length in the first trimester, became widely available starting in the 1970s and is now considered more accurate than LMP-based dating alone, especially for people with irregular cycles — most modern prenatal care uses both methods together.

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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Due Date Calculator

Estimate pregnancy due date using Naegeles Rule with cycle length adjustment, trimester tracking, and progress bar.

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

Naegele's Rule (Standard)

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.

Modified Naegele's (Cycle Adjustment)

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).

Ultrasound Dating Methods

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 Due Date Calculation

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

~400 BC

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.

1744

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.

1812

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.

1958

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.

1990

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.

2013

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?▼

LMP-based calculations are accurate to ±2 weeks for about 80% of pregnancies. First-trimester ultrasound narrows this to ±5-7 days. Combined methods (LMP + ultrasound) provide the best estimate. Remember, only 5% of babies arrive on the exact EDD.

Can my due date change?▼

Yes. If a first-trimester ultrasound differs from the LMP-based date by more than 7 days, your healthcare provider will typically adopt the ultrasound date. Due dates should rarely be changed after the first trimester.

What if I don't know my last period date?▼

If LMP is unknown or unreliable (irregular cycles, recent contraception use), a first-trimester ultrasound measuring crown-rump length (CRL) is the primary dating method. This is accurate to ±5 days between 8-13 weeks.

How is gestational age different from fetal age?▼

Gestational age is counted from the first day of the LMP (about 2 weeks before conception). Fetal age (embryonic age) is counted from actual conception. A 10-week gestational age = approximately 8-week fetal age.

When is the first trimester ultrasound done?▼

Dating ultrasounds are typically performed between 8-13 weeks gestational age. The crown-rump length (CRL) measurement is most precise at this stage. Earlier ultrasounds (5-6 weeks) can confirm viability but are less accurate for dating.

What is considered premature?▼

Preterm birth is before 37 weeks. It's further classified as: extremely preterm (<28 weeks), very preterm (28-32), moderate preterm (32-34), and late preterm (34-36). Survival rates have improved dramatically — 90% at 28 weeks in modern NICUs.

Is there a difference in due date for twins?▼

Twin pregnancies average 36-37 weeks, not 40 weeks. Due dates are still calculated from LMP/ultrasound, but delivery planning typically targets 37-38 weeks for dichorionic twins and 36-37 weeks for monochorionic twins to reduce complications.

What happens if I go past my due date?▼

Most providers recommend induction between 41-42 weeks. Beyond 42 weeks (post-term), risks increase: placental insufficiency, meconium aspiration, macrosomia, and stillbirth risk doubles from 1/1000 to 2/1000 per week. Fetal monitoring increases after 41 weeks.

Does sex of the baby affect due date?▼

Slightly. Studies show male babies are born, on average, 1-2 days later than female babies. The difference is small and clinically insignificant — it shouldn't influence EDD calculations.

How do conception date calculators work?▼

They estimate the approximate date of conception by subtracting 266 days from the estimated due date, or by adding 14 days to the LMP. This gives the approximate ovulation/fertilization window, useful for tracking but not precise enough for legal purposes.

What is the ARRIVE trial?▼

The ARRIVE trial (2018) showed that elective induction at 39 weeks in healthy first-time mothers reduced cesarean rates (from 22% to 19%) without increasing adverse neonatal outcomes. It led many providers to offer 39-week induction to low-risk patients.

Can stress affect my due date?▼

Chronic stress is associated with preterm birth (before 37 weeks) through cortisol-mediated pathways. However, normal daily stress doesn't change your due date. Severe psychological stress may increase preterm risk by 25-60% according to some studies.

References

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Methodology, sources & limitations

This calculator uses a published formula and the values entered in the calculator to generate the result.

Formula: EDD = LMP + 280 days
Jurisdiction & units: Intended for US, IN, EU, GB, Global. Supports both units where applicable.
Assumptions & limitations: Results are screening or educational estimates based on the inputs you provide. They do not diagnose, treat, or replace advice from a qualified clinician.
Review status: Formula and automated QA completed June 21, 2026; independent professional review is pending. Next scheduled review: September 21, 2026.

Validation sources

ACOG Practice Bulletin
WHO Pregnancy Guidelines

Due Date Calculator — Answer & Method

Calculate pregnancy due date from last menstrual period or conception date.

Formula: Due Date (Naegele)

EDD = LMP + 280 days

LMP = Last Menstrual Period (date)

Example Calculation

LMP of March 1, 2026 gives estimated due date of December 6, 2026.

Important limitation

This is an educational estimate, not a diagnosis or a substitute for professional medical advice.

Key Facts

Full-term pregnancy is 40 weeks (280 days) from the first day of the last period.
Only about 5% of babies are born on their exact due date.

Sources & Validation

ACOG Practice BulletinWHO Pregnancy Guidelines

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About This Calculator

Related Articles

Pregnancy Due Date Calculator Guide

🤰 Due Date Calculator — Complete Guide

How Due Dates Are Calculated

Pregnancy Term Definitions

Pregnancy Dating Methods Compared

Due Date Myths vs Facts

History of Due Date Estimation

Frequently Asked Questions

References & Further Reading

Related Calculators

Explore All Health Calculators

Due Date Calculator

Due Date Calculator — Complete Guide

How Due Date Calculation Works

Due Date Formulas & Methods

Pregnancy Trimester Breakdown

History of Due Date Science

Hippocrates — First Gestation Estimates

Boerhaave Develops the Calculation

Naegele's Rule Published

Ultrasound in Obstetrics Begins

Mittendorf Study Challenges 280 Days

ACOG Redefines 'Term' Pregnancy

Key Research & Data

Natural Variability in Human Gestation

Methods for Estimating Due Date (2014)

Cross-National Gestational Length Study

Defining 'Term' Pregnancy

Pregnancy Myths vs. Facts

Frequently Asked Questions

References

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Methodology, sources & limitations

Validation sources

Due Date Calculator — Answer & Method

Formula: Due Date (Naegele)

Example Calculation

Important limitation

Key Facts

Sources & Validation

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Pregnancy Due Date Calculator Guide

See Also

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Pregnancy Due Date Calculator Guide

🤰 Due Date Calculator — Complete Guide

How Due Dates Are Calculated

Pregnancy Term Definitions

Pregnancy Dating Methods Compared

Due Date Myths vs Facts

History of Due Date Estimation

Frequently Asked Questions

References & Further Reading

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