LMP is the most common method
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Enter the first day of your last menstrual period
Used to adjust ovulation timing
Sets LMP to that many days before today
Estimated Due Date
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⚠️ Disclaimer: This calculator provides an estimate only. Only about 5% of babies are born on their exact due date. Consult your obstetrician or midwife for an official due date and personalized prenatal care guidance.
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Sources & Methodology
Due date calculations use Naegele's Rule as standardized by the American College of Obstetricians and Gynecologists (ACOG) and consistent with NIH MedlinePlus pregnancy dating guidelines.
ACOG — How Your Baby Grows During Pregnancy
American College of Obstetricians and Gynecologists guidance on pregnancy dating methods, trimester definitions, and gestational age milestones used in this calculator.
NIH MedlinePlus — Calculating Your Due Date
NIH explanation of Naegele's Rule, the standard formula for estimating due dates from LMP. Used to verify formula accuracy and cycle-length adjustments.
CDC — Pregnancy Health
CDC data on pregnancy health, preterm birth definitions, and standard gestational age ranges referenced for trimester classifications and milestone definitions.
Methodology: LMP method (Naegele's Rule): EDD = LMP + 280 days, adjusted for cycle length: EDD = LMP + 280 + (cycle length − 28) days. Conception method: EDD = conception date + 266 days. Ultrasound method: EDD = ultrasound date + (280 − gestational weeks × 7) days. Weeks pregnant = (today − LMP) ÷ 7. Trimester 1: weeks 1–13. Trimester 2: weeks 14–26. Trimester 3: weeks 27–40.
⏱ Last reviewed: April 2026
How to Calculate Your Pregnancy Due Date
Your estimated due date (EDD) is one of the first and most important pieces of information in prenatal care. It determines the timing of all your prenatal appointments, screenings, and ultrasounds. Understanding how it is calculated helps you interpret your results and communicate clearly with your healthcare provider.
Most pregnancies last approximately 280 days (40 weeks) from the first day of the last menstrual period. This counting method is used because ovulation — and therefore conception — is harder to pinpoint than a menstrual period. The 40-week count includes approximately 2 weeks before conception actually occurred.
Naegele's Rule — The Standard Due Date Formula
EDD = LMP + 280 days (40 weeks)
Example: LMP = January 15, 2025
EDD = January 15 + 280 days = October 22, 2025
Alternative: LMP + 1 year − 3 months + 7 days = same result
EDD = January 15 + 280 days = October 22, 2025
Alternative: LMP + 1 year − 3 months + 7 days = same result
Cycle Adjustment: EDD = LMP + 280 + (Cycle Length − 28) days
Example (35-day cycle): LMP + 280 + (35 − 28) = LMP + 287 days
Example (21-day cycle): LMP + 280 + (21 − 28) = LMP + 273 days
Longer cycles push the due date later; shorter cycles make it earlier.
Example (21-day cycle): LMP + 280 + (21 − 28) = LMP + 273 days
Longer cycles push the due date later; shorter cycles make it earlier.
Pregnancy Trimesters and Key Milestones
| Trimester | Weeks | Key Developments | Screenings |
|---|---|---|---|
| First | 1–13 | Heart forms (wk 6), organs develop, embryo becomes fetus (wk 10) | Blood tests, nuchal translucency scan (wk 11–13) |
| Second | 14–26 | Movement felt (wk 18–20), viability reached (wk 24) | Anatomy ultrasound (wk 18–22), glucose test (wk 24–28) |
| Third | 27–40 | Rapid weight gain, lung maturation, full term at 39 wks | Group B strep test (wk 35–37), weekly visits from wk 36 |
What Term Definitions Mean
| Term | Gestational Age | Description |
|---|---|---|
| Preterm | Before 37 weeks | Requires special care; highest risk before 34 weeks |
| Early Term | 37w 0d – 38w 6d | Outcomes slightly better than preterm |
| Full Term | 39w 0d – 40w 6d | Optimal timing for most planned deliveries |
| Late Term | 41w 0d – 41w 6d | Induction often discussed with provider |
| Post Term | 42 weeks + | Induction typically recommended |
Which Calculation Method Is Most Accurate?
A first-trimester ultrasound (before 14 weeks) is the most accurate way to confirm gestational age, typically accurate to within 5 to 7 days. An ultrasound after 20 weeks is less precise (within 10 to 14 days). The LMP method works well for women with regular 28-day cycles, but becomes less reliable with irregular cycles.
ACOG recommends confirming the LMP-based due date with an ultrasound at the first prenatal visit. If there is a discrepancy of more than 7 days in the first trimester or more than 14 days in the second trimester, the due date is typically adjusted to match the ultrasound measurement.
💡 Only 5% of babies are born on their due date. Most births occur within two weeks before or after the EDD. The normal delivery window is 39 weeks 0 days to 40 weeks 6 days (full term). Do not be alarmed if your baby arrives a week or two early or late — this is completely normal. Contact your provider if you reach 41 weeks without labor beginning.
Frequently Asked Questions
The standard method uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. For example, if your LMP was January 1, your estimated due date is October 8. If your cycle is longer or shorter than 28 days, the due date is adjusted accordingly. Our calculator handles all methods including conception date and ultrasound.
Pregnancy is counted from the first day of your last menstrual period, not from conception. To find how many weeks pregnant you are, count the days from your LMP to today and divide by 7. If your LMP was 70 days ago, you are 10 weeks pregnant. Our calculator displays your exact weeks and days pregnant when you enter your LMP date.
A first-trimester ultrasound (before 14 weeks) is the most accurate dating method, typically within 5 to 7 days. The LMP method is accurate for women with regular 28-day cycles. If your cycles are irregular, ultrasound measurement of the crown-rump length in the first trimester provides the most reliable estimate. ACOG recommends confirming LMP dates with an early ultrasound.
The first trimester is weeks 1 through 13. The second trimester is weeks 14 through 26. The third trimester is weeks 27 through 40. A baby born before 37 weeks is preterm. Full term is 39 weeks 0 days to 40 weeks 6 days. Our calculator shows your current trimester based on your LMP or conception date and today's date.
Yes. Add 266 days (38 weeks) to your known conception date to get your EDD. Conception typically occurs about 14 days after the first day of your LMP in a 28-day cycle. This is why the LMP method adds 280 days while the conception method adds only 266 — the difference accounts for the approximately two weeks between your period and ovulation.
Naegele's Rule is the standard obstetric formula: take the first day of your last menstrual period, add 1 year, subtract 3 months, and add 7 days. This equals adding 280 days (40 weeks) to the LMP. The formula assumes a 28-day menstrual cycle with ovulation on day 14. Women with longer or shorter cycles need a cycle-length adjustment for accuracy.
Only about 5% of babies are born exactly on their estimated due date. Most births occur within two weeks before or after the EDD. ACOG considers a normal delivery to be anytime between 39 weeks 0 days and 40 weeks 6 days (full term). Preterm is before 37 weeks and post-term is after 42 weeks. Your due date is an estimate, not a prediction.
The first trimester runs from week 1 to the end of week 13. The second trimester runs from week 14 to the end of week 26. The third trimester runs from week 27 until delivery around week 40. Key milestones include a detectable heartbeat around week 6, the anatomy ultrasound around week 20, and viability reached at approximately week 24.
Full term is 39 weeks 0 days to 40 weeks 6 days according to ACOG. Early term is 37 to 38 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term is 42 weeks and beyond. Babies born between 34 and 36 weeks 6 days are called late preterm. If you reach 41 weeks without labor, your provider will likely discuss induction options.
Find the first day of your most recent menstrual period, then add 280 days (40 weeks) to that date. If your cycle is longer than 28 days, add the extra days to 280. For example, with a 35-day cycle, add 287 days. Use our calculator above — select Last Menstrual Period, enter the date, choose your cycle length, and your EDD is calculated instantly.
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