A pregnancy due date is an estimate, not a promise. The most common way to calculate it is from the first day of your last menstrual period, then add 280 days, which equals 40 weeks.

That method is often called Naegele’s rule. In practice, your clinician may adjust the estimate if your menstrual cycle is shorter or longer than average, or if an early ultrasound suggests a different gestational age. For general background on pregnancy timing, you can also use the NHS pregnancy information.

The basic rule behind the due date

Naegele’s rule starts from the first day of your last menstrual period, often shortened to LMP. From that date, pregnancy is estimated at 280 days or 40 weeks.

The logic is simple:

  1. Find the first day of your LMP.
  2. Add 280 days.
  3. The result is your estimated due date.

This assumes a typical cycle length and ovulation timing. It is a standard starting point, not a guarantee of when labor will begin.

A worked example from a hypothetical LMP

Suppose your hypothetical LMP started on April 10.

Using the basic rule, the estimated due date is:

April 10 + 280 days

If you want to see the calculation in week terms, the same estimate is:

40 weeks from the LMP

That is why the due date is often described as 40 weeks from the first day of your last period.

Why cycle length can change the estimate

Naegele’s rule works best when your cycle is close to the average length assumed by the method. If your cycle is longer or shorter, ovulation may happen later or earlier than the default assumption.

That means your due date may need to shift. The basic idea is:

  • Longer cycles can push the estimate later.
  • Shorter cycles can pull the estimate earlier.

A common adjustment is to move the due date by the difference between your cycle length and the standard cycle length used by the calculation method. For example, if your cycle is longer than the assumed length, the estimate is usually moved forward by that difference. If it is shorter, the estimate is moved backward.

This matters because the due date is tied to estimated ovulation, not just calendar math.

How ultrasound dating fits in

An early ultrasound can be used to estimate gestational age by measuring the developing pregnancy and comparing it with expected growth patterns. When the ultrasound result and the LMP-based estimate do not match, the earlier or more reliable information may be used to date the pregnancy.

Ultrasound dating is especially useful when:

  • your periods are irregular,
  • you do not know your exact LMP,
  • your cycle length is not typical,
  • the LMP-based estimate and the scan do not line up well.

In the first part of pregnancy, ultrasound dating is often more precise than later scans, because early growth differences are smaller and easier to interpret. After that, scans are still useful, but they are usually less exact for changing the due date.

Why the due date is only an estimate

The due date is a target date, not a deadline. Labor can begin before or after the estimate, and that is normal.

There are several reasons the date is approximate:

  • Ovulation does not always happen on the same day each cycle.
  • Implantation timing can vary.
  • Sperm can survive for a while after sex, which can make conception timing less exact.
  • Growth differences can show up even in otherwise healthy pregnancies.

So if your due date changes after a scan or cycle-length adjustment, that does not mean anything is wrong by itself. It usually means the estimate has been refined with better information.

Practical ways to use the due date

You do not usually need to calculate everything by hand, but it helps to understand the logic behind the date your clinician gives you.

Here is a practical approach:

  1. Write down the first day of your LMP.
  2. Estimate your usual cycle length.
  3. Add 280 days as the default starting point.
  4. Adjust if your cycle is clearly longer or shorter than average.
  5. Compare the result with any early ultrasound date.

If you want to work with the calendar directly, a date calculator can help you add or subtract days from a starting date. That is useful if you are checking whether a scan date, LMP date, or appointment date fits the same timeline.

When to use a calculator and when to ask for medical guidance

A calculator is useful for estimating dates, but it cannot replace medical assessment. If your cycles are irregular, if you are not sure about your LMP, or if an ultrasound result is different from your expected timing, a healthcare professional can explain which date is being used and why.

You may also want to ask for guidance if:

  • you are not sure which period count to start from,
  • you have had bleeding that may not have been a true period,
  • you conceived after fertility treatment,
  • your scan dates and menstrual dates differ a lot.

If you are tracking pregnancy stages, using a calculator can still help you understand the timeline, but medical staff should make the final call on the official due date. A pregnancy week calculator can also help you see how a date maps to gestational age.

How to read the date once you have it

Once the due date is set, it becomes the reference point for the rest of the pregnancy timeline. It helps estimate which week you are in, when tests or scans are due, and when you are approaching full term.

A simple way to think about it is:

  • LMP gives the starting point.
  • Cycle adjustment refines the estimate.
  • Ultrasound may confirm or replace the estimate.
  • The final due date is still an estimate of when birth may happen.

If you want to check where you are in the pregnancy timeline right now, your next step is to enter your LMP or scan date into a calculator and compare the result with the date your clinician gave you.

Frequently asked questions

What is Naegele’s rule?
Naegele’s rule is a standard way to estimate a pregnancy due date from the first day of your last menstrual period. The basic method adds 280 days, or 40 weeks, to that date.
Why do doctors use the last menstrual period instead of conception date?
The first day of the last menstrual period is usually easier to know than the exact day of conception. It gives a consistent starting point for dating the pregnancy.
What if my cycle is not 28 days?
Then the due date may need adjustment. A longer cycle can shift the estimate later, while a shorter cycle can shift it earlier, because ovulation may happen on a different schedule.
Can ultrasound change my due date?
Yes. An early ultrasound can give a better estimate of gestational age, especially if your periods are irregular or you are not sure about your LMP. That scan date may be used instead of the menstrual estimate.
Is the due date the day I will give birth?
Usually not. It is an estimated date, and labor can begin before or after it. The due date is mainly a planning tool.
How do I calculate the due date by hand?
Start with the first day of your LMP and add 280 days. If your cycle is clearly longer or shorter than average, adjust the estimate based on the cycle difference or use an early ultrasound result.
Should I rely only on a calculator for pregnancy dating?
A calculator is helpful for a quick estimate, but it should not replace medical advice. If your dates are uncertain or your scan and period dates do not match, ask a healthcare professional which date should be used.

About this article

Written by Yahav Atia, WSO – the digital agency (wso.co.il) that owns and founded this site, with the help of AI, and passed automated quality checks before publication. Numbers in the worked examples were computed in code, not estimated. This is general information, not medical or nutritional advice; for personal questions talk to a qualified clinician.