Due Date Calculator
Calculate your baby's estimated due date using last menstrual period, ultrasound date, conception date, or IVF transfer date. Supports custom cycle lengths and multiple pregnancies. Track pregnancy progress.
Key Features
- •Four estimation methods: Last Period, Ultrasound, Conception Date, IVF Transfer
- •Optional 'Estimate Based On' toggle — defaults to LMP when off
- •Average cycle length adjustment (22–44 days) for non-standard cycles
- •LMP method (Naegele's Rule: minus 3 months + 7 days, cycle-length adjusted)
- •Ultrasound date method (enter scan date for adjusted EDD)
- •Conception date method (add 266 days / 38 weeks)
- •IVF/ART method (Day 3 or Day 5 blastocyst transfer options)
- •Optional multiple pregnancy toggle (twins −3 weeks, triplets −4 weeks)
- •Current pregnancy progress (weeks + days gestation)
- •Trimester tracker (First, Second, Third boundaries)
- •Important dates list (NT scan, anatomy scan, glucose test, Tdap, GBS)
- •Delivery probability chart (by week from 37–42)
About Due Date Calculator
Your Estimated Due Date (EDD) is the projected date of delivery. While only 5% of babies arrive exactly on their due date, having a target helps you plan prenatal care, prepare for maternity leave, and arrange family support. Our Due Date Calculator uses four standard clinical methods plus custom cycle length adjustment and multiple gestation support.
Estimate Based On: Four Methods
Method 1: Last Period / Naegele's Rule — Standard Obstetric Practice
Named after Franz Karl Naegele (1778-1851), this is the most widely used formula:
- Start with first day of Last Menstrual Period (LMP)
- Subtract 3 months
- Add 7 days
- Adjust year if necessary
Example: LMP = January 15, 2026
- Subtract 3 months = October 15, 2025
- Add 7 days = October 22, 2025
- Add one year: October 22, 2026 (due date)
Formula version: EDD = LMP + 280 days (40 weeks)
Assumptions of Naegele's Rule:
- 28-day menstrual cycle (adjustable via cycle length setting)
- Ovulation on day 14
- 38 weeks from conception to delivery
- Regular cycles (± 7 days)
Average Cycle Length Adjustment
Standard Naegele's Rule assumes a 28-day cycle. If your cycles are shorter or longer (22–44 days), the calculator automatically adjusts your EDD:
- EDD = LMP + (280 + (your cycle length − 28)) days
- Example: 32-day cycle → EDD = LMP + 284 days
- Example: 24-day cycle → EDD = LMP + 276 days
This adjustment ensures your due date accurately reflects your personal ovulation timing.
Method 2: Ultrasound Date — Gold Standard
First trimester ultrasound (8–13 weeks, 6 days) measuring Crown-Rump Length (CRL) is the most accurate dating method (±3–5 days). If ultrasound and LMP dates differ by more than 5 days, providers typically use the ultrasound EDD. Enter the date of your ultrasound scan and the calculator applies the standard 280-day formula from the equivalent LMP date.
Method 3: Conception Date — For Known Ovulation or Timed Intercourse
If you know your conception date (via ovulation tracking, IUI, or known intercourse date):
- EDD = Conception Date + 266 days (38 weeks)
Method 4: IVF Transfer — For Assisted Reproduction
For IVF pregnancies, the transfer date is used directly:
- Day 5 (blastocyst) transfer: EDD = Transfer Date + 261 days
- Day 3 embryo transfer: EDD = Transfer Date + 263 days
- Donor egg (fresh): EDD = Retrieval date + 266 days
- Frozen embryo: EDD = Transfer date + embryo age adjustment
Multiple Pregnancy (Optional)
Due dates for multiples are earlier due to the increased demands of carrying more than one baby:
- Twins: EDD adjusted earlier by 3 weeks (average delivery 35–37 weeks)
- Triplets: EDD adjusted earlier by 4 weeks (average delivery 32–34 weeks)
Enable the Multiple Pregnancy toggle to apply this adjustment to any of the four dating methods.
Pregnancy Timeline: Key Weeks
| Weeks | Gestational Age | What's Happening |
|---|---|---|
| 0–4 | 1 month | Implantation, positive test |
| 4–8 | 2 months | Heartbeat visible, major organs form |
| 8–12 | 3 months | Nuchal translucency screen, miscarriage risk drops |
| 12–16 | 4 months | Sex organs visible, may feel flutters |
| 16–20 | 5 months | Anatomy scan, feel definite kicks |
| 20–24 | 6 months | Viability (25% survival), glucose test |
| 24–28 | 7 months | Third trimester begins, baby moves a lot |
| 28–32 | 8 months | Growth spurt, baby turns head down |
| 32–36 | 9 months | Lungs mature, Braxton Hicks |
| 36–40 | 10 months | Full term, baby could arrive any day |
| 40–41 | Post-term | Induction discussed |
| 41–42 | Late-term | Induction recommended |
Probability of Delivery by Week
| Week | Cumulative % Delivered | Notes |
|---|---|---|
| 37 weeks | 10% | "Early term" |
| 38 weeks | 25% | |
| 39 weeks | 50% | Most common week |
| 40 weeks (EDD) | 75% | Only 5% deliver exactly on EDD |
| 41 weeks | 90% | |
| 42 weeks | 98% | Induction standard of care |
When Due Dates Change
Your due date may be adjusted by your provider if:
- First trimester ultrasound discrepancy >5 days: Provider switches to ultrasound EDD
- Second trimester discrepancy >10–14 days: Provider may adjust (less accurate now)
- Third trimester: Too late for dating; provider keeps original EDD
- IVF pregnancy: EDD set at embryo transfer (no changes)
Using Due Date for Planning
- Maternity leave: Most US FMLA leave starts at delivery; some states allow 4 weeks pre-delivery
- Baby shower: Schedule 4–6 weeks before due date (36–37 weeks)
- Birth classes: Complete by week 34–36
- Hospital bag: Pack by week 35–36
- C-section scheduling: Typically week 39 for elective, week 37–38 for medical necessity
- Travel restrictions: Most providers discourage travel after week 34–36
Induction and Post-Dates Pregnancy
If you go past your due date:
- Week 41 (41+0): Discuss induction with provider, increased fetal monitoring (NST, BPP)
- Week 41+3 to 42+0: Induction recommended (ACOG guidelines) due to increased risk of stillbirth, meconium aspiration, and macrosomia
- Week 42+0: Maximum recommended gestation; induction strongly advised
FAQ: Due Date Calculator
Which method should I use?
Use Last Period if you know your LMP and have regular cycles — adjust the cycle length if yours differs from 28 days. Use Ultrasound if you've had a first-trimester scan, as it's the most accurate. Use Conception Date if you tracked ovulation. Use IVF Transfer for assisted reproduction.
Can my due date be wrong?
Yes, especially if based solely on LMP with irregular cycles. The cycle length adjustment helps account for this. Early ultrasound is accurate within 3–5 days.
Why is pregnancy measured in weeks from LMP when conception is later?
Historical convention. Before modern ovulation tracking, LMP was the only known date. The 40-week count includes ~2 weeks before conception.
What's the difference between gestational age and fetal age?
Gestational age = weeks since LMP (clinical standard). Fetal age = weeks since conception (2 weeks less).
I can't remember my LMP.
Early ultrasound (8–12 weeks) is most accurate. If unavailable, providers estimate based on fundal height or first fetal heartbeat detection (Doppler at 10–12 weeks).
Due Date Calculator is optimized for fast browser-based use, so you can test multiple scenarios in seconds.
Formula & Logic
- 01Last Period (Naegele's Rule): EDD = LMP Date + (280 + (cycleLength − 28)) days.
- 02Ultrasound method: EDD = Scan Date + 280 days (standard LMP equivalent).
- 03Conception method: EDD = Conception Date + 266 days (38 weeks).
- 04IVF Day 5 blastocyst: EDD = Transfer Date + 261 days.
- 05IVF Day 3 embryo: EDD = Transfer Date + 263 days.
- 06Twins adjustment: EDD − 21 days (3 weeks earlier).
- 07Triplets adjustment: EDD − 28 days (4 weeks earlier).
- 08Current gestational age = (Today's Date − LMP equivalent) in days ÷ 7.
- 09Days remaining = (EDD − Today's Date) in days.
Practical Examples
- 01Baseline check: Use realistic inputs in Due Date Calculator to generate a first-pass estimate.
- 02Sensitivity check: Change one key input at a time to compare how the output shifts.
- 03Decision check: Save two or more scenarios and use the differences to choose the better option.
Important Limitations
- •Results depend on the accuracy of your inputs.
- •Displayed values may be rounded for readability.
- •Health-related outputs are educational estimates and should not replace medical advice.
Frequently Asked Questions
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