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Reviewed by CalculatorApp.me Health Team
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
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.
Enter values above to see results.
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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.
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
| 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 |
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.
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.
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.
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.
BJOG — Jukic et al. (2013)
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
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.
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)
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).
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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.
| 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 |
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 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.
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.
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.