# Pregnancy due date calculator: how the 280-day rule works, week-by-week dates and how accurate it is

> How is a due date worked out from your last period? Why a pregnancy is counted as 40 weeks when it lasts about 38 from conception, what ‘term’ really means, how many babies arrive on the day, and when a scan changes the date. A plain guide with an example calendar.

Published 2026-10-22 by Toolcloak. 4 minute read. Original: https://toolcloak.com/articles/pregnancy-due-date-calculator-how-the-280-day-rule-works-week-by-week-dates-and-how-accurate-it-is
Related free tool: [Pregnancy Due Date Calculator](https://toolcloak.com/tools/due-date-calculator)

A due date is an **estimate**, not a deadline. It is worked out from the first day of the last menstrual period (LMP) by adding 280 days, which is 40 weeks. Only a small share of babies arrive exactly on that day, and the normal range of a healthy delivery is several weeks wide. This guide shows how the number is made, what each week means, and why your doctor may move it.

> **In short**
> - **Due date = first day of last period + 280 days (40 weeks).** It assumes a 28-day cycle with ovulation on day 14.
> - **Weeks pregnant are counted from the last period,** about two weeks before conception. A pregnancy is therefore about 38 weeks from conception.
> - **Only about 1 baby in 20 arrives on the due date.** A birth from 37 to 42 weeks is considered within the usual range.
> - **An early ultrasound can correct the date,** especially if your cycles are irregular. Your doctor’s date is the one to use.

## The calculation in three lines

**First day of last period: 1 March 2026**

1. **Take the first day of your last period.** 1 March 2026. Not the last day, and not the date of intercourse.
2. **Add 280 days.** 40 weeks from 1 March 2026 is **6 December 2026**.
3. **Estimate conception.** About 14 days after the period started: roughly **15 March 2026**.
4. **Read the result as a window.** Most births fall within two weeks either side, so late November to late December.

*An older method, Naegele’s rule, says to add one year, subtract three months and add seven days. It lands within a day or two of the 280-day count. For this example it gives 8 December.*

Try it: [Pregnancy Due Date Calculator](https://toolcloak.com/tools/due-date-calculator). Enter the first day of your last period to see the estimated due date, the estimated conception date and how many weeks pregnant you are today. This is not medical advice.

## Why 40 weeks when conception is two weeks later?

Doctors count from the last period because it is a date most people can remember. Ovulation and conception happen about two weeks later, so the time from conception to birth is about **38 weeks**. The extra two weeks are counted as pregnancy even though you were not yet pregnant. This is why a woman who is “4 weeks pregnant” when she misses a period is only about 2 weeks past conception.

**The 40 weeks, split into two parts**

|  | Before conception (counted from last period) | After conception |
| --- | --- | --- |
| 40 weeks counted | 2 wk | 38 weeks |

*Grey is the two weeks before conception that still count. Colour is the roughly 38 weeks that the baby has actually been developing.*

## Week-by-week calendar for the example

Here is how the weeks map to dates for a last period starting 1 March 2026. Use it as a guide to the shape of the pregnancy.

| Milestone | Week | Date (example) |
| --- | --- | --- |
| First day of last period | 0 | 1 March 2026 |
| Estimated conception | 2 | 15 March 2026 |
| End of first trimester | 13 weeks 6 days | 6 June 2026 |
| Mid-pregnancy scan window | about 20 | 19 July 2026 |
| Third trimester begins | 28 | 13 September 2026 |
| Early term begins | 37 | 15 November 2026 |
| Full term begins | 39 | 29 November 2026 |
| Estimated due date | 40 | 6 December 2026 |
| Late term | 41 | 13 December 2026 |
| Post-term begins | 42 | 20 December 2026 |

*Dates are 7 times the week number added to the first day of the last period.*

**The three trimesters**

| When | What |
| --- | --- |
| Weeks 1 to 13 | First trimester. Early development. Many people start prenatal care and folic acid here. |
| Weeks 14 to 27 | Second trimester. Often the most comfortable stretch. The anatomy scan is usually done around week 18 to 22. |
| Weeks 28 to birth | Third trimester. Rapid growth. Checks become more frequent towards the end. |

*Exact boundaries vary slightly between guides. Your care provider’s counting is the one that matters.*

## What “term” means

**Week of birth and what it is called**

| Zone | Range |
| --- | --- |
| Preterm | before 37 |
| Early term | 37 to 38+6 |
| Full term | 39 to 40+6 |
| Late term | 41 to 41+6 |
| Post-term | 42 and over |

*Obstetric groups use these names. A baby born at 39 weeks is full term, even if the original due date was a week later.*

## How accurate is a due date?

Quite loose. Studies of births show that **only around 4 to 5 percent of babies arrive on the estimated due date,** and the great majority arrive within a couple of weeks either side. A due date is best read as the middle of a window.

**Out of 20 babies, about how many arrive on the due date?**

| Group | Count |
| --- | --- |
| On the due date | about 1 |
| Another day | about 19 |

*Roughly 1 in 20. The rest come on the days before and after, which is why the date is a guide and not a deadline.*

## When the due date changes

- **Irregular or long cycles.** If you ovulate on day 21, not day 14, the real conception date is a week later than the LMP method assumes.
- **Early ultrasound.** A scan in the first trimester measures the baby and is more accurate than the LMP date when the two differ by more than about a week. Your doctor will then use the scan date.
- **Unsure of your last period.** Say so at your first visit. A scan can date the pregnancy.
- **IVF or other assisted conception.** The date is set from the transfer or egg collection date, which is known exactly.

> **A calculator is not a doctor:** This page and the tool give an estimate for planning. They cannot tell you whether your pregnancy is healthy. If you think you are pregnant, see a doctor or midwife for confirmation and care. Contact them urgently for bleeding, severe pain, reduced baby movements later in pregnancy or anything that worries you.

Planning around the date? The [age calculator guide](https://toolcloak.com/articles/how-to-calculate-your-exact-age-in-years-months-and-days-and-why-leap-years-and-cut-off-dates-matter) shows how date counting works, and the [working days calculator](https://toolcloak.com/tools/working-days-calculator) helps count leave.

## Frequently asked questions

### How do I calculate my due date from my last period?

Add 280 days (40 weeks) to the first day of your last menstrual period. For a period that began on 1 March 2026, the estimated due date is 6 December 2026.

### How many weeks pregnant am I?

Count the full weeks from the first day of your last period to today. If that was 70 days ago you are 10 weeks pregnant. A pregnancy tool does this count for you.

### What is Naegele’s rule?

It is a quick way to estimate the due date: add one year, subtract three months and add seven days to the first day of the last period. It gives nearly the same date as adding 280 days.

### How accurate is the due date?

It is an estimate. Only around 1 in 20 babies is born on the exact date, and most arrive within two weeks of it. A first-trimester scan can improve the accuracy.

### Why is pregnancy counted as 40 weeks and not 9 months?

Weeks are used because they are more precise, and the count starts at the last period, about two weeks before conception. Forty weeks is a little over nine calendar months, about 9 months and 1 week.

### What is full term?

A pregnancy from 39 weeks to 40 weeks 6 days is called full term. Birth from 37 to 38 weeks 6 days is early term, and 42 weeks onward is post-term.

### Does the due date change with an ultrasound?

It can. If an early scan measures the baby more than about a week away from the date given by your last period, doctors usually use the scan date. Follow your doctor’s date.

### What if my cycles are irregular?

The 280-day method assumes a 28-day cycle, so the date may be off. Ask for a dating scan. The date from the scan is usually more reliable than the date from a calculator.