Quick Answer (Featured Snippet)

How is a pregnancy due date calculated?
A typical pregnancy lasts approximately 40 weeks — counted from the first day of your last menstrual period (LMP), not from conception.
The standard formula is:
Estimated Due Date (EDD) = First day of LMP + 280 days
If you know your conception date instead, the formula is:
EDD = Conception date + 266 days
Most pregnancy due date calculators use one of these two methods to produce your estimated due date. The result is an estimate — only about 5% of babies are born exactly on their due date.

What Is a Pregnancy Due Date?

A pregnancy due date — also called the estimated due date or EDD — is the date around which your baby is expected to be born. It's calculated as 40 weeks from the first day of your last menstrual period, which is the universal medical standard used by obstetricians and midwives in the USA, UK, Canada, Australia, and most of the world.
The EDD is an estimate, not a deadline. It marks the midpoint of a normal delivery window, which runs from 37 weeks to 42 weeks of gestation. A baby born anywhere in this window is considered full term.
Several things are important to understand about your estimated due date:

• It is based on a 28-day average menstrual cycle. Women with shorter or longer cycles may have a different actual ovulation date, which affects when conception occurred.
• It assumes ovulation happened on day 14 of the cycle — which is average, not universal.
• Ultrasound measurements, particularly those done in the first trimester, can refine your due date more precisely than LMP calculation alone.
• The due date is a planning target. Your care team uses it to schedule scans, monitor development milestones, and determine when any intervention might be needed.

Think of your EDD as the most likely birth window, centered on a specific date — not a fixed appointment your baby has agreed to keep.

How a Pregnancy Due Date Calculator Works

A pregnancy due date calculator is a tool that takes one or two key inputs and applies a standardized obstetric formula to produce your EDD.
Most calculators ask for one of the following:
Input Method 1 — Last Menstrual Period (LMP)
You enter the first day of your most recent period. The calculator adds 280 days (40 weeks) to that date to produce your EDD.
Input Method 2 — Conception Date
If you know or strongly suspect when you conceived — because you tracked ovulation, used an ovulation predictor kit, or had a single date of intercourse — you enter that date. The calculator adds 266 days (38 weeks from conception, which equals 40 weeks from LMP).
Input Method 3 — IVF Transfer Date
For pregnancies achieved through IVF, calculators use the embryo transfer date. For a Day 5 blastocyst transfer, the calculator adds 261 days. For a Day 3 transfer, it adds 263 days.
After processing your input, a good pregnancy due date calculator will also tell you:

• Your current gestational age in weeks and days
• Which trimester you're in
• Key milestone dates (end of first trimester, anomaly scan window, third trimester start)
• Your conception date (if you entered LMP) or LMP (if you entered conception date)

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How to Calculate Due Date by LMP

The LMP method is the standard approach used in clinical practice worldwide. It is taught in medical schools, used by the WHO, and built into every obstetric software system.
Why count from LMP rather than conception?
Because most women don't know their exact conception date — but they do know when their last period started. The LMP date is a reliable, consistently recorded starting point. Medical convention therefore counts all 40 weeks of pregnancy from day one of the LMP, even though fertilization doesn't actually occur until roughly day 14.
The LMP due date formula:
EDD = First day of LMP + 280 days
Step-by-step example:

• LMP: January 10, 2025
• Add 280 days
• Estimated due date: October 17, 2025

Naegele's Rule is the original version of this calculation. It works like this:

1. Take the first day of your LMP
2. Add one year
3. Subtract three months
4. Add seven days

Using the same January 10 example:

• January 10, 2025 + 1 year = January 10, 2026
• Subtract 3 months = October 10, 2025
• Add 7 days = October 17, 2025 ✓

Both methods produce the same result. The formula-based calculator is faster, but Naegele's Rule helps you understand why the number is what it is.
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How to Calculate Due Date by Conception

If you know your conception date with reasonable confidence, you can use a conception-based due date calculator for a potentially more accurate estimate — particularly if your cycles are irregular or significantly shorter or longer than 28 days.
The conception date formula:
EDD = Conception date + 266 days (38 weeks)
Conception is the moment of fertilization, which typically occurs 12–24 hours after ovulation. Sperm can survive in the fallopian tubes for up to 5 days, meaning conception can occur up to 5 days after intercourse. This is why "conception date" is sometimes a range rather than a precise moment.
Step-by-step example:

• Confirmed ovulation / conception: January 24, 2025
• Add 266 days
• Estimated due date: October 17, 2025

Notice that the LMP example (LMP of January 10) and the conception example (conception on January 24, which is 14 days later) produce the same due date — which is exactly what you'd expect when ovulation falls on day 14 of a regular 28-day cycle.
Where the two methods diverge is when ovulation doesn't fall on day 14. If a woman with a 35-day cycle ovulates on day 21, conception-based calculation will give a later EDD than LMP-based calculation, and in that case, the conception-based date is more accurate.
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How to Calculate My Due Date Manually (Step by Step)

If you don't have access to a calculator, here is how to calculate your due date by hand using the LMP method.
Step 1: Write down the first day of your last menstrual period. Be specific — not the day you think it started, but the actual first day of bleeding.
Step 2: Add 7 days to that date.
Step 3: Subtract 3 months from the result.
Step 4: Add 1 year.
Example:

• LMP: March 15, 2025
• Add 7 days: March 22, 2025
• Subtract 3 months: December 22, 2025
• Add 1 year: December 22, 2026

Wait — that's not right. Let's correct: when your LMP is March 15, you don't add a year unless the result drops into a previous year. Let's redo:

• LMP: March 15, 2025
• Add 7 days → March 22
• Subtract 3 months → December 22, 2024
• Add 1 year → December 22, 2025 ✓

This is the manual version of Naegele's Rule. It works cleanly for most dates. A due date calculator simply automates these steps and accounts for month-length variations automatically.

Pregnancy Week Calculator: Tracking Your Progress Week by Week

A pregnancy week calculator tells you how many weeks and days pregnant you are right now, based on your LMP or EDD.
How it works:
Take today's date and subtract your LMP date. The result in days, divided by 7, gives your gestational age in weeks.
Example:

• LMP: January 10, 2025
• Today: April 10, 2025
• Days elapsed: 90 days
• Gestational age: 90 ÷ 7 = 12 weeks and 6 days

This places you at the end of your first trimester.
Trimester breakdown:

• First trimester: Weeks 1–12 — Organ development, highest miscarriage risk, nausea common
• Second trimester: Weeks 13–27 — Energy often returns, baby movements begin, anatomy scan occurs
• Third trimester: Weeks 28–40+ — Rapid growth, birth preparation, increased monitoring

Tracking your week-by-week progress helps you know which scans to expect, when to schedule midwife or OB appointments, and which developmental milestones your baby is reaching.
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Calculating Your Current Pregnancy Week from Your Due Date

Sometimes you know your due date — perhaps from an early scan — but want to work backward to understand how many weeks pregnant you are today.
Reverse calculation formula:
Current gestational age = 40 weeks − weeks remaining until EDD
Example:

• Due date: October 17, 2025
• Today: April 10, 2025
• Weeks remaining: approximately 27 weeks
• Current gestational age: 40 − 27 = approximately 13 weeks

This tells you that you are at the very start of your second trimester.
You can also run this in days:

• Count the days between today and your EDD
• Subtract from 280
• Divide by 7 to get current weeks

Both approaches give you the same answer and are useful when you want to understand where you are in your pregnancy without having your LMP date to hand.

How Accurate Is a Pregnancy Due Date?

This is one of the most important things to understand about any due date — whether it comes from a calculator, your doctor, or a scan.
Only about 4–5% of babies are born on their actual due date.
Research consistently shows that most births occur within a window of two weeks before to two weeks after the EDD. Specifically:

• Approximately 70% of births occur within 10 days of the EDD
• About 90% occur within 2 weeks either side
• First-time mothers tend to deliver slightly later than the EDD on average
• Mothers who have given birth before tend to deliver slightly earlier

Why isn't the due date more precise?

• The LMP method assumes a 28-day cycle and ovulation on day 14 — which doesn't apply to everyone
• Implantation timing varies between pregnancies
• Genetic factors influence gestation length
• The natural variation in human pregnancy length is genuinely wide

How ultrasound improves accuracy:
A dating scan performed between 8 and 14 weeks — ideally around 10–13 weeks — measures the baby's crown-rump length (CRL) and can establish gestational age to within 5–7 days. If the scan date differs from the LMP-calculated date by more than 5–7 days, most clinicians will update the EDD to reflect the scan measurement.
Third-trimester scans are less accurate for dating because individual size variation between babies becomes large. First-trimester ultrasound remains the gold standard for accurate due date determination.

Real-Life Due Date Examples

Example 1: Regular Cycle — LMP Method

• Woman with a regular 28-day cycle
• LMP: February 1, 2025
• EDD calculation: February 1 + 280 days = November 8, 2025
• She is currently 10 weeks pregnant on April 10, 2025

Example 2: Irregular Cycle — Conception Method

• Woman with a 35-day cycle who tracked ovulation and confirmed it on day 21
• Ovulation/conception date: February 22, 2025
• EDD calculation: February 22 + 266 days = November 15, 2025
• If she had used LMP (February 1) without adjusting for her long cycle, her LMP-based EDD would be November 8 — 7 days earlier and less accurate for her situation

Example 3: IVF Pregnancy — Transfer Date Method

• Woman who underwent IVF with a Day 5 blastocyst transfer
• Transfer date: February 10, 2025
• EDD calculation: February 10 + 261 days = October 29, 2025
• IVF pregnancies use transfer date rather than LMP because conception timing is precisely known

Factors That Can Affect Your Due Date

Irregular menstrual cycles
The LMP method assumes 28-day cycles. Women with cycles consistently shorter than 28 days (say, 21 days) will ovulate earlier and may deliver earlier than the LMP-based EDD suggests. Women with longer cycles (35+ days) ovulate later and may deliver later.
Uncertain LMP
Some women have spotting or light bleeding that they mistake for a period. Using an incorrect LMP date shifts the entire EDD calculation. If you're unsure, an early ultrasound gives a more reliable foundation.
Ovulation timing variation
Even in women with regular 28-day cycles, ovulation doesn't always fall precisely on day 14. Stress, illness, and lifestyle changes can shift it by several days, which shifts conception and therefore the EDD.
Multiple pregnancies
Twins and higher-order multiples typically arrive earlier than singletons. The standard 40-week EDD is less applicable — most twin pregnancies result in delivery around 36–37 weeks.
First trimester ultrasound
The most reliable due date adjustment comes from a first trimester dating scan. If the scan-based date differs from your LMP-based date, your healthcare provider will typically adjust your EDD accordingly.
Ethnicity and genetic factors
Research suggests modest differences in average gestation length across different ethnic groups. These differences are small but real, and ongoing research continues to refine our understanding of what's normal.

Common Due Date Calculation Mistakes

Using the wrong LMP date
The most frequent error. The LMP is the first day of bleeding — not the last day, not the middle, and not when you suspect your period might have started. Even being one week off on your LMP shifts your EDD by a full week.
Confusing conception date with intercourse date
Conception (fertilization) doesn't happen at the moment of intercourse. It occurs up to 5 days later when sperm meets egg. Using the intercourse date as the conception date can misplace your EDD by several days.
Assuming the due date is the birth date
Especially for first-time parents, the due date can feel like a firm deadline. Mentally planning around a two-week window — 38 to 42 weeks — is more realistic and reduces anxiety if the exact date passes without delivery.
Not updating the EDD after an ultrasound
If your dating scan produces a different date from your LMP calculation, use the scan date. Scan-based dating in the first trimester is more accurate than calendar calculation in most cases.
Using a due date calculator without inputting accurate cycle length
Some advanced pregnancy due date calculators ask for your average cycle length. Leaving this at the default 28 days when your cycles are actually 35 days will produce a less accurate result.

Medical Note

Everything in this guide is educational. A pregnancy due date calculator — whether online, in an app, or calculated manually — provides an estimate based on the information you input. It is not a medical diagnosis or clinical assessment.
Your healthcare provider — obstetrician, midwife, or general practitioner — will establish your official EDD using a combination of your menstrual history, a clinical examination, and ultrasound findings. The due date may be revised at any point during pregnancy if new information suggests a different gestational age.

Always attend your scheduled prenatal appointments. Report any concerns — unusual symptoms, reduced fetal movement, spotting or bleeding — to your care team promptly rather than relying on a calculator or website for guidance.

Conclusion

Your estimated due date is one of the most meaningful numbers of your pregnancy — a fixed point around which to plan, prepare, and count down. A pregnancy due date calculator gives you that number quickly and reliably, using either your last menstrual period or your conception date as the starting point.
The key things to remember: pregnancy lasts approximately 40 weeks from LMP, your due date is an estimate rather than a guarantee, and a first trimester ultrasound will give your care team the most accurate dating information available.
Use this guide to understand the calculation — and use your prenatal appointments to confirm the details with the professionals caring for you and your baby.
This content is for informational purposes only and does not constitute medical advice. Always consult your doctor, midwife, or qualified healthcare provider for guidance specific to your pregnancy.