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Pregnancy Due Date Calculator (EDD)

Calculate your Estimated Due Date (EDD) and current trimester based on your Last Menstrual Period (LMP). Track your exact pregnancy timeline with Naegele's Rule, adjusted for your personal cycle length.

Pregnancy Due Date Calculator (EDD)

Calculate your Estimated Due Date using any of four clinical methods — LMP, known conception date, or IVF embryo transfer. Based on Naegele's Rule, adjusted for your personal cycle length where applicable.

Standard 28 days — each day ± shifts EDD by 1 day

Adjusted gestation = 280 + (28 − 28) = 280 days
EDD = 2026-02-21 + 280 days = Sat, November 28, 2026
Gestational age = 44 days elapsed = 6w 2d
Estimated Due Date
Sat, November 28, 2026
280-day gestation
Gestational Age
6w 2d
from LMP
Days Remaining
236
until EDD
First Trimester — 1–13 wks

Major organ formation. Morning sickness peaks. Critical period for neural tube development.

Conception / LMP16% through pregnancyEDD
1st (0–13w)2nd (14–27w)3rd (28w+)
Key Milestone Dates
First Heartbeat (6 wks)
Sat, April 4, 2026
End of First Trimester (13 wks)
Sat, May 23, 2026
Anatomy Scan window (18–20 wks)
Sat, July 11, 2026
Viability Milestone (24 wks)
Sat, August 8, 2026
Third Trimester begins (28 wks)
Sat, September 5, 2026
Full Term (39 wks)
Sat, November 21, 2026
Estimated Due Date (40 wks)
Sat, November 28, 2026

Practical Example

LMP = January 1, standard 28-day cycle:
EDD = January 1 + 280 days = October 8

Same conception date, Day-5 IVF transfer:
Transfer date ≈ January 19 (ovulation day 14 + 5 days culture).
EDD = January 19 + 261 days = October 7 (one day earlier — embryo culture offset).

Why "9 months" is a myth: Pregnancy is tracked as 40 weeks (280 days) from LMP. Since ovulation occurs ~Day 14, the embryo on the day of conception is already clinically 2 weeks pregnant by gestational age. You are never "0 weeks pregnant" on conception day.

💡 Field Notes

  • IVF vs. natural conception dating: In IVF, the embryo age is precisely known. A Day-5 blastocyst transfer means fertilization occurred exactly 5 days ago. Adding these 5 days to the 266-day embryonic gestation gives 261 days added to the transfer date. This is why IVF-conceived pregnancies often have more accurate EDDs than naturally-conceived pregnancies — the fertilization date is lab-documented, eliminating LMP recall uncertainty and cycle length variability entirely.
  • Cycle length adjustment is clinically important: Women with 35-day cycles ovulate around Day 21 instead of Day 14 — 7 days later than assumed. Without cycle adjustment, the LMP method predicts a due date 7 days too early, causing these women to appear "overdue" when they're on schedule. This calculator adds (CycleLength − 28) days to the standard 280-day gestation for LMP calculations only (conception and IVF methods don't need it — fertilization date is already known precisely).
  • Only 4–5% of babies are born on their EDD: The EDD is the peak of a statistical distribution, not a hard deadline. "Term" spans 37–42 weeks. Most hospitals consider induction if pregnancy extends past 41 weeks 0 days to reduce risks of placental insufficiency. If early ultrasound (8–12 weeks) crown-rump length disagrees with the calculated EDD by more than 5–7 days, the ultrasound EDD supersedes the LMP-derived date in clinical management.
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What is Obstetrics and Naegele's Rule?

The Estimated Due Date (EDD) is one of the most consequential calculations in medicine — it determines the clinical management of the entire pregnancy, sets induction thresholds, guides fetal development monitoring, and informs decisions about preterm birth risk. Despite the precision implied by a specific date, the EDD is probabilistic: only about 5% of babies are born on the exact estimated due date. Understanding how the EDD is calculated, why the '9 months' concept is imprecise, and how cycle length affects the estimate is essential for any pregnant person managing their own clinical timeline.

Mathematical Foundation

Naegele's Rule (Standard 28-Day Cycle)
EDD=LMP+280 days\text{EDD} = \text{LMP} + 280 \text{ days}
LMPLMP= Last Menstrual Period — the first day of bleeding in the most recent menstrual cycle before pregnancy. Crucially, this is NOT the date of conception (which typically occurs around Day 14 of a 28-day cycle).
280280= Standard gestation in days from LMP. This equals exactly 40 weeks. Naegele derived this number in 1850 from observation of birth timing in a hospital population.
Cycle-Length Adjusted Naegele's Rule
EDD=LMP+(280+(C−28))\text{EDD} = \text{LMP} + (280 + (C - 28))
C−28C - 28= Adjustment in days for non-standard cycle length. A woman with a 35-day cycle ovulates approximately 7 days later than a woman with a 28-day cycle (around Day 21 vs. Day 14). This shifts the entire timeline.
LMP+AdjustedGestationLMP + Adjusted Gestation= The total days from LMP to EDD, now accounting for the shift in ovulation timing. Ranges from 260 days (20-day cycle) to 297 days (45-day cycle) in this calculator's input range.
Gestational Age Calculation
Gestational Age=⌊Today−LMP86400000⌋ days=weeks+remainder days\text{Gestational Age} = \left\lfloor \frac{\text{Today} - \text{LMP}}{86400000} \right\rfloor \text{ days} = \text{weeks} + \text{remainder days}
8640000086400000= Milliseconds in one day (1000 ms × 60 s × 60 min × 24 hr). JavaScript's Date objects store time in milliseconds.
⌊⌋\lfloor \rfloor= Floor function — always rounds DOWN to the nearest whole day/week. A pregnancy at '14 weeks, 0 days' is distinct from '13 weeks, 6 days' clinically.

Laws & Principles

  • Why '9 Months' Is a Myth — The 40-Week Reality: Most people believe pregnancy lasts 9 months. Biologically, this is imprecise. First, 40 weeks = 280 days, which spans roughly 9.2 calendar months. Second, pregnancy is always counted from LMP, not conception — adding approximately 2 weeks to the beginning.
  • Term Pregnancy Definition — The 37–42 Week Window: Medical consensus defines 'term' pregnancy as a range, not a point: Early Term: 37–38 weeks 6 days. Full Term: 39–40 weeks 6 days. Late Term: 41–41 weeks 6 days. Postterm: 42+ weeks.
  • Ultrasound Dating vs. LMP Dating — Which Wins: In clinical practice, if an early ultrasound (performed before 14 weeks) shows a crown-rump length inconsistent with the LMP-derived gestational age by more than 5–7 days, the sonographic EDD typically supersedes the LMP-based EDD.

Step-by-Step Example Walkthrough

" LMP = January 1, 2025. Standard 28-day cycle. "

  1. 1. Adjusted gestation = 280 + (28 − 28) = 280 days.
  2. 2. EDD = January 1, 2025 + 280 days.
  3. 3. January has 31 days → 31 days used, 249 remaining after January.
  4. 4. Adding month by month: Feb(28)+Mar(31)+Apr(30)+... = October 8, 2025.
  5. 5. If cycle = 35 days: adjusted gestation = 280 + 7 = 287 days. EDD = January 1 + 287 days = October 15, 2025 — one week later.
  6. 6. At gestational age 14 weeks 0 days: entering Second Trimester.
Final Result: EDD = October 8, 2025 for a standard 28-day cycle. A 35-day cycle shifts the due date to October 15. At the time of a January 1 LMP, conception occurred around January 14–15 (Day 14 ovulation).

Quick Answer: How exactly is my Estimated Due Date determined?

Your Estimated Due Date (EDD) is mathematically calculated using a modified version of Naegele's Rule. You simply take the first day of your Last Menstrual Period (LMP) and add precisely 280 days (or 40 full weeks). This engine accounts for your unique cycle length to algorithmically correct the ovulation offset, granting a pinpoint clinical timeline spanning all three trimesters.

Core Mechanics

The Naegele protocol structures human gestation as a universally standardized 40-week timeline anchored to the first day of menstruation, not biological conception.

EDD = LMP + 280 Days (+ Cycle Length Variation)
Gestational Age = (Current Date - LMP) / 7 Weeks
Term Threshold = 37 Weeks, 0 Days

By injecting a cycle length shift parameter `(Cycle Length - 28)`, the equation dynamically aligns the date of presumed conception with your extended or truncated follicular phase.

Real-World Scenarios

✅ Adjusted Menstrual Sequencing

A patient experiences consistent 35-day cycles. Standard Naegele math assumes standard ovulation and calculates a rigid 40-week EDD. But our exact adjusted calculator recognizes ovulation occurred on Day 21, appending 7 days to the target EDD and safeguarding her from an unwanted "overdue" induction.

❌ The "9-Month" Miscalculation

A mother simply adds 9 calendar months to the day she took her positive pregnancy test. Because the test occurs up to 5 weeks into her clinical gestation, she misses the universal LMP anchor by a profound margin, derailing the scheduling of her critical 20-week anatomy scan.

Gestational Term Definitions

Clinical Definition Timeframe Range Medical Significance
Preterm Prior to 37w0d Elevated risk of NICU admittance due to underdeveloped lung surfactant.
Early Term 37w0d - 38w6d Considered 'at term' legally, but physiological maturation continues.
Full Term 39w0d - 40w6d Optimal delivery window. The target span of the predictive EDD algorithm.
Late Term 41w0d - 41w6d Frequent fetal monitoring begins at this phase to ensure placental viability.
Postterm 42w0d + Clinical consensus generally advises induction to mitigate placental degradation.

Pro Tips & Common Mistakes

Do This

  • ✓Pinpoint your exact cycle length. Do not blindly assume a 28-day cycle. Using the exact distance between your menses guarantees the ovulation parameter is mathematically correctly aligned.
  • ✓Defer to early ultrasound datums. If your 8-week dating scan contradicts your LMP formula by a span greater than 7 days, defer completely to the sonogram data as embryos grow at a universally fixed rate.

Avoid This

  • ✗Don't anchor to the conception date. Do not enter the day you think you conceived into an LMP form field; if you do so, you instantly throw the entire algorithm off by 14 days, resulting in massive clinical desync.
  • ✗Don't change your EDD late. Later term scans (20+ weeks) analyze fetal mass, not absolute age. Never allow a late-term growth scan to overwrite a confirmed early-term dating scan.

Frequently Asked Questions

Why am I considered 'two weeks pregnant' on the day of conception?

Medical terminology counts pregnancy from the first day of your Last Menstrual Period (LMP), which typically occurs two weeks before you actually ovulate and conceive. This means that during the first two weeks of a clinical 40-week pregnancy, the biological embryo does not even exist yet.

If my ultrasound date differs from my LMP date, which one is correct?

Early ultrasounds (conducted between 8 and 12 weeks) provide the most accurate assessment of fetal age. If an early sonogram measures a gestational age that alters your Estimated Due Date (EDD) by more than 7 days from your LMP calculation, the ultrasound date clinically supersedes it.

Does a 35-day cycle change my estimated due date?

Absolutely. Standard biological models assume a 28-day cycle with ovulation occurring on Day 14. If you have a 35-day cycle, you ovulate a full week later (around Day 21). This shifts conception forward 7 days, which systematically pushes your Estimated Due Date back exactly one week compared to a standard Naegele calculation.

What is the clinical definition of a 'Full Term' pregnancy?

A pregnancy is considered 'Full Term' starting mathematically at 39 weeks and 0 days, stretching until 40 weeks and 6 days. 'Early Term' is classified as 37 weeks to 38 weeks and 6 days, while delivering prior to 37 completed weeks is medically defined as 'Preterm'.

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