Due Date Calculator

How Does the Due Date Calculator Work?

Our due date calculator uses Naegele's Rule, the standard method used by healthcare providers worldwide. It estimates your due date (EDD, or estimated date of delivery) by adding 280 days — 40 weeks — to the first day of your last menstrual period (LMP). Because Naegele's Rule assumes a textbook 28-day cycle, our calculator also lets you enter your average cycle length and shifts the result by(cycle length − 28) days so it fits your body more closely.

How Is My Due Date Calculated From My Last Period?

The calculation is a two-step estimate:

Estimated due date: first day of your last period + 280 days, then adjusted for your cycle length.
Gestational age: how far along you are today, measured back from your due date and expressed as completed weeks + days (for example, “12 weeks 3 days”) — the same convention your doctor and ultrasound reports use.

Worked Example

Suppose your last period started on March 21 and you have a typical 28-day cycle:

Due date = March 21 + 280 days ≈ December 25
Estimated conception ≈ March 21 + 14 days = April 4
If today is June 13, you are 12 weeks 0 days pregnant (about 30% of the way through), in your first trimester.

With a longer 35-day cycle, ovulation happens about a week later, so the same last-period date pushes the due date to roughly January 1 — seven days later.

Pregnancy Weeks, Trimesters, and Key Milestones

Stage	Weeks (gestational)	What it marks
First trimester	0–13 weeks	Organs form; miscarriage risk drops after week 12
Second trimester	14–27 weeks	Anatomy scan (~20 weeks); first movements felt
Third trimester	28–40 weeks	Viability, rapid growth, full term at 37 weeks
Full term	37–42 weeks	Most babies arrive between 39 and 41 weeks

StageFirst trimester

Weeks (gestational)0–13 weeks

What it marksOrgans form; miscarriage risk drops after week 12

StageSecond trimester

Weeks (gestational)14–27 weeks

What it marksAnatomy scan (~20 weeks); first movements felt

StageThird trimester

Weeks (gestational)28–40 weeks

What it marksViability, rapid growth, full term at 37 weeks

StageFull term

Weeks (gestational)37–42 weeks

What it marksMost babies arrive between 39 and 41 weeks

Understanding Your Results

Due Date: the estimated day your baby will arrive. Only about 1 in 20 (5%) babies are born on their exact due date.
Gestational Age: how many completed weeks and days pregnant you are, counted from the start of your last period.
Trimester: pregnancy is divided into three trimesters — first (weeks 0–13), second (14–27), and third (28–40).

What Can Change Your Due Date?

This calculator gives an estimate based on your last period and cycle length. Your actual due date may shift because of:

The length and regularity of your menstrual cycle
Ultrasound measurements — an early scan (around 8–13 weeks) is the most accurate way to date a pregnancy
Individual variation in how long pregnancies last
Conception through IVF, where the transfer date sets the timing

Frequently Asked Questions

How accurate is a due date calculator?

A due date calculator gives a solid estimate based on average pregnancy length, but only about 4-5% of babies are born on their exact due date. Most arrive between 39 and 41 weeks. A first-trimester ultrasound is the most accurate way to date a pregnancy, so always confirm with your healthcare provider.

How is my due date calculated from my last period?

The calculator uses Naegele's Rule: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If you enter a cycle length other than 28 days, it shifts the due date by (cycle length minus 28) days to account for when you likely ovulated.

How many weeks pregnant am I?

Gestational age is counted from the first day of your last period and expressed as completed weeks plus days (for example, 12 weeks 3 days). If your last period was 84 days ago, you are 12 weeks 0 days pregnant. The calculator works this out automatically and also shows your current trimester.

What is Naegele's Rule?

Naegele's Rule is the standard method for estimating a due date. It adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, which is why entering your real cycle length makes the estimate more accurate.

Does cycle length change my due date?

Yes. Naegele's Rule assumes a 28-day cycle. If your cycle is longer, you ovulate later, so your due date moves later by the same number of days; a shorter cycle moves it earlier. For example, a 35-day cycle pushes the due date about a week later than a 28-day cycle.

Can my due date change during pregnancy?

Yes. Your due date may be adjusted after an early ultrasound, especially in the first trimester, which measures the baby directly and is more accurate than dating by your last period. Your healthcare provider will give you the most reliable estimate.

Methodology and Sources

This tool calculates the estimated due date with Naegele's Rule (last menstrual period + 280 days), adjusted for your reported cycle length, and dates gestational age backward from the due date — the convention recommended by the American College of Obstetricians and Gynecologists (ACOG). ACOG and the American Institute of Ultrasound in Medicine note that first-trimester ultrasound is the most accurate method for dating a pregnancy, so an LMP-based estimate is a useful starting point rather than a fixed date. Research consistently finds only about 4–5% of births occur on the estimated due date, with most occurring between 39 and 41 weeks.

Important Considerations

This calculator provides an estimate based on general formulas and average pregnancy length. Every pregnancy is unique, and your due date may be revised as your pregnancy progresses.

Always consult your healthcare provider for the most accurate due date and for any questions about your pregnancy. An early ultrasound gives the most reliable estimate.