Due Date Calculator (Pregnancy Estimator)
Estimate your baby's due date with our free Due Date Calculator. Simply enter the first day of your last menstrual period and your average cycle length to get your estimated due date, current pregnancy week, trimester breakdown, and weekly milestones. This tool uses the standard Naegele's rule for accurate pregnancy dating.
Due Date Calculator
Your Due Date Results
Pregnancy Timeline
| Week | Dates | Trimester | Milestone |
|---|
How to Use the Tool
1. Enter LMP
First day of your last period.
2. Set cycle length
Your average cycle length.
3. Get insights
Due date, week, trimester & timeline.
How It's Calculated
Key Formulas
- Ovulation Day: Cycle Length − 14
- Conception: LMP + Ovulation Day
- Due Date: Conception + 266 days
- Alternative: LMP + 280 days (Naegele's Rule)
Trimesters
- First Trimester: Weeks 1–13
- Second Trimester: Weeks 14–27
- Third Trimester: Weeks 28–40+
About Due Dates
What is a Due Date?
A due date is an estimate of when your baby is likely to be born. It is calculated based on the first day of your last menstrual period (LMP) and your average cycle length. The average pregnancy lasts about 280 days (40 weeks) from the first day of your LMP. Only about 5% of babies are born on their exact due date — most arrive between 37 and 42 weeks.
Key Facts:
- Naegele's Rule: LMP + 280 days = Due Date
- Full Term: 39 weeks to 40 weeks + 6 days
- Early Term: 37 weeks to 38 weeks + 6 days
- Post-Term: 42 weeks and beyond
Limitations of This Tool
- Assumes a regular 28-day cycle — irregular cycles may affect accuracy.
- Does not account for conception date variations or ovulation timing.
- Not a substitute for professional medical advice or ultrasound dating.
- Due dates are estimates — only about 5% of babies arrive on their due date.
Trust & Privacy
Your data stays in your browser. All calculations happen locally. We never send your data to any server.
No registration or login required. Your privacy is 100% protected.
Uses Naegele's rule & standard methods. Our tool follows established pregnancy dating principles.
No cookies used for tracking. Your history is stored locally in your browser's localStorage.
Real-World Examples
LMP: Jan 1, Cycle: 28 days
Due Date: Oct 7
LMP: Mar 15, Cycle: 30 days
Due Date: Dec 19
LMP: Jun 1, Cycle: 26 days
Due Date: Mar 7
Frequently Asked Questions
References
- American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. acog.org
- World Health Organization. Pregnancy, childbirth, postpartum and newborn care. who.int
- National Institutes of Health. Pregnancy and Childbirth. nichd.nih.gov
Related Tools
Your History
Editorial & Medical Review
Due Date Calculator: Calculate Your Estimated Baby Due Date
A Due Date Calculator helps estimate when your baby may be due based on the first day of your last menstrual period (LMP) and your menstrual-cycle information. The result is an estimated due date (EDD), not an exact prediction of the day your baby will be born. Pregnancy normally lasts around 37 to 42 weeks from the first day of the last menstrual period.
The Healthtus calculator is designed to give you a quick estimated delivery date while also helping you understand pregnancy weeks, trimesters, and the general pregnancy timeline.
Quick answer: A traditional due-date calculation estimates about 280 days, or 40 weeks, from the first day of the last menstrual period. Your healthcare professional may later adjust the estimated due date based on your menstrual history, cycle length, and ultrasound findings.
How to Use the Due Date Calculator
The Healthtus Due Date Calculator is designed to make the calculation simple.
1. Enter Your LMP
Enter the first day of your last menstrual period (LMP).
This is the first day of your most recent menstrual bleeding and is used as the starting point for calculating gestational age.
For example:
LMP: August 1, 2026
2. Set Your Cycle Length
Enter your usual average menstrual cycle length.
A standard 28-day cycle is commonly used in traditional due-date calculations, but cycle length can vary between people. NHS guidance also allows cycle length to be considered when estimating a due date. (nhs.uk)
For example:
Average cycle length: 28 days
3. Get Your Pregnancy Insights
After entering your information, select Calculate Due Date.
The tool can provide information such as:
- Estimated due date
- Current pregnancy timing
- Pregnancy weeks
- Trimester information
- Pregnancy timeline
The result is an estimate based on the information entered.
It does not confirm pregnancy, determine fetal health, or guarantee the exact date of delivery.
How Our Due Date Calculator Is Calculated
The Healthtus Due Date Calculator estimates your expected delivery date using the first day of your last menstrual period and your cycle information.
Key Formulas
The traditional calculation is:
Estimated Due Date = LMP + 280 days
For a standard 28-day menstrual cycle, this corresponds to approximately:
40 weeks
For example:
Last menstrual period: January 10, 2026
Estimated due date: October 17, 2026
A traditional alternative is Naegele’s Rule, which adds seven days to the first day of the LMP, subtracts three months, and adds one year.
Cycle-Length Adjustment
Traditional 280-day calculations assume a 28-day menstrual cycle.
When a person’s usual cycle is longer or shorter, an adjusted estimate may be used:
Adjusted Due Date = LMP + 280 days + (Cycle Length − 28 days)
For example, someone with a 32-day average cycle may have an estimated due date approximately four days later than the standard 28-day calculation.
Only use this formula if it matches the calculation actually implemented in the Healthtus tool.
Pregnancy Trimesters
The tool can also organize pregnancy into the three broad trimesters:
First Trimester: Approximately weeks 1–12
Second Trimester: Approximately weeks 13–26
Third Trimester: Approximately week 27 until birth
These ranges are useful for understanding the general pregnancy timeline, but healthcare providers may describe pregnancy timing slightly differently depending on the clinical context.
What Is a Due Date?
A due date, also called an estimated due date (EDD), is the approximate date when a pregnancy is expected to reach 40 weeks of gestation.
It is important to understand that the due date is an estimate.
Pregnancy does not normally end on exactly the calculated date for everyone. NHS guidance states that pregnancy normally lasts around 37 to 42 weeks from the first day of the last period.
So a due date should be thought of as a useful reference point rather than a guaranteed delivery date.
What Is the Last Menstrual Period (LMP)?
LMP stands for Last Menstrual Period.
For pregnancy dating, the first day of the last menstrual period is generally treated as the starting point of gestational age.
This can feel confusing because conception usually occurs approximately two weeks after the beginning of the menstrual cycle in a typical cycle.
That means pregnancy is traditionally dated from the LMP even though conception generally happens later.
For example:
LMP → Pregnancy dating begins
then approximately:
Ovulation → Possible conception
then:
Pregnancy continues → Estimated delivery date
The LMP is therefore primarily a dating reference point.
How Many Weeks Is Pregnancy?
Pregnancy is commonly described as approximately 40 weeks from the first day of the last menstrual period.
This is called gestational age.
Because gestational age starts before conception, the first approximately two weeks are generally counted as part of pregnancy dating even though conception has not yet occurred.
A pregnancy may therefore be described as:
6 weeks pregnant
even though conception likely occurred around four weeks earlier in a typical cycle.
This dating system is widely used in prenatal care.
How to Calculate Your Pregnancy Week
Pregnancy week is generally calculated from the first day of your last menstrual period.
For example, if your LMP was:
July 1
and the current date is:
August 12
the estimated gestational age would be approximately:
6 weeks
The Healthtus calculator can use the LMP and current date to provide an estimated pregnancy timeline.
This is a calendar estimate.
An ultrasound can sometimes provide a more accurate assessment of gestational age, especially when the LMP is uncertain or menstrual cycles are irregular.
How Accurate Is a Due Date Calculator?
A due date calculator gives an estimated date, not a guaranteed birth date.
Several factors can affect accuracy:
- Uncertainty about the LMP
- Irregular menstrual cycles
- Variation in ovulation timing
- Different cycle lengths
- Conception occurring earlier or later than expected
- Early ultrasound findings
NHS guidance notes that a dating scan can provide a more accurate estimate of how many weeks pregnant someone is and can result in a revised due date.
Mayo Clinic also explains that the traditional calculation provides an initial estimate and that healthcare providers may use ultrasound and other information to establish or adjust the estimated due date.
Can an Ultrasound Change Your Due Date?
Yes.
An early pregnancy ultrasound, particularly a dating scan, may provide a more accurate estimate of gestational age than a calculation based solely on the LMP.
This can be especially useful when:
- You don’t remember your LMP
- Your periods are irregular
- Your cycle length varies considerably
- You are unsure when ovulation occurred
NHS guidance states that the dating scan can estimate how many weeks pregnant you are and may provide an adjusted estimated due date.
Your healthcare professional will decide how the due date should be established based on the available clinical information.
Due Date Calculator for Irregular Cycles
A due date calculator can still provide an estimate if your periods are irregular, but the result may be less reliable.
If you do not ovulate around the expected time, a calculation based only on cycle length may not accurately represent the timing of conception.
This is why an early ultrasound can be particularly useful when menstrual dates are uncertain.
If your cycles are consistently irregular, discuss your pregnancy dating with your obstetrician, midwife, or other qualified healthcare professional.
How Does Cycle Length Affect Your Due Date?
The traditional calculation assumes a 28-day menstrual cycle.
But not everyone has a 28-day cycle.
If your cycle is usually longer than 28 days, ovulation may occur later than the traditional calculation assumes.
If your cycle is shorter, ovulation may occur earlier.
For this reason, cycle length can be considered when estimating the expected delivery date.
For example:
28-day cycle: Standard reference
32-day cycle: Approximately 4 days longer
24-day cycle: Approximately 4 days shorter
The adjustment is an estimate and should not replace clinical pregnancy dating.
Understanding the Three Pregnancy Trimesters
Pregnancy is commonly divided into three trimesters.
First Trimester
The first trimester covers the early part of pregnancy, when major early developmental changes occur.
This is also when pregnancy is often first confirmed and prenatal care is established.
Second Trimester
The second trimester follows the first and is often associated with continued fetal growth and development.
Many routine prenatal assessments occur during this period.
Third Trimester
The third trimester is the final part of pregnancy leading toward delivery.
Prenatal appointments become increasingly focused on maternal and fetal wellbeing and preparation for birth.
The exact timing of pregnancy milestones varies from pregnancy to pregnancy.
Pregnancy Timeline
A due date calculator is useful because it provides a simple reference point for understanding where you are in pregnancy.
A general timeline looks like this:
| Pregnancy Period | Approximate Timing |
|---|---|
| First trimester | Weeks 1–12 |
| Second trimester | Weeks 13–26 |
| Third trimester | Week 27 until birth |
| Estimated due date | Around 40 weeks from LMP |
These are broad pregnancy timeframes, not a prediction of exactly when individual events will occur.
Your healthcare provider may use gestational age, ultrasound findings, and other clinical information to track pregnancy progress.
What If You Don’t Know Your Last Period?
If you don’t know the first day of your last menstrual period, your calculator estimate may be less reliable.
Possible reasons include:
- Irregular periods
- Light or unusual bleeding
- Recently stopping hormonal contraception
- Uncertainty about the date
- Not tracking previous periods
Don’t guess if you are unsure.
A pregnancy test can help determine whether pregnancy is present, while an ultrasound or clinical assessment may help establish gestational age.
Can a Pregnancy Test Tell You Your Due Date?
No.
A pregnancy test can help determine whether pregnancy hormones are present, but it does not directly determine gestational age or the baby’s estimated delivery date.
Once pregnancy is confirmed, the due date can be estimated using your LMP, cycle information, and, when appropriate, ultrasound findings.
Is Your Due Date the Exact Day Your Baby Will Be Born?
No.
The estimated due date is a reference point rather than an exact prediction.
NHS guidance states that pregnancy normally lasts around 37 to 42 weeks from the first day of the last menstrual period, meaning delivery can occur before or after the estimated due date.
A due date helps healthcare professionals organize prenatal care and monitor pregnancy progress.
It does not tell you exactly when labor will begin.
When Should You Contact a Healthcare Professional?
You should arrange appropriate prenatal care if you believe you are pregnant or receive a positive pregnancy test.
You should also discuss your situation with a healthcare professional if:
- You are unsure about your LMP
- Your menstrual cycles are very irregular
- You are unsure about your gestational age
- You have vaginal bleeding during pregnancy
- You experience severe abdominal or pelvic pain
- You have concerns about your pregnancy
- You have been given conflicting due dates
A calculator cannot evaluate your pregnancy’s health.
Seek urgent medical care for serious symptoms such as severe abdominal pain, heavy bleeding, fainting, or other concerning pregnancy-related symptoms.
Frequently Asked Questions
What is a due date calculator?
A due date calculator estimates your baby’s expected delivery date using information such as the first day of your last menstrual period and menstrual-cycle length.
How is my due date calculated?
A traditional calculation estimates 40 weeks, or 280 days, from the first day of the LMP.
What is the first day of my last period?
It is the first day of your most recent menstrual bleeding. This date is commonly used as the starting point for calculating gestational age.
How many weeks is pregnancy?
Pregnancy is traditionally dated as approximately 40 weeks from the first day of the last menstrual period.
How accurate is a due date calculator?
It provides an estimate. Accuracy depends partly on how accurately the LMP is known, cycle regularity, ovulation timing, and whether ultrasound dating is available.
Can an ultrasound change my due date?
Yes. An early pregnancy ultrasound can sometimes provide a more accurate estimate of gestational age and may lead your healthcare professional to revise the estimated due date.
What if I don’t remember my last period?
A calendar-based due-date calculation may be less reliable. Your healthcare professional may use ultrasound and other clinical information to estimate gestational age.
Can I calculate my due date from my conception date?
Yes, an estimate can be made from a known conception date, but conception is often difficult to determine precisely. Clinical pregnancy dating commonly uses the LMP and/or ultrasound.
How does cycle length affect my due date?
Traditional due-date calculations assume a 28-day cycle. A significantly shorter or longer cycle may shift the estimated due date because ovulation may occur earlier or later.
What is gestational age?
Gestational age is the age of the pregnancy measured from the first day of the last menstrual period rather than from conception.
Is my due date the exact day my baby will be born?
No. It is an estimated delivery date. Babies may be born before or after the estimated due date.
Can I use a due date calculator if my periods are irregular?
Yes, but the result may be less accurate. An early ultrasound may provide a better estimate of gestational age when menstrual dates are uncertain.
How many weeks pregnant am I?
Your estimated pregnancy week can be calculated from the first day of your LMP and the current date. A healthcare professional can confirm gestational age using your medical history and, when appropriate, ultrasound.
Related Tools
- Period Calculator
- Ovulation Calculator
- Fertile Window Calculator
- Pregnancy Calculator
- Pregnancy Weight Gain Calculator
- Daily Water Intake Calculator
- BMI Calculator
- Blood Pressure Calculator
Key Takeaways
- A Due Date Calculator estimates your baby’s expected delivery date.
- Traditional pregnancy dating uses approximately 280 days or 40 weeks from the first day of the LMP.
- The estimated due date is not the exact date your baby will be born.
- Cycle length can affect the estimated date when it differs substantially from the traditional 28-day assumption.
- Pregnancy is dated from the LMP even though conception usually occurs later.
- An early ultrasound can sometimes provide a more accurate estimate of gestational age.
- Irregular periods or an uncertain LMP can make calendar-based estimates less reliable.
- A due date calculator does not replace prenatal care or clinical pregnancy dating.
References
- NHS. Due date calculator.
NHS: Pregnancy Due Date (nhs.uk) - Mayo Clinic. Due date calculator.
Mayo Clinic: Due Date Calculator (mayoclinic.org) - American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date.
ACOG: Methods for Estimating the Due Date - ACOG. Committee Opinion: Methods for Estimating the Due Date.
ACOG: Pregnancy Dating Guidance - NHS. 12-week scan.
NHS: 12-Week Dating Scan (nhs.uk) - Mayo Clinic. Pregnancy week by week.
Mayo Clinic: Pregnancy Information
Medical Disclaimer
This Due Date Calculator provides an estimated delivery date for educational and informational purposes. It does not diagnose pregnancy, determine fetal health, or replace prenatal care.
Your estimated due date may be established or adjusted by a healthcare professional using your menstrual history, cycle length, ultrasound measurements, and other clinical information. Do not make medical decisions based solely on this calculator.
If you are pregnant, arrange appropriate prenatal care with a qualified healthcare professional.
Author Box
Written by: Suvojit Datta, M.Pharm
Last reviewed: August 2026