How is a due date calculated?

By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, or equivalently subtract 3 months and add 7 days. An LMP of 1 January gives a due date around 8 October. It assumes a regular 28-day cycle.

Why is pregnancy counted from the last period?

Because the LMP is a date you can actually know, while the exact day of conception usually isn't. Conception happens about two weeks after the LMP, so 'weeks pregnant' includes roughly two weeks before you were technically pregnant - a standard clinical convention.

How accurate is the due date?

It's an estimate - only about 4-5% of babies are born on the exact date. Most arrive within a week or two either side. An early ultrasound (weeks 8-13) measures the baby directly and is more accurate than the LMP method, so clinicians may adjust the date.

What is Naegele's rule?

The standard formula for estimating the due date: first day of the last period minus 3 months plus 7 days (plus a year), equivalent to adding 280 days. It assumes a 28-day cycle with ovulation on day 14, so it's less accurate for irregular cycles.

How many weeks is a full-term pregnancy?

40 weeks is the average, but 37 to 42 weeks is all considered full or term. Babies born before 37 weeks are preterm; after 42, post-term. So arriving a week or two before or after the due date is completely normal.

Can the due date change?

Yes - especially after an early dating ultrasound, which is more accurate than the LMP calculation. If the scan measurement differs meaningfully from the LMP-based date, your clinician will usually revise the due date to match the scan.

Does cycle length affect the due date?

Yes. Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer, you likely ovulate later, so the true due date is a few days later than the standard calculation; shorter cycles shift it earlier.

What are the three trimesters?

First (weeks 1-12): organs form and early pregnancy symptoms appear. Second (13-27): often the most comfortable, with the anatomy scan and first movements. Third (28-40): rapid growth and birth preparation, with term reached at 37 weeks.

When will I feel the baby move?

Usually between weeks 16 and 25, often earlier in later pregnancies. Early flutters ('quickening') build into clear movements in the second trimester. Reduced movement in the third trimester should always be reported to your care provider.

Is an ultrasound or LMP more accurate for dating?

An early ultrasound (first trimester) is more accurate, because it measures the baby's size directly rather than relying on cycle assumptions. The LMP method is a good starting estimate; when the two disagree, clinicians generally trust the early scan.

Can I calculate the due date from the conception date?

Yes - add 266 days (38 weeks) to the conception date, since conception is about two weeks after the LMP. But conception dates are usually uncertain, which is why the LMP method (280 days) is the clinical default. See the conception date calculator for the reverse.

Why do so few babies arrive on the due date?

Because natural variation in cycle length, ovulation timing and each pregnancy's length means birth is spread across several weeks. The due date marks the statistical centre, but the full-term window of 37-42 weeks is wide, so most births fall around - not on - the date.

What if I don't know my last period date?

An early ultrasound can date the pregnancy accurately without the LMP, by measuring the baby. If you're unsure of your LMP or have irregular cycles, a first-trimester scan is the reliable way to establish the due date.

Does the due date affect my care schedule?

Yes - it dates scans, screening tests, growth checks and decisions about induction if pregnancy goes past term. That's why establishing an accurate date early (often via ultrasound) matters for the whole course of care.

How accurate is this due date calculator?

It applies Naegele's rule exactly to your LMP, giving a solid standard estimate for regular cycles. It's approximate by nature - your clinician's assessment and an early scan are more accurate, so use this as a helpful starting point and confirm with your care provider.

Pregnancy Due Date Calculator – EDD from Last Period

Complete guide

Reviewed July 2026

From the moment a pregnancy is confirmed, one question follows every parent-to-be: when is the baby due? The estimated due date (EDD) anchors the whole pregnancy - it dates scans, schedules tests, tracks the baby's growth and frames the countdown. Yet it's exactly that: an estimate, and only about one in twenty babies actually arrives on it.

This calculator estimates your due date using Naegele's rule from the first day of your last menstrual period (LMP). Below you'll find how the 40-week count works, why pregnancy is measured from before conception, how accurate the date really is, the trimester breakdown, and the factors that shift it.

This is general health information, not medical advice. Your doctor or midwife will confirm and may adjust your due date - especially after an early ultrasound, which is more accurate than the LMP method.

How the due date is calculated

Naegele's rule (EDD from LMP):
EDD = first day of last period + 280 days (40 weeks)

Equivalently: LMP date - 3 months + 7 days + 1 year

Assumes a regular 28-day cycle with ovulation on day 14

Pregnancy is counted as 40 weeks (280 days) from the first day of your last period - not from conception. This seems odd, since conception happens about two weeks later, but the LMP is a date you can actually know, whereas the exact conception day usually isn't. So 'weeks pregnant' already includes about two weeks before you were technically pregnant.

Pregnancy is dated 40 weeks from the last period; conception is around week 2.

Worked examples

LMP 1 January: EDD = 1 January + 280 days = about 8 October.
Naegele shortcut: 1 January - 3 months + 7 days = 8 October (same result).
Longer cycles: if you ovulate later than day 14 (e.g. a 32-day cycle), the true due date is a few days later - add the extra cycle days.
An early dating ultrasound (weeks 8-13) is more accurate and may adjust the EDD by several days.

Accuracy, trimesters and why timing varies

Only about 4-5% of babies are born on their exact due date. A full-term birth is anywhere from 37 to 42 weeks, and most arrive within a week or two either side of the EDD. The date is best thought of as the centre of a due window, not a deadline.

The three trimesters
Trimester	Weeks	Highlights
First	1-12	Organs form; morning sickness common; highest miscarriage risk early
Second	13-27	Often the most comfortable; movements felt; anatomy scan
Third	28-40	Rapid growth; preparation for birth; term reached at 37 weeks

The LMP method assumes a regular 28-day cycle and ovulation on day 14. If your cycles are longer, shorter or irregular, the LMP-based date can be off by several days. An early ultrasound measures the baby directly and is the more accurate dating method - if it differs meaningfully from the LMP date, clinicians usually go with the scan.

Using this calculator

Enter the first day of your last menstrual period.
Read your estimated due date and current pregnancy week.
Treat it as the centre of a due window (37-42 weeks is full term).
Confirm with your doctor or midwife, who may adjust the date after an early scan.

Common mistakes

Treating the due date as a fixed deadline rather than an estimate - only ~1 in 20 babies arrive on it.
Counting from conception instead of the last period - pregnancy weeks start ~2 weeks earlier.
Ignoring cycle length; longer or irregular cycles shift the true date.
Preferring the LMP date over an early ultrasound when they differ - the scan is usually more accurate.
Forgetting that 37-42 weeks is all full term, so 'early' or 'late' by a week or two is normal.

Frequently asked questions

Glossary

EDD: Estimated Due Date - the projected date of birth, about 40 weeks from the LMP.
LMP: Last Menstrual Period - the first day of your last period, the dating anchor.
Naegele's rule: The standard method: LMP + 280 days, or - 3 months + 7 days.
Gestational age: How far along the pregnancy is, counted in weeks from the LMP.
Trimester: One of the three roughly-13-week stages of pregnancy.
Term: 37-42 weeks - the normal full-term range for birth.
Dating ultrasound: An early scan that measures the baby to date the pregnancy accurately.
Quickening: The first felt fetal movements, usually in the second trimester.

Key takeaways

The due date is estimated as 40 weeks (280 days) from the first day of your last period using Naegele's rule - counted from before conception because the LMP is a knowable date. It's the centre of a due window, not a deadline: full term is 37-42 weeks and only about 1 in 20 babies arrive exactly on time. Cycle length shifts it, and an early ultrasound is the more accurate dating method. Always confirm with your care provider.

Enter the first day of your last period above for your estimated due date and current week; then confirm it with your doctor and an early dating scan.

Pregnancy Due Date Calculator

Enter your details

Complete guide

How the due date is calculated

Worked examples

Accuracy, trimesters and why timing varies

Using this calculator

Common mistakes

Frequently asked questions

Glossary

Key takeaways

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