Can I use the due date calculator if I don't know my LMP?

Yes. Use either the Conception Date mode (if you know when conception likely occurred) or the Ultrasound mode (if you have had a scan with a gestational age measurement). The conception date method adds 266 days to the estimated conception date. The ultrasound method uses the gestational age at the scan date to project forward to 280 days from LMP.

What is Naegele's rule and how was it derived?

Naegele's rule was proposed by German obstetrician Franz Karl Naegele in 1806: add 1 year, subtract 3 months, and add 7 days to the LMP. This is equivalent to adding 280 days (40 weeks). It was based on the observation that most pregnancies last approximately 10 lunar months (of 28 days each). Although the rule is over 200 years old, it remains the standard clinical dating method worldwide because it is simple and reliable for women with regular cycles.

What are the key pregnancy milestones by week?

The main milestones by gestational week are: Week 6-8 (heartbeat detectable on ultrasound), Week 12-13 (end of first trimester, miscarriage risk drops significantly), Week 18-20 (anatomy scan to check fetal development), Week 24 (viability threshold, when survival outside the womb becomes possible), Week 27 (end of second trimester), Week 37 (early term, lungs typically mature), Week 39-40 (full term, optimal delivery window), Week 42 (post-term, induction typically recommended).

What does trimester mean in pregnancy?

Pregnancy is divided into three trimesters. The first trimester spans weeks 1 to 12 (3 months), covering early embryonic development. The second trimester spans weeks 13 to 26, during which the fetus grows rapidly and the pregnant person typically feels more energy. The third trimester spans weeks 27 to 40 (and beyond), with the fetus reaching full size. Most complications occur in the first trimester; the second trimester is often called the most comfortable period.

How does cycle length affect the due date calculation?

Cycle length shifts the estimated ovulation date. The LMP-to-ovulation gap equals cycle length minus 14 days (the luteal phase is consistently 14 days). A woman with a 32-day cycle ovulates around day 18, four days later than a 28-day cycle woman. This shifts conception (and thus the due date) by the same 4 days. Naegele's rule assumes 28-day cycles, so this calculator adds or subtracts the cycle-length adjustment automatically.

What is the full-term gestation period?

Full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days (per the American College of Obstetricians and Gynecologists, 2013). Early-term is 37 to 38 weeks; late-term is 41 weeks; post-term is 42 weeks or beyond. Babies born before 37 weeks are considered preterm. The highest fetal readiness across all organ systems occurs in the full-term window, which is why induction or elective cesarean before 39 weeks is generally discouraged for uncomplicated pregnancies.

How do I calculate my due date from a conception date?

Add 266 days (38 weeks) to the estimated conception date. Since conception typically occurs around day 14 of a 28-day cycle, this is equivalent to LMP + 280 days (40 weeks). Example: conception on February 14 gives an EDD of February 14 + 266 days = November 7. Use the Conception Date mode in this calculator for an instant result. Note that pinpointing the exact conception date is difficult; a window of plus or minus 2 to 5 days is typical.

Due Date Calculator

Q: How accurate is the due date calculator?

LMP-based due dates are accurate to within about 1 to 2 weeks for women with regular 28-day cycles. An early first-trimester ultrasound (8 to 12 weeks) is the gold standard, accurate to plus or minus 5 days. Second-trimester ultrasounds (13 to 26 weeks) are accurate to plus or minus 10 to 14 days. LMP dating becomes less reliable with irregular cycles or uncertain period dates.

Q: What is the difference between gestational age and fetal age?

Gestational age counts from the first day of the last menstrual period (LMP) and is used by all doctors and ultrasound reports. Fetal age (embryonic age) counts from conception, which typically occurs 14 days after LMP. At 10 weeks gestational age, the embryo is approximately 8 weeks old by fetal age. Always use gestational age (from LMP) when communicating with your healthcare provider.

Find your estimated due date from your last menstrual period, conception date, or ultrasound gestational age. Shows current week, trimester, and milestone dates.

First Day of Last Menstrual Period

Cycle Length28 days

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Days Remaining

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Milestone	Date	Countdown

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Gestational Age

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Estimated Due Date

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Current Gestational Age

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🤰 What is a Due Date Calculator?

A due date calculator estimates the Estimated Due Date (EDD) of a pregnancy using clinically validated obstetric dating methods. It accepts three types of input: the first day of the last menstrual period (LMP), an estimated conception date, or a gestational age from an ultrasound scan. All three approaches are based on the same underlying principle: human pregnancy lasts approximately 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from conception.

The most widely used method is the LMP approach with Naegele's rule, which adds 280 days to the first day of the last period. This is the method used by obstetricians, midwives, and hospital records in most countries. The calculator also adjusts for non-28-day cycles: if your cycle is 32 days, ovulation occurs 4 days later than in a standard cycle, so the EDD is pushed 4 days later. Women who have had an early ultrasound (before 12 weeks) often get a more accurate dating because the scan directly measures the fetal crown-to-rump length, which correlates precisely with gestational age at that stage.

Beyond the EDD, this calculator shows the current gestational week and trimester, the number of days remaining until the due date, and (in LMP mode) a full milestone timeline covering the end of the first trimester, the anatomy scan window, the viability threshold at 24 weeks, the full-term boundary at 39 weeks, and the EDD itself. These milestones are important reference points for scheduling prenatal care appointments and understanding the developmental stages of pregnancy.

A common misconception is that the due date is a precise delivery target. In reality, only about 5% of births occur on the exact EDD. A delivery within the 37 to 42 week window is considered normal. The EDD is the midpoint of the expected delivery window, not a deadline. Another common confusion is between gestational age (counted from LMP) and fetal age (counted from conception). Healthcare providers universally use gestational age, which is 2 weeks longer than fetal age. At 10 weeks gestational age, the fetus is only 8 weeks old by fetal development timing.

📐 Formula

EDD (from LMP) = LMP + 280 days + (Cycle − 28)

EDD = Estimated Due Date

LMP = first day of Last Menstrual Period

280 days = 40 weeks (standard Naegele's rule for 28-day cycle)

Cycle = your actual menstrual cycle length in days

Cycle adjustment = adds (Cycle − 28) days for non-standard cycles

From Conception: EDD = Conception Date + 266 days (38 weeks)

From Ultrasound: EDD = Scan Date + (280 − GA at scan in days)

Example (LMP): LMP = Feb 1, cycle = 28 days. EDD = Feb 1 + 280 = November 8.

Example (32-day cycle): LMP = Feb 1, cycle = 32. EDD = Feb 1 + 284 = November 12 (4 days later).

Gestational Age at any point: Days since LMP ÷ 7 = weeks + remainder days.

Naegele's rule was described by German obstetrician Franz Karl Naegele in 1806 and is still the primary dating method used globally. It is equivalent to adding 1 year, subtracting 3 months, and adding 7 days to the LMP. The conception-based method (266 days) reflects that ovulation typically occurs 14 days before the next expected period, so the LMP-to-conception gap is approximately 14 days: 280 = 14 + 266. The ultrasound method extrapolates the EDD by calculating how many days remain until 280 days from LMP, using the gestational age at the scan as the known anchor point.

📖 How to Use This Calculator

Steps

Choose your dating method - Select By Last Period (most common), By Conception Date (if you know when conception likely occurred), or By Ultrasound (if you have a scan report listing gestational age).

Enter the date - Type or pick the relevant date in the date field. For LMP mode, also adjust the Cycle Length slider if your period is not every 28 days.

Click Calculate - The results instantly show your EDD, current gestational week, days remaining, and trimester. LMP mode also populates the milestone table below the main results.

Review milestones (LMP mode) - The milestone table shows key dates (end of 1st trimester, anatomy scan, viability, full term, EDD) with countdown days, so you can plan prenatal appointments in advance.

💡 Example Calculations

Example 1 - Standard 28-Day Cycle

LMP February 1, 2026, cycle 28 days

EDD = LMP + 280 days = February 1 + 280 days = November 8, 2026.

As of May 17, 2026: days since LMP = 105 days = 15 weeks + 0 days (2nd trimester).

Days remaining = November 8 minus May 17 = 175 days. Key milestone: end of 1st trimester was May 3, 2026 (13 weeks from LMP).

EDD = November 8, 2026 | Currently at Week 15

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Example 2 - Longer Cycle (32 Days)

LMP March 10, 2026, cycle 32 days

Cycle adjustment = 32 - 28 = +4 days. EDD = March 10 + 284 days = January 17, 2027.

As of May 17: days since LMP = 68 days = 9 weeks + 5 days (1st trimester). Standard 28-day rule would give January 13, 2027 (4 days earlier), which would be incorrect for this person.

Days remaining = January 17 minus May 17 = about 245 days.

EDD = January 17, 2027 | Currently at Week 9

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Example 3 - By Conception Date

Estimated conception February 14, 2026 (Valentine's Day)

EDD = Conception + 266 days = February 14 + 266 days = November 7, 2026.

Gestational age as of May 17: days since conception = 92 days. Gestational age = 92 + 14 = 106 days = 15 weeks + 1 day.

Days remaining = November 7 minus May 17 = 174 days. Trimester: 2nd (weeks 13 to 26).

EDD = November 7, 2026 | Gestational Age = Week 15 + 1 day

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Example 4 - By Ultrasound Scan

Scan on May 3, 2026, showing 12 weeks and 3 days gestational age

GA at scan = 12 weeks + 3 days = 87 days. EDD = Scan Date + (280 - 87) = May 3 + 193 days.

May 3 + 193 days = November 12, 2026.

As of May 17 (14 days after scan): Current GA = 87 + 14 = 101 days = 14 weeks + 3 days. Still 1st trimester (up to 12 weeks) just crossed into 2nd.

EDD = November 12, 2026 | Currently at Week 14 + 3 days

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❓ Frequently Asked Questions

How is the pregnancy due date calculated from LMP?+

The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. For non-28-day cycles, adjust by the difference from 28: a 32-day cycle adds 4 days, a 25-day cycle subtracts 3 days. The 280-day figure assumes ovulation on day 14, so conception occurs around day 14 and pregnancy lasts 266 days from conception (38 weeks).

How accurate is a due date calculator?+

LMP-based due dates are accurate to within 1 to 2 weeks for women with regular 28-day cycles. An early first-trimester ultrasound (8 to 12 weeks) is the gold standard, accurate to plus or minus 5 days. Second-trimester scans (13 to 26 weeks) are accurate to plus or minus 10 to 14 days. Only about 5% of babies are born on their exact EDD; normal delivery ranges from 37 to 42 weeks.

What is Naegele's rule for calculating due date?+

Naegele's rule: add 1 year, subtract 3 months, and add 7 days to the LMP. This is equivalent to adding 280 days (40 weeks). It was proposed by Franz Karl Naegele in 1806 and is still the universal obstetric standard. For example, LMP on January 15 gives an EDD of October 22 (adding 280 days). The rule assumes a 28-day cycle; this calculator adjusts for other cycle lengths automatically.

What is the difference between gestational age and fetal age?+

Gestational age counts from the first day of the LMP and is the universal clinical standard used by all doctors and ultrasound reports. Fetal age (embryonic age) counts from conception, typically 14 days after LMP. At 10 weeks gestational age, the embryo is approximately 8 weeks old by fetal development timing. Always use gestational age when speaking with your healthcare provider to avoid confusion.

Can I calculate a due date if I do not know my LMP?+

Yes. If you know your conception date, use Mode 2 (By Conception Date): EDD = conception date + 266 days. If you have had an ultrasound, use Mode 3 (By Ultrasound): enter the scan date and the gestational age shown on the report (in weeks and days). The calculator works backwards from the scan gestational age to determine the EDD. An early scan is generally more reliable than a conception estimate if periods are irregular.

How does cycle length affect the due date?+

Cycle length determines when ovulation occurs. Ovulation happens at cycle length minus 14 days (the luteal phase is consistently 14 days). A 32-day cycle means ovulation on day 18, four days later than in a 28-day cycle. This shifts the conception date (and EDD) by 4 days later. This calculator adds (your cycle length minus 28) to the standard 280-day EDD, so a 32-day cycle gives EDD at 284 days from LMP.

What are the trimesters and when do they start and end?+

The first trimester is weeks 1 to 12, covering early embryonic development and the highest miscarriage risk. The second trimester is weeks 13 to 26, typically the most comfortable period. The third trimester is weeks 27 to 40 (and beyond), during which the fetus reaches full size. Some sources place the second trimester end at week 27 or 28; definitions vary slightly, but weeks 13 to 26 is the most common clinical standard.

What are key pregnancy milestones by week?+

Key milestones: Week 6 to 8 (fetal heartbeat visible on ultrasound), Week 10 to 13 (first-trimester screening, NIPT), Week 12 to 13 (end of first trimester, reduced miscarriage risk), Week 18 to 20 (anatomy scan), Week 24 (viability threshold), Week 27 (end of second trimester), Week 37 (early term), Week 39 to 40 (full term, optimal delivery window), Week 42 (post-term, induction typically recommended).

Why do doctors change the due date after an ultrasound?+

If the baby's measurements at an early scan differ from LMP-based dating by more than 5 to 7 days (first trimester) or 10 to 14 days (second trimester), the provider may revise the EDD based on the scan. This is most common in women with irregular cycles, uncertain LMP dates, or where ovulation occurred at a different time than expected. The revised EDD from an early scan is generally considered more accurate than the LMP-based date.

Is it normal to deliver before or after the due date?+

Yes. About 50% of babies are born after the EDD, and only about 5% arrive on the exact date. Delivery is considered normal anywhere from 37 weeks (early term) to 42 weeks (post-term). First-time mothers tend to deliver later than the EDD on average. Most providers recommend induction at 41 weeks (or by 42 weeks at the latest) if labor has not started, to reduce the small but increasing risk of complications in post-term pregnancies.

What is the viability threshold in pregnancy?+

Viability refers to the gestational age at which a premature baby has a realistic chance of survival outside the womb with medical intervention. This is generally considered to be 22 to 24 weeks of gestational age. At 24 weeks, survival rates with intensive care are approximately 50 to 70%. By 28 weeks, survival rates exceed 90%. Full organ system maturity, including lung surfactant production, occurs around 36 to 39 weeks, which is why full-term delivery (39 to 40 weeks) is recommended for uncomplicated pregnancies.

🔗 Related Calculators

📌 Quick Tips

💡Naegele's rule (LMP + 280 days) assumes a 28-day cycle. If your cycle is longer or shorter, the calculator automatically adjusts by adding or subtracting the difference from 28 days. A 32-day cycle pushes the due date 4 days later than the standard calculation.

💡Only about 5% of babies are born on their exact estimated due date. A normal delivery can occur anytime from 37 to 42 weeks of gestation. The EDD is a midpoint estimate, not a precise delivery target.

💡An early ultrasound at 8 to 12 weeks is the most accurate method for dating a pregnancy, with a margin of error of plus or minus 5 days. LMP-based dating assumes regular cycles; if yours are irregular, the scan date is more reliable.

💡Gestational age (counted from LMP) is used universally by healthcare providers. Fetal age (counted from conception) is approximately 2 weeks less. Always use gestational age when communicating with your doctor or midwife.

💡The anatomy scan, typically performed between weeks 18 and 20, can sometimes slightly revise the due date if fetal measurements differ significantly from LMP-based dating. If the discrepancy is more than 10 to 14 days, the scan estimate may be preferred.